The Susceptibility of Sorghum Varieties to Rhizopertha dominica (F.) (Coleoptera: Bostrychidae). Laboratory Investigations

CONTENTS

Page

Declaration

Abstract

Acknowledgement

List of Figures

List of Tables

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW
1.1 Sorghum
1.2 Storage Pests of Grains
1.3 Storage Losses of Grains
1.4 Differences in Varietal Susceptibilities
1.5 The Biology of Rhizopertha dominica (F.)
1.6 Aims and Objectives,of this study

CHAPTER 2 MATERIALS AND METHODS
2.1 The Sorghum Varieties
2.2 The Stock ‘Culture
2.3 Obtaining Insects of Known Age
2.4 F1 Progeny Count
2.5 Weight of Frass Produced
2.6 Weight Loss of Grains
2.7 Total and Median Developmental Periods
2.8 Index of Susceptibility (i)
2.9 Preference Test
2.10 Moisture Content
2.11 Grain Hardness
2.12 Structure of Endosperm
2.13 Grain Size
2.14 Nutrients Analyses

CHAPTM 3 RESETS
3.1 Insect Performance
3.2 Grain Damage
3.3 Preference
3.4 Factors of Susceptibility

CHAPTM 4 DISCUSSION

SWUARY

REFERENCES

APPENDICES

I Storage Pests of Grains :
II Temperature and Relative H^idity Readings Recorded during tte Progeny Count Experiment
III Metoods of Preparation of the reagents used in the nutrients’ analyses
IVA Absorbance of variaus concentrations of D-glucose standard solution and the standard curve in the determination of starch 68
IVB Absorbance readings and x-values of six sorghm varieties in the deteraination of starch
V Absorbance readings of six sorghm in the determination of amylose
VI varieties varieties the determination of phospholipids
VII Titre volue in the determination < of protein
VIII Indices of susceptibility of 12 sorgho varieties calculated using the total developmental period.

LIST OF FIGURES

Figure 1: R. dominica Adult + Larva

Figure 2: aA plan of the choice-chamber used in the preference test

Figure 2: bArrangement of the six sorghm varieties used in the preference test

Figure 3: Nabers of progeny of R.dominica toat emerged from 12 sorghm varieties

Figure 4: Indices of susceptibility of 12 sorgho varieties. it

Figure 5: Weights of frass produced by R, dominica in 12 sorgho varieties during development

Figure 6: Loss in weight of 12 sorghum varieties due to the development of R. dominica

Figure 7: the n^bers of grains with the toree types of endosperm show, in the key found in 100- grain samples of twelves sorgho varieties

LIST OF TABLES

Table 1: A list of 12 sorghum varieties under study

Table 2: Scale for rating endosperm type in sorghum grain

Table 3: A summary of the numbers of progeny of R. dominica produced, indices of susceptibility, weights of frass produced and losses in weight in 12 sorghum varieties.

Table 4: Total and median developmental periods of R. dominica in 12 sorghum varieties

Table 5: Choice of 6 sorghum varieties by R. dominica in three experiments.

Table 6: Grain size, hardness and moisture content of 12 sorghum varieties.

Table 7: The numbers of grains with the five types of endosperm found in 100-grain samples of 12 sorghum varieties.

Table 8: Amounts of primary nutrients in 6 sorghum varieties

Table 9: Amounts of amylose and phospholipids in 6 sorghum varieties.

Table 10: Correlation table.

ABSTRACT

The susceptibility of 12 sorgum varieties to a storage insect pest, toizopertha dominica (F.) was investigated in the laboratoty. These included six improved varieties (L.187, L533, L.1499, 3^912, FF.BL and A.9025) and six local varieties (Farfara, Kurkura, Mori, Yalam, Balankwasa and Ribdaful). For the insect under investigation, the number of F^ progeny produced, length of development, weight of frass produced and loss in weight in each sorghum variety were taken as indicators of grain susceptibility.

Factors suspected to contribute to the susceptibility of grains to storage pests were also investigated. These included grain size, hardness, moisture content and nutrients.

Though none of the varieties was completely resistant, they were found to differ in their susceptibility to the pest in question. Balankwasa was least susceptible with the least number of insect progeny (mean, 10.2), smallest amount of frass (mean, 54.9mg) and lowest loss in weight (mean, 0.57%). On the other hand, FF.BL was most susceptible wito the highest number of insect progeny (mean, 224.2) the largest amount of frass (mean, 96l,5mg) and the highest loss in weight (mean, 4.30%). The other varieties fell in between these two in the said parameters.

Among the suspected factors of susceptibility only amylose was found to affect the susceptibility.of the varieties. This was negatively correlated with the number of progeny and weight of frass produced.

ACKNOWLEDGEMENTS

My sincere thanks are due to my present and former supervisors, Drs. M. Aminu-Kano and J.N. Ayertey, respectively, for their guidance, useful suggestions and untiring assistance, without which this work would not have materialised. I also wish to thank Alhaji M.Y. Gwarzo, of the Department of Biochemistry, for his help in the nutrients’ analyses and Dr. C. Kawagga and Mr.I.G. Elekwa of the Departments of Biological Sciences and Computer Ser­ vices .respectively for their assistance in the statistical analyses. The computer programme used in most of these analyses was. provided by Dr. J.T. Ayodele of the Department of Chemistry. The permission granted me to use the micro­computer of the Department of Chemistry is hereby acknow­ledged.

I also wish to thank the Institute for Agricultural Research, Ahmadu Bello University, Zaria, for providing the improved sorghum varieties used in this study and for permission to use the Institute’s Library.

The research reported here was jointly financed by the Department of Biological Sciences, Bayero University, Kano and ai grant by the Usmanu Danfodio University (U.D.U), Sokoto.

Finally my sincere thanks are due to the Department of Biological Sciences, UsmanuDanfodio University, Sokoto, for releasing me to undertake this study, the entire members of the Department of Biological Sciences and Mai. Abdullahi Chiroma of the Faculty of Law, Bayero University, Kano,, for making my stay in Kano a pleasant one.

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

1.1 Sorghum

Sorghum (Guinea corn, Sorghum bicolor) is one of the most important food crops irj the tropics. In Nigeria sorghum is mainly cultivated in the northern part of the country where it is the most important food crop (it provides 52.37% & 51.17%,respectively,of total protein and calorie intake of rural population, Simmons,1976) and is grown throughout the region, though to a lesser extent in the drier upper north where it is superceded by millet (Webster, 1966). It is planted in May/June and harvested in November/December (Obilana, 1983).

World production of sorghum was put at a little over sixty seven million tonnes out of which Nigeria, the largest producer in Africa and the sixth largest producer in the world, had about four million tonnes, with a yield of 631kg/hectare, grown on six million hectares of farm­land (Anon, 1979 and Obilana, 1983).

Apart from food, sorghum is used in preparing beer in some communities and is also an important feed for livestock crop/(both grain and vegetative parts). The stalk is used in the construction of huts, granaries, fences and similar structures, especially in the rural areas.

In northern Nigeria, sorghum is usually stored unthreshed, at the farm level, in granaries made of mud or plant materials, but the former are better in attack reducing storage pests/(Giles, 1965 and Hays, 1975). Hays (1975) observed that this mode of storage was adequate, efficient and cheap. Threshed sorghum is stored in pots, bags or similar vessels at home or stored in bulk or in silos bagged in ware houses by large corporations.

1.2 Storage Pests of Grains

The most important pests of stored grain are insects, fungi, rodents and mites (Haines, 1982). The more common storage insects and mites are shown in Appendix I.

Howe (1952) reported Rhizopertha dominica (F.) and Sitophilus oryzae (L.) as aommon pests of sorghum in Kano, Nigeria. Giles (1964) reported S. oryzae, Sitotroga cerealella (Oliv.) and R. dominica as important pests of sorghum in northern Nigeria. Halliday (1967 )reported that R. dominica was more common in sorghum samples, taken from a Kano market, than other pests. Other pests of sorghum are (Giles, 1965): Cryptolestes ugandae Steel and Howe, Oryzaephilus mercator Fauv., Tribolium confusum Duval, T. castaneum Herbst, Lasioderma serricone (F.), Carpophilus spp and Attagenus gloriosae (F.).

Storage insect pests gain access to sorghum (and indeed other grains) in one or more of the three ways identified by Giles (1965):-

1. as the crop matures in the field or after it has been cut and laid out to dry, insects may come from nearby granaries;
2. residual infestation when sorghum is stored in old granaries; and
3. movement of insects between granaries during storage.

1.3 Storage losses in.Grain

Food loss is defined as "any change in the availa­bility, edibility, wholesomeness or quality of the food that prevents it from being consumed by people". (Harris and Linblad, 1978).

The major causes of storage losses in grain may be grouped into twot prestorage handling of the grain and factors found in the storage environment.

Prestorage handling of grain

Ripe grain left in the field longer than necessary is more liable to attack by storage insects (Anon, 1978; Giles, 1965 and Allwood and Li, 1982). Grain harvested pre-maturely deteriorates faster during storage, than ripe grain. This is due to the high moisture and enzyme activity in the former. Grain damaged as a result of improper harvesting and threshing techniques is easily attacked by insects and fungi.(Anon, 1978). The same applies to poorly dried grain.

The storage environment

This consists of the biotic (grain, insects, fungi, rodents) and the abiotic (temperature, humidity) components of the store. These components are not independent of one another :they work as a complex system, one influencing another.' Grain respiration and activities of microorganisms may raise the store temperature and humidity and this may result in conditions favourable for insect and fungal growth. High temperatures during the day time followed by low temperatures at night may lead to condensation of moisture on the surface of bulk grain and this will promote fungal growth and caking (Anon, 1978 and Ayertey, 1986).

The major types of grain loss are reduction in dry weight, nutritional value, viability and quality (Tyler and Dendy, 1978). Reduction in dry weight may be caused arthropod by the feeding of / pasts or by spillage from storage structures,damaged by rodents or termites, or by grain respiration. Selective feeding by some insects, where the grain is attacked through the germ, reduces the nutritional value and destroys the germinative power of the grain. Fungi are known to produce toxins (e.g. the fungus, Aspergillus flavus, produces aflatoxin) in the grain. Grain which contains insects (both live and dead) with their excreta and skin casts or rodent excreta and hairs may be unacceptable to consumers (Anon, 1978).

The quantitative loss of sorghum to storage insects in Nigeria has not been accuratelly assessed. Whereas Giles (1964) estimated it at 4%, Caswell (1978) reported a much lower figure (V4%).

1.4 Difference in varietal susceptibilities

It has been reported by many workers that varieties of the same crop may differ in their susceptibilities to storage insects. Varieties of maize (Hinds, 1914), wheat (Rahman, 1942), sorghum (Samuel and Chatterji, 1953 1 and Giles, 1965), rice (Fernando, 1957), barley (Sinha, 1969, 1971) and legumes (Brewer and Horber, 198?) have been shown to differ in their susceptibilities to various storage insects.

The characteristics of grains that have been implicated in their susceptibility (or resistance) to insects include grain hardness, size, smoothness, Chemical composition and equilibrium moisture content. In the field the grains may be offered additional protection by structures such as husks (maize), glumes (sorghum) and pods (legumes) (Dobie, 1977 and Rogers and Mills, 1974a).

Doggette (1957) attributed the resistance of some sorghum varieties to the thickness of the hard outer corneous endosperm and grain size - varieties with comparatively more corneous than the soft mealy endosperm were less damaged than those with more mealy endosperm. Similary, small grains were less damaged than large ones. The corneous endosperm was thought to be less suited to the insects’ need (for food) than the mealy one. This was confirmed by Davey (1965) who found that adults of Sitophilus oryzae (L.) consumed more of a sorghum variety the UR477, in which/majority of the seeds had a predominantly mealy endosperm than of another variety, ’Wiru, in which the majority of the seeds had a predominantly corneous endosperm. Furthermore higher oviposition and adult emergence were recorded on the former variety than the latter.

Khalifa (1962) suggested that large sorghum grains were more susceptible to Trogoderma granarium Everts than small ones because the large grains offered a larger proportion of starchy endosperm which was more suitable for larval growth. Russell(1962 and 1966) found a negative correlation between the hardness and size of grains of some varieties of sorghum, and the life span of adults of S. oryzae produced. Similarly Fadelmula and Horber (1983) found a negative correlation between grain hardness and number of progeny and weight of frass produced, by S. oryzae and Sitotroga cerealella (Oliv.), on some Sudanese and American sorghum varieties.

The chemical components of grain likely to affect varietal susceptibility are nutrients (carbohydrates, fats and proteins), tannins and toxins (enzyme inhibitors). Sugar and starch contents of maize kernels were found to be highly positively correlated with number and weight of S. oryzae offspring produced while there was a negative correlation with fat content, When fat content was correlated with insect weight and reproductive rate in combination with other factors of susceptibility (eg hardness, sugar etc), the contribution of fats to susceptibility was found to be negligible (Sing and McCain, 1963). Although Rhine and Staples (1968) reported that the amylose content of maize did not affect either larval nutrition or survival of Rhizopertha dominica (F.) and Tribolium castaneum Herbst, they found a negative correlation between high amylose content and weight of adults of S. oryzae, Sitophilus granarius (L.) and S. cerealella. Tannin has been known to offer cereals some protection against bird damage

(Rooney and Sullins, 1976) but its effect on storage insects has not been established. Fadelmula and Horber (1983) found that high tannin content did not contribute to resistance of sorghum grains to S. oryzae and S. cerealella. Resistance of cowpea to Callosobruchus chinensis (L.) was probably as a result of high levels of trypsin and chymotrypsin inhibitors in the bean (Brewer and Horber, 1983).

Wheatly (1973) observed that maize varieties having higher equilibrium moisture content (E.M.C) were more susceptible to Sitophilus zeamais Motsch. than those having lower E.M.C. but Dobie (1974) observed that the E.M.C. was not significant in the resistance of maize to this pest. Rogers and Mills (1974b) assessed reactions of some sorghum varieties to S. zeamais under three relative humidities (R.H) and found that resistance was not associated with the ability of a variety to equilibrate at a lower moisture content when exposed to a given R.H.

The factors contributing to the resistance of grain to storage insects, discussed in the foregoing, are gene­tically controlled (Widstrom al, 1983) but they may be modified by environmental conditions e.g. the nature of the soil on which the crop is cultivated, seasonal fluctuations in rainfall, etc (Bhatia, 1976 and Dobie, 1977).

1.5 The Biology of Rhizopertha dominica (F.)

R. dominica, commonly refered to as the lesser grain borer, is a cylindrical, dark brown to black beetle about 2,5 to 3.0mm' long and 0.7mm wide (Allwood and Li, 1982) (fig,1). It bores into and feeds on the interior of virtually all grains and may even bore into wood and paper (Metcalf and Flint, 1962). The female lays 200 - 400 eggs. These are laid singly or in clusters amongst the grain and, after hatching, the larvae bore into the grains where they mature. The life cycle is completed in 4 to 7 weeks and the adults live up to 3 months (Allwood and Li, 1982).

Thomson (1966) reported that the life cycle of R.dominica .was completed in 38 days (egg 9, larva 16, pre-pupa and pupa 7 and pre-oviposition period 6 days) when reared on milo grains of 12% moisture content at 25+0.5°C and 75+5% R.H. The oviposition period was 27 to 37 days and the female lived for 35 - 53 days.

The first instar larva was unable to penetrate grain with a moisture content of less than 8%. He also reported that the adults were able to mate within 24 hours of eclosion and they could mate several times.

Birch (1945a) reported that R. dominica could complete development at any temperature within the range of 18.2 to 39.0°C when reared on wheat grains of moisture content 14% but the optimum temperature was 34°C. Oviposition dropped markedly when R. dominica was fed wheat grains with a moisture content of less than 9%. No eggs were laid in grains with moisture content of less than 8% (Birch, 1945a).

Abbildung in dieser Leseprobe nicht enthalten

Fig 1: R. dominica ADULIT + LARVA

It is apparent from the literature that the longevity by, of R. dominica. is affected/ among other factors,nature of diet on which it is fed. Koura (1971) reported that the adults liv^for 62, 46 and 21 days respectively when fed somd maize, wheat and.rough rice grains. Similar results were obtained when the insects were fed different preparations of the same cereal - the longevity was 31» 46, 51 and 102 days respectively on wheat straw,;, somd wheat grains, crushed wheat grains and whole wheat flour. During starvation, the adults liv^for 11 days (Koura et al. 1971) rnd the first, second, third and fourth instar la^ae for 14, 16, 15 and 17 days respectively (toare, 1965).

1.6 Aims and Objectives of this Study

The methods employed in reducing grain losses during storage can be classified into four - cultural, physical, chemical and biological. The cultural methods are slow and labour intensive. Ayertey (1986) has highlighted the major defficiencies of the other 'techniques.

He observed thatthe high cost and expertise involved in the physical methods make them impractical in developing countries while the need to treat large numbers of insects and the expertise and research requirement of the sterile insect technique and the use of pheromones eliminate the feasibility of biological control using these metoods.

In the same report, Ayertey also identified the problems associated with chemical control as toxicity to non-target organisms, risks to operators, insect resistance and the accumulation of pesticide residues in treated grain. In addition,seed fumigation may reduce the growth rate of seedlings arising from such seeds as in Caswell and Clifford (1960).

A more feasible biological control method is the breeding and cultivation of varieties resistant to the pests under consideration. Apart from the specificity of this technique to the target organism it has the advantage of being neither labour - nor capital-intensive. Although local crop varieties, in general, have been found to be more resistant to storage pests than im­proved varieties (e.g. Doggette, 1957; Dobie, 1977 and Anon, 1978); this is likely to be because improved varieties are bred for higher potential yields with little consideration for their susceptbility to storage pests. Consequently, pest-resistant as well as high-yielding crops can be produced if the former trait is incorporated into plant breeding programs.

This project aims to carry out a laboratory assessment of six local and six improved varieties of sorghum with a view to identifying,

a) the relative susceptibility of each variety to the lesser grain borer, Rhizopertha dominica (F.)
b) the factors responsible for any differences that might be found from the above.

The first part of the study will indicate whether improved varieties are more susceptible to infestation by the grain borer than local varieties. Furthermore, resistant varieties identified from the study will be recommended for widespread cultivation so as to reduce storage losses and consequently increase the quantity of sorghum available in Nigeria.

Identification of the factors that confer resistance will help in evolving effective control strategies. If they are genetic, plant breeders might be able to reproduce them in later varieties.

R. dominica has been chosen for this study because, apart from being an important pest of sorghum, it is more tolerant of the unfavourable ambient temperature and humidity condition characteristic of the study area* than most of the other storage pests (Ayertey and Ibitoye, 1986; Birch, 1945b and Allwood and Li, 1982).

The choice of sorghum is because it is an important food crop and it is increasingly becoming a source of raw material for a number of industries (e.g. breweries) in Nigeria. In addition,the country is the largest producer of the crop in Africa and the sixth largest producer in the world (Anon, 1979). Moreover, the Ahmadu Bello University, Institute for Agricultural Research in Zaria (about 150km away) has a sorghum breeding program where improved varieties could be obtained while local varieties are easily obtainable in the study area*.

A description of the sorghum varieties under study is given in table 1.

* Kano, Nigeria.

Table 1: A list of 12 sorghum varieties under study

Abbildung in dieser Leseprobe nicht enthalten

*Institute for Agriculture Research, Ahmadu Bello University, Zaria.

CHAPTER 2

MATERIALS AND METHODS

2.1 The sorghum varieties

Details of the sorghum varieties, used in this study, are given in table 1. The grains and glass­ware used in sections 2.2 and 2.4 were sterilized by heating in hot air oven at 80°C for 3 hours. The grains were then stored in a deep freezer until when they were needed for use.

2.2 The stock culture

Adult Rhizopertha dominica (F.), obtained from the Nigerian Stored Products Research Institute (N.S.P.R.I.), Kanotwere used in raising the stock culture from which experimental insects were taken. The insects were reared on a local sorghum variety, ’’Kaura*’, in one litre canning jars, covered with muslin cloth, under ambient laboratory conditions.

2.3 Obtaining insects of known age

Grain was taken from culture jars and sieved, using a sieve of 10 mesh to the inch, to remove frass and adult insects. Individual grains harbouring adults were hand-picked. These grains, the frass and the adults were returned to the culture and the grains that were free of adults were placed in fresh glass jars (covered with muslih cloth). The sieving process was repeated for three consecutive days to make sure that all adults were removed from the grain.

The sieved grain was kept mder ambient conditions for 7 days and then sieved again to remove the adults that had emerged, these adults, which were 0 to 7 days old, were kept in fresh glass jars and fed ”Kaura[1]’.

2.4 Progeny Comt

Seven replicates of 50g samples were collected for each variety. The grain was placed in l40cm3 clear glass jars with metal screw-top covers. A hole, 1.5cm in diameter, was cut through each cover, to provide ventilation. A. 5.5cm quality Ti Whatman filter paper was placed inside each cover to prevent the insects from escaping and to prevent mwanted infestation. Twenty, 7 to 18 days old, ad^t R. dominica, obtained from the stock culture as described earlier (section 2.5), were introduced into each of five replicates of all the varieties. The other two replicates for each variety were used as control. All the jars were placed on a laboratory bench under ambient conditions. Temperature and relative hmidity were recorded by a theraohygregraph placed on the same bench (the readings are given in Appendix II). the insects were removed after eight days and the jars left mdisturbed mtil the begining of emergence of the FT progeny. Merged adults were cornted daily until the end of emergence.

2.5 Weight of toass Produced

At the end of emergence in the experiment above, the grain in each replicate was sieved with a 40 mesh to the inch sieve and the frass collected was weighed.

2.6 Weight Loss of Grains

The replicates were re-weighed at the end of the experiment (section 2,4), The difference between the final weight of a replicate of any variety and the mean of the final weights of the two control replicates of toat variety was regarded as the loss in weight due to insect damage to that replicate, tois loss was expressed as a percentage from, loss in weight (%) = (W1) x 100 Were = mean of final weights of control replicates W2 = final weight of replicate

2.7 Total and Median tevelopmental Periods

The period from the begining of oviposition to the end of adult emergence, in the progeny comt experiment (section 2.4), was refered to as the total developmental period. The median developmental period was the period between the begining of oviposition and the emergence of 50% of the adults. The two periods were recorded in days, for each replicate.

2.8 Index of Susceptibility (I)

This was calculated using the formula (Dobie, 1974)

Abbildung in dieser Leseprobe nicht enthalten

where F is the number of progeny and D is the median developmental period in days (see section 2.7).

Another index of susceptibility was calculated taking D as the total developmental period (see section 2.7).

2.9 Preference Test

Six, out of the twelve sorghum varieties, were selected for the preference test. The selection was based on the results of the progeny count experiment (section 2.4) so that two of them (FF.BL. and L.187) were judged ’highly susceptibletwo (L.1499 and BAL) ’slightly susceptible’and two (SK5912 and KUR) ’moderately susceptible’.

A choice chamber was improvised, for this experiment, as described below.

A 400g powdered milk ("Nido”) tin was obtained (diameter, 9.5cm and height, 13cm). Six holes, 5.0cm apart and 0.8cm in diameter, were cut round the base.

Six condensed milk tins (diameter, 6.0cm and height, 6.0cm) were obtained and each treated as follows:

The top was cut open and a hole (diameter, 0.8cm) was 2.0cm from the base. A hole (diameter 0.8cm) was cut cut/5.0cm on either side of and at the same height with the first hole. A rigid PVC tube (length, 7.0cm, external diameter, 0.8cm and internal diameter, 0.5cm), was inserted into the middle hole. The other end of the tube was; inserted into one of the holes on the ”Nido” tin described above. The six condensed milk tins were then interconnected with flexible PVC tubes .(eseh about 10#0cm long and with the same external and internal diameters as the rigid PVC tubes mentioned earlier) through the side holes. Thus the resulting structure, the choice chamber had the following features:

(1) The central vessel, formed by the "Nido" tin.
(2) The peripheral vessels (six), formed by the condensed milk tins.
(3) Radial passages, linking the peripheral vessels with the central vessel, foraed by the rigid PVC tubes.
(4) The peripheral passage foraed by the Hexible PVC tubes.

Details of the choice ch^ber are shown in fig.2a. Five replicates of the choice chamber were made as described above.

The six peripheral vessels were filled, to the level of the holes, with a different sorghm variety each, and covered with Whatman quality 1 filter paper (size, 11.0cm) held with masking tape. 120 adult insects were released into the central vessel and the vessel covered with the same filter paper described for the peripheral vessels. The remaining four replicates of the choice chamber were similarly treated making sure that no one variety was bomd, on both sides, by the same variety in more toan one of the replicates (fig. 2b).

The apparatus was examined after 48 hours and the insects fomd in the various vessels comted and recorded.

The experiment was repeated two more times.

2.10 Moisture Content

5 samples of 100 grains each (for each variety) were collected, weighed and heated in watch glasses in ventilated hot air oven for 4 hours at 120°C (Pixton, 1967). After cooling (in the oven) the samples were

Abbildung in dieser Leseprobe nicht enthalten

Fig. 2a: A PLAN OF THE CHOICE CHAMBER USED IN THE PREFERENCE TEST (Section 2.9)

Abbildung in dieser Leseprobe nicht enthalten

KEY

1 L.187

2 L. 1499

3 SK 5912

4 FFBL

5 KUR

6 BAL

Fig. 2b: ARRANGEMENT OF THE SIX SORGHUM VARIETIES IN THE FIVE CHOICE CHAMBERS USED IN THE PREFERENCE TEST (Section 2-9)

transfered into dry, previously weighed, glass vials and re-weighed. Moisture content (M.C) was then calculated, as a percentage of the wet weight, from, M.C - (WrW2) x 1£)0 * —ST where W1 = the weight of sample before heating and W2 =the weight of sample after heating.

2.11 Grain Hardness

The grinding and sieving method, similar to that used by Davey (1965), was employed, in this experiment, as an indirect way of measuring grain hardness. The theory of this method is that when equal quantities of grains of different hardness are ground and sieved under the same conditions more of the softer grain will pass through the sieve than the harder one.

Five 10g-replicates were separated from each variety. Each of the replicates was ground with a domestic coffee grinder (PREDOM-PRESPOL TYP 53 SM) for 60 seconds and sieved with a 40 mesh to the inch sieve. The portion that passed through (A) and the one that was retained (B) by the sieve were weighed. The ratio of the retained to the total amount was calculated as a percentage using the relation, % Retained = B/(A+B) x [100]

2.12 Structure of Endosperm

100-grain samples were collected from each variety. The grains were soaked in water for 24 hours, removed and disected, longitudinally, using a sharp razor blade.

The sections were studied ^der adisecting microscope (mastication, x10) and the proportions of the corneous and mealy endosperms were rated according to the scale below (Davey, 1965).

Abbildung in dieser Leseprobe nicht enthalten

TABLE 2: SCALE FOR RATING ENDOSP^M TYPE IN < SORGHUM GANS

2.13 Grain Size.

Five 100-grain samples were taken from each variety. Each sample was dropped into a 25ml ^ass measuring cylinder, filled with water upto the 10ml mark. The volw of water displaced by each sample was taken as its val^e.

2.14 Nutrients[1] Malyses.

A.100g portion of grain was taken from each of the six sorgho varieties used in section 2.^ This portions were repeatedly ground to fine powder with the same coffee grinder used in section 2.11 and sieved wito a 40 mesh to the inch sieve. The powder was stored in glass jars for all the analyses carried out in tois section.

* Due to the shortageof BAL;MOR was used in its place.

The starch, amylose, total fat, phospholipid and protein contents of the six varieties were deterained. The metoods of preparation of the reagents, used in these determinations, are given in Appendix III.

2.14.1 Determination of Starch

The starch content of the grains was determined according to the method in Wistler (1964). It involved the extraction of soluble sugars from the grain and a subsequent extraction of the starch using perchloric acid, the starch was precipitated with iodine and the iodine­starch complex was then decomposed wito alkali and the liberated starch was determined, colorimetrically, using anthrone reagent. Details of the procedure are set out below.

a) Ectraction of soluble sugars

A. 50mg sample of the flour was weighed for each of the test varieties and each was placed in a 50ml conical centrifuge tube. A.few drops of 80% ethanol, followed by 5ml of distilled water, were added while ; stirring* The tubes were imersed in a hot water bath and 25ml of hot 80% ethanol added to each.

Stirring- was continued for several minutes, the tubes were centrifuged at 2000 r.p.m. for five minutes, the supernatant liquid was decanted and the tubes returned to the water bath for a repetition of the process.

b) Extraction of starch

5ml of distilled water were added to the sugar- free residue in each of the tubes and the tubes were placed in a boiling water bath for 15 minutesafter cooling to room temperature, 6.5ml of 52% perchloric acid were added to every tube while stirring. Stirring was continued for five minutes. The tubes were stood for 15 minutes with occasional stirring. 20ml of distilled water were added to each of the tubes and they were centrifuged at 2000 r.p.m for 5 minutes and the supernatant liquid of each was decanted into a 50ml volumetric flask. A further 6.5ml of the perchloric acid were added to each tube, whiles timing as beforehand the contents added to the flasks. Tubes and stirring- rods were washed with distilled water and the washings added to the flasks. The contents of the flasks were diluted to the mark, returned to the tubes and centrifuged as before. The supernatant was decanted through glass wool into 50ml conical flasks.

c) Precipitation of starch with iodine

A 10ml aliquot of starch extract (for each variety) was pipetted into a 50ml conical centrifuge tube and 5ml of 20% sodium chloride solution and 3ml of iodine solution1(Appendix IIIA) added to each. The contents were mixed and allowed to stand for 30 minutes. The tubes were centrifuged at 2000 r.p.m for five minutes and the supernatant liquid was removed. 5ml of ethanolic sodium chloride wash solution (Appendix IIIC) were added to each tube and the preci­pitate washed. The tubes were centrifuged as before and the wash solution discarded. The washing process was repeated once. 23

d) Decomposition of the starch-iodine complex

2ml of ethanolic sodium hydroxide solution (Appendix IIID) were added to each tube. The tubes were stood for 15-20 minutes with occasional shaking until the blue colour of the precipitate was completely discharged. They were then centrifuged at 2000 r.p.m for 5 minutes’ and the supernatant liquid discarded.

The liberated starch was washed, two times, with ethanolic sodium chloride solution (Appendix IIIC) as in (C). The starch was dissolved in 5ml of distilled water and transfered into 50ml volumetric flasks. The tubes and stirring rods were washed with distilled water and the washings were added to the flasks. The contents were mixed thoroughly and diluted to the mark.

e) Colorimetry

A 1ml aliquot of the starch solution from (d) above was pipetted into a sulphuric acid-rinsed test tube for each variety and this was diluted with 1ml of distilled water. A series of sulphuric acid-rinsed test tubes containing 0 to 200JJg of 0.01% D-glucose in 2ml of solution were prepared (Appendix IVA). These tubes ( and those of the test varieties) were immersed in cold water and 10ml of the enthrone reagent (Appendix IIIE) were added to each. They were thoroughly mixed and placed in » boiling water bath until a permanent deep green colouration developed (after about 20 minutes). The tubes were cooled by immersion in cold water and the absorbance was read at 620 an on a Beckman 35 spectrophotometer. The absorbance readings of the D-glucose solution are given in Appendix IVA. Three absorbance readings were taken for each variety of sorghum (Appendix IVB).

f) Calculations

A. standard curve was obtained by plotting absorbance against concentration for the various concentrations of the 0.01% D-glucose solution (Appendix IVA). The amount of starch in the test samples was obtained, by extra­polation, as outlined below.

I) The x-values corresponding to the absorbance of the test samples were calculated from the regression equation of the D-glucose standard curve (Appendix IVA). These values are given in Appendix IVB,
II) The x-values were multiplied by a factor of 0.90 to obtain the weight of starch in the 1ml aliquot of the test samples used in colorimetry. Thus, the weight of starch in 1ml aliquot = 0.90x Ug.
III) The weight of starch in the 1ml aliquot was multiplied by 50 to give the weight of starch in the 50ml solution from which the aliquot was taken. Thus, the weight of starch in 50ml = (0.90k) x 50Ug.
IV) The last value was multiplied by 5 to give the weight of the total extracted starch (because the 50ml were equivalent to one fifth of the total extracted starch)• Thus, the weight of total starch = (0.90x)x50x5/1000 mg

V) The percentage of starch in the sample was calculated, from, starch(%)

Abbildung in dieser Leseprobe nicht enthalten

2.14.2 Determination of Amylose

The iodine sorption (blue value) method was employed in this determination. The method estimates the quantity of amylose in each sample as the blue value. Theoritical background on the method can be found in Whistler (1964). The procedure is outlined below.

A. 2ml aliquot of starch solution, extracted in the last section (2.14.1 (d)?was pipetted into a 50ml volumetric flask (one for each variety), 0.5ml of 1M sodium hydroxide was added and the solution heated for 3 minutes in a boiling water bath. After cooling, 0.5ml of 1M hydrochloric acid and about 0,1g of potassium hydrogen tartarate (Appendix IIIF) were added. Distilled water was then added to a volume of about 45ml followed by 0,5ml of iodine solution2 (Appendix IIIB), The solution was diluted to the mark and shaken. After standing for 20 minutes, at room temperature, absorbance was read at 680nm on a Spectronic 20 spectro­photometer (Appendix V). A reference solution was prepared by mixing all the reagents that were added to the test samples in 50ml solution.

The blue value was calculated from the relation, Blue value (B.V.) = Absorbance (A) x 4 C (mg/IO'Omlj where C = carbohydrate content of the solution, C was calculated as follows:

The x value in Appendix IVB were multiplied by a factor of 0.90 to obtain the carbohydrate (starch) content of a 1ml aliquot of the starch solution in mg. But 2ml were used in this test.

Abbildung in dieser Leseprobe nicht enthalten

The 2 ml aliquot was made to 50ml giving a concentration of (0.90x) x 2 JJg/50ml

Abbildung in dieser Leseprobe nicht enthalten

substituting C in the formula

Abbildung in dieser Leseprobe nicht enthalten

would give

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2.14.3 Determination of Total Fat

This involved the extration of fat from the sample by stirring it in a mixture of chloroform, methanol and water. Two layers separated; a lower chloroform layer (containing the fat) and on upper aqueous layer.

The latter was removed and the former was dried with anhydrous sodium sulphate. The chloroform was evapourated off leaving the fat residue. The procedure,, from Osborne and Voogt (1978), is given below.

A 5g sample of flour was weighed into a 50ml centrifuge tube and mixed with 5ml of water. Six glass beads, 5ml of chloroform, 10ml of methanol were addei into each tube, these were followed by 0.06ml of 20% magnesium, chloride solution to reduce emulsification, the contents were mixed on a whirlimixer for two minutes. A further 5ml of chloroform were added and the contents mixed for another two minutes. 4ml of distilled water were added and the contents mixed for another half a minute. The extract was filtered, torough sinteral glass (porosity No.1), into 50ml conical centrifuge tubes, using gentle suction. Extraction tubes and residue were washed toree times wito 2.5ml portions of chlorofora. The filtrate was then centrifuged at 1500 r.p.m. for 5 minutes. Most of the upper aqueous layer was removed wito a Pasteur pipette attached to a suction line. 10ml of 0.1% sodi^ chloride solution were added to the remaining chloro­form extract and the contents mixed by gentle inversion several times. The extract was again centrifuged and the upper layer was removed as before, the extract was dried by shaking vigorously wito powdered anhydrous sodi^ sulphate (1.5g per sample) and filtered torough sintered glass (porosity No.4), using gentle suction, into dry test tubes. Tubes and sinteres were washed toree times with 2.5ml portions of chloroform, the dried chlorofora extracts were transfered into 50ml ^at bottomed flasks of known weights and the test tubes washed toree times with 2.5ml portions of chloroform. the washings were added to the extracts. The flasks were placed on a- steam bath until all of the chloroform evaporated and only the yellow fat residue was left, the flasks were then placed in an oven for 5 minutes at 100°C and afterwards cooled in a dessicator and re-weighed. This analysis was repeated once.

The percentage of total fat in each sample was estimated from,

Abbildung in dieser Leseprobe nicht enthalten

where W1 » weight of sample in grams

W2 = weight of empty flask in grams

W3 = weight of flask + fat in grams.

2.14.4 Determination of Phospholipids

This involved the extraction of fat from the sample, using chloroform (this excluded inorganic substances). Phos­pholipids were then determined from the extract as "organic” phosphorus. The details of the procedure from Osborne and Voogt (1978) and King and Wootton (1959) are given below.

Fat was extracted from the samples as in section 2.14.3^* and a 3ml aliquot was pipetted from the chloroform^† extract (for each variety) into a test tube. 0.5ml of 52% perchloric acid and 3 glass beads (to reduce bumping) were added to each tube and the tubes were heated gently on an electric heater for a few minutes. Thereafter, a drop of concentrated nitric acid was added to each tube and the heating continued until the mixture cleared. The tubes were then cooled and 5ml of distilled water and 0.4ml of 5% ammonium molybdate solution were added to each. 0.2ml of the reducing agent (Appendix IIIG) were added to each of the tube and the standard solution (Appendix IIIH) and, after 10 minutes, absorbance was read, on a Spectronic 20 spectrophotometer, at 660nm (Appendix VI), The reference solution was made of 5ml of distilled water, 0.4ml of the molybdate and 0.2ml of the reducing agent.

The test was repeated once.

The amount of "organic” phosphorus in each sample was; calculated from the relation,

Phosphorus (P) = Absorbanc.e of Test Sample (T) x q,02x10 (in mg/10ml) Absorbance of standard (s)

where f = volume of fat extract, used in the test, in ml and 0.02 = weight of phosphorus in the standard, in mg

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2.14.5 Determination of Protein

The Kjeldahl principle was employed in this deter­mination - it involved the digestion of the sample with concentrated sulphuric acid, in the presence of a copper catalyst and potassium sulphate (to raise the reflux temperature), to convert aminoid nitrogen into ammonium sulphate. The latter was converted to ammonia (which was distilled into boric acid solution) by the addition of sodium hydroxide solution. The distillate was titrated with hydrochloric acid to determine the amount of nitrogen in the samplethis was used in estimating the amount of protein. Details of the procedure,found in King and Wootton (1959), Whistler (1964) and Welcher (1975) are given below:

a) Digestion

A 1g sample of flour was weighed for each variety. 1g of potassium sulphate and 0,1g of copper sulphate were, added to each. Each sample was then transfered to a 800ml Kjeldahl :Qask and 2ml each of distilled water and concentrated sulphuric acid (specific gravity, 1.84) were added. Six anti-h^ping granules were dropped into each flask and the -contents gently boiled for 10 minutes.

b) Distillation

After cooling, each digest ws'diluted wito 20ml of distilled water. 10ml of the standadised boric acid solution (Appendix IIIK) were accurately measured into each of six 50ml conical flasks and each was placed mder a condencer of the Kjeldahl apparatus so that the tip of the latter was dipped into the acid. 10ml of 40% sodium hydroxide solution were added into each of the Kjeldahl flasks (along the sides). Each flask was connected to the condencer, immediately after the addition of sodi^ hydroxide, to avoid loss of ammonia. Power was turned on and the distillation continued gently for 15 minutes. The conical flasks were lowered and the tips of the condencers rihsed wito little distilled water.

c) Titration

The colour of the boric acid changed from grey to green on receiving the distillate. It was back titrated to grey wito 0.1M hydrochloric acid and the titer vol^e recorded (Appendix VII). the test was repeated twice. A blank determination was carried out substituting D-glucose for the sample.

The amount of protein in the samples was calculated from,

(i) Nitrogen(%)= (A-B)x< Molarity of Hcl x 1.4

sample weight (g)

where A = sample titre .(ml of Hcl)

B = blank litre (ml of Hcl)

(ii) Protein (%) = % Nitrogen x 5.70

in this study

% protein = (A-0.8) x 0,14 x 5.70

= Cä-0.8) x 0.798

CHAPTER 3

RESULTS

3.1 Insect Performance

Insect performance was assessed as the number of adult insects that emerged from each of the test varieties (section 2.4) and the corresponding total and median developmental periods (section 2.7).

As shown in Fig.3 and Table 3, the highest number of adults emerged from an improved variety, FF.BL (mean, 224.2) and the lowest from a local variety, BAL (mean, 10.2). The number of adults in FF.BL was not only more than twenty times that in BAL but was more than double that in L.187 (mean, 79*2), the second largest. An analysis of variance (ANOVA) on these results revealed that differences in the insect number of/ progeny between the varieties were significant (p<0.001).

The total and median developmental periods (TDP and MDP, respectively) are shown in Table 4. The highest MDP was in A-9025 (mean, 44.8 days). Both TDP and MDP (mean, 41.8 and 33-4 days respectively) were lowest in BAL. The differences in TDP between the varieties were significant (ANOVA, p<0.01) while MDP did not differ significantly between the varieties (ANOVA, p>0.05).

It is apparent from the indices of susceptibility (Fig.4 and Table 3) that FF.BL was the most susceptible while BAL was the least. The other varieties fell in between these two. ANOVA revealed that the varieties differed significantly in their susceptibility to the insect (p<0.001).

The indices of susceptibility calculated using TDP are given in Appendix VIII and were also significant (ANOVA, PC0.001).

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Fig 3: NUMBERS OF PROGENY OF R.DOMINIC THAT EMERGED FROM 12 SORGHUM VARIRTIES

Abbildung in dieser Leseprobe nicht enthalten

Fig 4: INDICES OF SUSCEPTIBILITY OF 12 SORGHUM VARITIES

Table 3; A sumary of the nmbers of progeny of R. dominica produced, indices of susceptibility, weights of frass (mg) produced and losses in weight (%) in 12 sorghm varieties.

Abbildung in dieser Leseprobe nicht enthalten

*S.E.M: Standard Error of the mean

**Means followed by the same letter are not significantly different from each other at p=0.05 according to Duncan's Multiple Test (DMRT).

Table 4: Total and median developmental periods of R. dominica (in days) in 12 sorghum varieties.

Abbildung in dieser Leseprobe nicht enthalten

* Means followed by the same letter are not significantly different from each other at p=0.05 according to DMRT.

3.2 Grain Damage

The weight of frass produced by the insects in each variety and the corresponding loss in weight (as compared to controls) were taken as indicators of damage.

The largest amount of frass (mean, 961.5mg) was obtained from FF.-BL and this was about twenty times that of the lowest,which was BAL (mean, 54.9mg)^and three times that of the second largest^which was L.187 (mean, 297.3mg) (Fig.5 and Table 3). ANOVA indicated that the amounts of frass produced in the varieties were significantly different (p<0.001).

The loss in weight results are given in Fig.6 and Table 3) and were significantly different from one another (ANOVA pcO.OOH). They showed a trend similar to that shown by progeny and frass results with FF.BL suffering the highest loss in weight (mean, 4.30%) and BAL the lowest (mean, 0.57%).

3.3 Preference

The results of the preference test are given in Table 5. Wen subjected to a 3-factor ANOVA, the differences between varieties were not significant (p>0.05) suggesting that the insects did not show prefe­rence for any of the varieties.

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Fig. 5: WEIGHTS OF FRASS PRODUCED BY R. DOMINICA IN 12 SORGHUM VARIETIES DURING DEVELOPMENT

Abbildung in dieser Leseprobe nicht enthalten

Fig. 6: LOSS IN WEIGHT OF 12 SORGHUM VARIETIES DUE TO THE DEVELOPMENTWEIGHT OF R. DOMONICA

Table 5: Choice of 6 sorghum varieties by R. dominica in three experiments (as % of insects put into the choice chamber).

Abbildung in dieser Leseprobe nicht enthalten

3.4 Factors of susceptibility

As highlighted in section 1, the factors suspected to affect the susceptibility of grains to storage insects are physical factors such as grain size, hardness and moisture content (e.g. Davey, 1965) and nutrients (e.g. Sing and McCain, 1963).

The results of the investigations on the physical factors are given in Table 6. BAL had the largest grains, followed by MOR, while L.187 had the smallest, followed by L.533. Also, the grains of BAL were the hardest, again followed by those of MOR, while those of RIB were the softest, followed by those of SK5912. Varietal differences in both size and hardness were highly significant (ANOVA, p<0.001). The moisture contents of the varieties did not differ significantly (ANOVA, p>0.05).

As grain hardness depends on the structure of the endo- spermjlin investigation on the latter was carried out and the results are given in Table 7. The grains ranged from those with 25% mealy endosperm (=»75% corneous endosperm) to those that were entirely mealy. No grain was entirely corneous, L.187 had the largest number of entirely mealy grains (43%) and this was followed by FF.BL (40%). None of the grains in BAL was entirely mealy. The results in Table 7 were summa­rised into three categories, as shown in the key, and presented in Fig.7.

Table 6Grain size (volume of 100 grains, in cnr[5]), hardness (% retained part after grinding and sieving) and moisture content (% of wet weight) of 12 sorghum varieties.

Abbildung in dieser Leseprobe nicht enthalten

*Means followed by the same letter are not significantly different from each other according to DMRT, at p=0.05.

Abbildung in dieser Leseprobe nicht enthalten

Table 7: The numbers of grains with the five types of endosperm found in 100-grain samples of 12 sorghum varieties.

Abbildung in dieser Leseprobe nicht enthalten

The highest amounts of starch, protein and fat, respectively, were recorded in L.1499 (mean, 78.98%), KUR (mean, 20.65%) and MOR (mean, 3.36%). The lowest amounts of the said nutrients were recorded in KUR (mean, 59.88), L.1499 (mean, 9.45%) and L.187 (mean, 2.22%) (Table 8). The varieties differed significantly (ANOVA, p<0.001) in their contents of each of these nutrients.

MOR had the highest amylose content (0.2689)- toile FF.BL had tte lowest (0.0692). MOR also had the hipest phospholipid content (mean, 0.02^) while L.187 had the lowest (mean, 0.0108). Amylose and phospholipid contents of the .varieties are given in Table 9. Phospholipid contents were significantly different between the varieties (ANOVA, p<0.001). The amylose results, were not analysed as the test was not replicated due-to shortage of chemicals.

Table 8: Amounts of primary nutrients in 6 sorghm varieties (% of ’.vet weight).

Abbildung in dieser Leseprobe nicht enthalten

*Means followed by the same letter are not significantly different from each otherat p=0.05 according-to DMRT.

Table 9: Amounts Xylose (as Blue Value) and phospholipids (as mg of phosphorus per gram of grain) in 6 sorghum varieties.

Abbildung in dieser Leseprobe nicht enthalten

Linear regression and correlation aralyses (Table 10) revealed a positive correlation between the number of insect progeny and the weight of frass produced in the varieties (r=.993O; p<0.01). Similarly there was a positive insect correlation between the number of/progeny and the loss in weight of grain (r= p<0.0l). Mong the suspected physical causes of susceptibility only grain size was correlated to the index of susceptibility computed using the total developmental period^** (r=-.6j64; p<0.05). However there was no correlation between grainsize and the index of susceptibility computed using themedian of the physics factors developmental period. The rest/were not significantly correlated with the n^ber of progeny, weight of frass, total developmental period or index of susceptibility.

Mylose was negatively correlated with the number of progeny (r=-.8182; p<0.05),weight of frass (r=,8221; p<0.05) and index of susceptibility crnputed using the median developmental period (r= • -8534; p<£>.05). The other nutrients were not significantly correlated with any of the parameters used.

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KEY

NS Non-significant at p=0.05

* Signifcant at p=0.05

** Significant at p=0.01

(+) Positive correlation

(-) Negative correlation

x - independent variable

y - dependent variable

CHAPTER 4

DISCUSSION

Herbivores can be grouped into specialist and generalist feeders. Generalist feeders exploit a number of plant species while specialist feeders feed on only a. few species of plants. In this sense, toizopertoa, dominica (R) can be classed as a specialist feeder for it has only been reported to subsist on a few cereal grains such as sorghum, maize, millet, e.t.c.

However even within the food species of R. dominica and other stored products’ insect pests, the insects have been taown to perfora better on or show preference for some varieties rather toan others. For instance, larval feeding and survival of R. dominica. were found to differ significantly when the insect was fed on different varieties of maize (Rhine and Staples, 1968).

In tois study, the relative susceptibility of twelve sorghum varieties to R. dominica was assessed. The number of F1 progeny toat emerged from each variety and the [1] grain developmental period were taken as indicators ©insusce­ptibility. The weight of frass produced in and the loss in weight suffered by the varieties were used to indicate the extent of damage. These were additional indicators of susceptibility.

The varieties differed in the nw.ber of progeny toat emerged. There was a strong positive correlation between frass weight and the number of progeny toat emerged as reported by Fadelmula and Horber (1983). There was also a positive correlation between grain loss in weight and the nmber of progeny.

Though Thomson (1966) reported toat R. dominica completed development in 38 days, the total developmental period in the present study ranged between 66.6 days in to 41,8 days in BM.. the desparity with Thomson’s results might be due to differing environmental conditions and or diet* However the developmental period in tois study conforms with toat reported by Golebiowska (1969) where the developmental period of R. dominica ranged between 84 and 41 days. There was a positive correlation between the total developmental period and grain loss in weight suggesting that larval feeding is a major cause of grain damage.

The significant differences in the number of progeny and the developmental period of the insect and in grain damage clearly show that there were differences in the susceptibility to R. .dominica infestation between the varieties used in this study. Consequently it is possible to attempt a classification of the varieties into three follows

Group I (’highly susceptibTe*)':FF.BL"' and'L.187.

Group II (’moderately susceptible’)FAR,KUR,YAL,RIB,L.533 and SK5912.

Group III (’sligttly susceptible’)BAL,A-9O25, L1499, MOR. This classification is only a broad division as the varieties did not conform to it in all the parameters assessed. However, it is a useful guide as a majority of the parameters support it.

The basic properties of plants that render them less susceptible to insect exploitation are antibiosis and antixenosis (i.e non-preference). the preference test carried out in tois study showed toat when given a.choice, R. dominica adults selected varieties at random. Consequently it can be concluded toat the property respon­sible for differential susceptibility in this study is antibiosis. To determine the possible mechanisms of antibiosisja nmber of physical factors and the content of certain nutrients in each variety were assessed. The physical factors investigated were grain size, moisture content and hardness.

The varieties ranged from toose with predominantly large grains (e.g. BAL) to toose wito predominantly small grains (e.g L.187). Though grain size was not significantly correlated with either the n^ber of progeny that emerged or the developmental period, it was negatively correlated with the index of susceptibility, suggesting that large grains are less susceptible than small ones, contrary to earlier reports (Doggette, 1957? toalifa, 1962 Russel, 1962) toat small grains are more resistant toan large on^ because they (smally grains) offer less food and a smaller surface area for oviposition. As tois res^t contradicts those of earlier workers and because there was no additional evidence to support to®' conclusion that large sorgho grains are less susceptible to R. dominica, it is probable toat the negative correlation between grain size and susceptibility observed in tois study was not causal; rather it could arise as a result of both factors being associated wito a toird factor.

The varieties did not differ significantly in their moisture content and moisture content was not correlated with varietal susceptibility, agreeing with earlier reports (Dobie, 1974 and Rogers and Mills, 1974b).

Grain hardness has often been attributed to the outer corneous endosperm of the grain (Doggette, 1957 and Davey, 1965). In this study too a positive correlation, which was insignificant under the two tailed test adapted in this study, but significant under the one-tailed test was fomd between the corneous endosperm and grain hardness. It has earlier been reported toat the harder the grain, the fewer the nmber of progeny toat emerged from the grains (Doggette,,1957; Davey, 1965 and Fadelmula and Horber, 1983) and the less the amomt of frass produced (Fadelmula and Horber, 1983). This is explained by the fact that hard grains are more difficult to bore by the insects and consequently, provide less food, oviposition site and shelter (for larval development). In the present study, the highest nmber of progeny, amomt of frass and loss in weight were recorded in FF.BL. These parameters were lowest in B^. However boto varieties were among the hardest. Furthermore statistical analysis revealed toat grain hardness was not correlated with any of these parameters, suggesting toat grain hardness is not an important factor in the susceptibility of sorghum varieties to R. dominica.

The apparent disagreement between what was reported by the above-mentioned workers and the findings of the present study might be due to differences in the biology of the insects used rather than the hardness of the grain itself. The said workers worked on Sitophilus oryzae, (L.) expcept Fadelmula and Horber (1983) who worked on Sitotroga cerealella (Oliv.) as well. S. oryzae, like most other weevils, hardly attacks the grain through the germ (Doggette, 1957) but through the corneous endosperm, for feeding and oviposition. Thus the hardness of this layer would certainly affect the weevil’s performance. On the other hand, adult R, dominica devour the germ and lay eggs amongst the grains (Appert, 1987; Alwood and Li, 1982 and Golebbwska, 1969). Furthermore the first and second instar larvae of R. dominica feed on frass, produced by adults’ feeding, and on damaged grains before the sound grains are invaded by the third instar larvae (Golebiowska, 1969); consequently the presence of a hard corneous outer shell does not necessarily affect the performance of R. dominica. This assertion is supported by an earlier report by Khokar and Gupta (1974) that grain hardness was not an important factor in the susceptibility of wheat varieties to R. dominica. It was also reported by Khalifa (1962) that since the larvae of Trogoderma granarium (Everts) attack sorghum grains through the germ to reach the mealy endosperm the outer corneous layer of the grains was not a barrier to this pest.

The nutrients investigated as possible factors related to susceptibility were starch, protein, fat, amylose and phospholipids. Though the varieties differed in their contents of the primary nutrients (starch, protein and fat) noneof these nutrients was significantly correlated with the number of progeny, developmental period* amount of frass or loss in weight, contrary to the finding by Sing and McCain (1963) that starch content correlated positively with the number of progeny of S. oryzae produced in maize varieties. The present finding on protein content, however, agrees with what was reported by Khokar and Gupta (1974) that the protein content of wheat varieties did not affect the development of R. dominica.

Though Rhine and Staples (1968) found that R. dominica was not affected by amylose content of some maize varieties, the present work revealed a negative correlation between amylose content and the number of progeny that emerged from the test varieties. They (Rhine and Staples) suggested that R. dominica might have an enzyme system that enabled it to digest amylose which other pests, such as S. oryzae, lacked. This suggestion would not be valid in the light of the present finding. Amylose was also negatively correlated with the indices of susceptibility of and the amount of frass produced in the varieties. The present finding supports the work of Eickmeier (1965) who found that the resistance of maize to S. cerealella was due to high amylose content. However his claim that amylose content was a factor in grain hardness is not supported (at least in sorghum) by the present finding because amylose content of the grains did not correlate with grain hardness.

In conclusion, it can be stated that the sorghum varieties tested in this study differ in their susceptibility and damage to infestation/by R. dominica. It is possible to categorise them into highly susceptible’, .’’moderately susceptible’ and slightly susceptible*varieties. However, the relative susceptibilities of improved and local varieties does not present a clear- cut picture. Of the six local varieties, four fell into susceptible’ the ’moderately / category of susceptibility while the remaining two were classed as ’slightly susceptible’. No local variety fell under the ’highly susceptible’ category. On the other hand, of the six improved varieties^ two were highly susceptible, two intermediate and the remaining two were least susceptible.

Investigations into the properties of the grains that are responsible for these differences in susceptibility revealed that they were not as a result of the insect exhibiting preference for some varieties as compared to others.

Correlation analyses indicated that physical properties of sorghum grains such as hardness and moisture content do not confer resistance to R. dominica. Although, grain size was negatively correlated to susceptibility, it is believed that this relationship is not causal. Further analyses showed that starch, protein, fat and phospholipid contents of the grains do not correlate with their susce­ptibility to the insect.

However, there is evidence, albeit correlative, that the susceptibility of sorghum grains to infestation by R. dominica is linked to the amylose content of the grains. The higher the amylose content in a variety, the less susceptible 'the variety.

Future work on the susceptibility of sorghum varieties to R. dominica infestation should include the design of experiments, probably, using artificial diets, that will investigate the relationship betheen the amylose content of grains and the susceptibility of such grains to this insect with a view to:

(a) providing confirmato^evidence for the conclusions of the present study,
(b) defining the relationship quantitatively and the
(c) elucidating precise mechanise by which amylose affects the performance of the insect.

In addition, field-based experiments using the varieties tested in tois study should be conducted to determine whether susceptibility ratings worked out in the laboratory can be extrapolated to field situations.

No successful achievement of the above objectives, sorghum varieties with high amylose contents, as well as having other required characteristic, could be bred with the . knowledge that they will be less susceptible to infestation by R. dominica_ in storage.

SUMMARY

The susceptibility of twelve sorgho varieties to the lesser grain borer Rizoper^a dominica, (F.) was investigated in the laboratory mder ambient conditions. These varieties included six-improved (L.187, L.555, L.1499, SK5912, FF.BL and A-9025) and six local varieties (F^, KUR, MOR, YAL, BAL and RIB).

Seven 50g replicates were collected for each variety and 20 adult 7 to 18 days old R, dominica, were put into each of five replicates of each variety. The otter two replicates for each variety were used as controls. The adults were removed after 8 days. The number of F. progeny that emerged, the developmental in weight period, amomt of frass produced and loss/in each variety grain were recorded and taken as indicators of/susceptibility. An index of susceptibility was computed for each variety based on the number of progeny produced the developmental period of the insect on that variety.

Though, none of the varieties was completely resistant, they were found to differ in their susceptibility to the pest in question. BM. was least susceptible with the least number of progeny (mean, 10.2), smallest amount of frass (mean, 54.9mg) and lowest loss in ' weight (mean, 0.57%). to the other hand, FF.BL was most susceptible witt. the highest n^ber of progeny (mean,224.2) the largest amomt of frass (mean, 961.5mg) and the highest loss in weight (mean, 4.50%). ^e other varieties fell in between these U^o in the said parameters.

The insect’s median developmental period on the varieties did not differ significantly but the total developmental period ranged from 66.6 days in KUR to 41.8 days in BAL.

A choice chamber was designed to test whether, given a choice, R. dominica would show preference for some varieties rather than others. Six varieties (L.187,L1499, SK5912, FF.BL, KUR and BAL) were used in the preference test and it was found that R. dominica chose the varieties at random.

Factors suspected to contribute to the susceptibility of grains to storage pests were also investigated.

These included grain size, hardness, structure of endosperm, moisture content and nutrients.

Grain size was measured as the volume of 100 grains. This was taken as the volume of water displaced by the grains in 25ml measuring cylinder. The largest grains were those of BAL (mean, 5.4cm ) the smanest were those of L.187 Z (mean, 1.7cm ).

Grain hardness was measured by grinding samples of grains with a coffee grinder and sieving through a 40-mesh to the inch sieve. The fraction of the sample that was retained by the sieve was used to estimate hardness. It was expressed as a percentage of the total amount that was ground, BAL was the hardest (22.73%) and Ribdafu the softest (9.62%).

Endosperm structure was determined by dissecting grains and rating them according to the relative proportion of the mealy to corneous endosperm. No grain was 100% corneous but L.187 had the highest number of entirely mealy grains (43%) while none of the grairS of BAL was entirely mealy.

Moisture content of the grains was determined as\. the weight loss by the grains after being oven-dried but the varieties did not differ significantly in their moisture content.

The contents of starch, protein, fat, amylose and phospholipids were determined for six of the varietiess L.187, L1499, SK5912, FF.BL, KUR and MOR.

Starch was determined by the anthrone reagent method; protein by the Kjeldahl method; fat by the chloroform method; amylose by the Blue Value method; and Phospolipid by the ’organic phosphorus’ method. The varieties differed in their contents of these nutrients.

Statistical analyses revealed that of the suspected factors of susceptibility only amylose was found to affect the susceptibility of the test varieties to R. dominlca. This is because it was negatively correlated with the number of progeny, weight of frass produced and the index of susceptibility.

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APPENDIX I

Storage Pests of Grains and Pulses (After Anon, 1978)

Abbildung in dieser Leseprobe nicht enthalten

*Major pest species

APPENDIX II

Abbildung in dieser Leseprobe nicht enthalten

Temperature and relative humidity readings recorded during the progeny comt experiment (section 2.4) from 31st August to 15th November, 1987.

*Readings were not taken due to equipment’s failure

APPENDIX III

METHODS OF PREPARATION OF THE REGENTS USED IN THE NUTRIENTS' ANALYSES

A. Iodine Solution 1

1.5g each of iodine and potassium iodide were mixed and gromd in 30ml of distilled water, the solution made to 50ml with more distilled water and filtered through glasswool.

B. Iodine Solution 2

20mg of iodine and 200mg of potassim iodide were grornd in 10ml of distilled water and the solution filtered through glasswool.

C. Ethanolic Sodim Chloride Solution

10ml of 20% sodim chloride solution were mixed with 30ml of 95% e^anol and 16ml of distilled water and the mixture made to 100ml with more distilled water.

D. Ethanolic Sodium Hydroxide Solution

1.25ml of 5M sodim hydroxide solution were mixed with 17.5ml of 95% ethanol and 5ml of distilled water and the mixture made to 25ml with more distilled water.

E. Anthrone Reagent

380ml of concentrated sulphuric acid (specific gravity, 1.84) were gradually added to 120ml of distilled water and the solution cooled. 0.5g of anthrone pure was then dissolved into the solution.

F. Potassim Hydrogen Tartarate

7.9770g of tartaric acid rnd 2.9773g of potassim hydroxide pelletes were separately dissolved in rater and the two solutions mixed. The resulting precipitate was centrifuged at 1500r.p.m for 5 minutes and the aqueous layer diseased, the precipitate (potassim hydrogen tartarate) was washed twice with distilled water, centrifuged and dried in a vacum oven at 95OC.

G. Reducing Agent for the Determination of Phospholipids tois was made of 0.2% 1:2:4-arninonaphthosulphonic acid in 12% sodim metabisulphite and 2.4% sodim sulphite.

H. The Standard Solution for the Determination of ^ospholipids 2ml of potassim dihydrogen phosphate solution (2.194g in 500ml), equivalent to 1mg of phosphorus per ml, were diluted to 500ml to obtain a concentration of phosphorus of 0.004mg/ml. To 5ml of this solution (= 0.02mg phosphorus.) were added 0.4ml each of 52% per chloric acid and 5% ammonium molybdate solutions and 5ml of distilled water.

I. Tashiro's Indicator

This indicator was prepared by dissolving 20mg of methylene blue and 80mg of methyl red in 10tol of absolute ethanol.

K. Standardised Boric Acid

20g of boric acid were dissolved in 50ml of distilled watery 10ml of Tashiro’s indicator (see J above) added and the solution made to one litre, with distilled water. The p.H. was adjusted to 4.8.

APENDIX IVA

Asortance of various concentrations of 0.01% D-glucose standard solution and the standard curve.

Abbildung in dieser Leseprobe nicht enthalten

Abbildung in dieser Leseprobe nicht enthalten

APPENDIX IVB

Abbildung in dieser Leseprobe nicht enthalten

Absorbance readings of six sorghum varieties, at 620nm, in the determination of starch.

Abbildung in dieser Leseprobe nicht enthalten

x - values (concentration in Ug per 2ml of solution) of six sorghum varieties, calculated from the regression equation of the D-glucose standard curve, in the determination of starch.

APPENDIX V

Abbildung in dieser Leseprobe nicht enthalten

Absorbance readings of six sorghum varieties, at 680nm , in the determination of amylose.

APPENDIX VI

Abbildung in dieser Leseprobe nicht enthalten

Absorbance readings of six sorghum varieties at 660 nm , in the determination of phospholipids.

APPENDIX VII

Abbildung in dieser Leseprobe nicht enthalten

Titre volume (ml of 0.1M hydrochloric acid) in the determination of protein.

APPENDIX VIII

Abbildung in dieser Leseprobe nicht enthalten

Indices of susceptibility of 12 sorghm varieties calculated using the total developmental period (section 2.8)

[...]

---

^* The process was stopped just before the addition of 0.1% sodium chloride solution.

^† Exactly 10ml of chloroform was used in this extraction.

29

^‡ S.E.MStandard Error of the mean

^§ Means followed by tte same letter are not significantly

different from each other at p=0.05 accotoing to. -s toner’s Multiple Range Test (DWT).

^** See Appendix VIII

f

44 i

^†† Means followed by the same letter are not significantly different from each other, at p=0.05, according to DMRT.