Host plant and varietal resistance to post harvest insect attack

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by R.L. Semple

INTRODUCTION

It is now well established that various staple cereals, such as rice, maize, wheat, sorghum, oats and barley, and legumes such as cowpeas, vary quite significantly in their inherent resistance or susceptibility to both field infestations, and to postharvest insect attack in storage by the more recognized grain storage insects.

The full yield potential of the growing crop is seldom realized due to the interaction of many factors (climatic, ecological and edaphic) of which pre-harvest insect infestation and consequent damage is one of the most important.

Studies in Nigeria by Raheja (1976), as reported by Dobie (1981), have revealed that field pest infestation in cowpeas can reduce the yield of that crop by 92-97% of the potential yield in the absence of insects, principally by the bruchid, Callosobruchus maculatus. Pimentel (1978) estimates crop losses due to pests to be around 35% worldwide, while post-harvest losses may range from (a maximum value of) 9% in the United States, to 20% or more in some developing countries, especially in the tropics. Total losses could therefore approach almost half of the potential yield of that particular crop, if certain restrictions on the development of pest populations is not implemented.

Plant breeders at the International Rice research Institute (IRRI) believe that rice intensification programmes can be achieved by:

  1. producing high yielding varieties (HYV's) with inherent disease and pest resistance characteristics with maximum protein content and enhanced taste which will make these varieties more acceptable to consumers than the traditional varieties;
  2. multiple cropping characteristics of introduced dwarf HYV's that allow two or more crops per growing season due to the shorter period required to maturation (15 weeks instead of 25 weeks);
  3. minimizing field crop losses by improved agronomic and cultural practices, water management, and improved fertilizer and pesticide usage in terms of frequency and timeliness of application.

The so-called "Green Revolution" has in fact intensified these losses by;

  1. increased susceptibility of HYV's whereby they have become biological weaklings unable to compete in a natural environment, or reach their full yield potential, because of their heavy dependence on fertilizers and pesticides for disease, insect and weed control;
  2. market specifications demanding product uniformity have all too frequently narrowed the gene pool from which we select our varieties, thus reducing the number of genotypes that naturally harboured alleles resistant to insects and other pests. Genetic uniformity tends to provide an ideal ecological environment for pathogens to evolve and severely attack these genotypes, and;
  3. continuous cropping techniques ("monoculture", or the establishment of "homoclines") provides a ready source for infestation by a range of pests such as insects, mites and nematodes, and infection by bacterial and fungal pathogens from one crop to the next.

One can conclude from these observations, that there has not been a reduced reliance on the use of applied pesticides with the introduction of improved varieties, but in fact a higher requirement for increased pesticide and fertilizer usage to adequately protect and sustain these crops (rice and cotton are too good examples).

To meet the growing demand for rice in Southeast Asia, increasing areas are being brought under irrigation (since the HYV's do not perform well under waterlogged conditions) and existing irrigation areas are being more intensively managed through the continued widespread introduction of HYV's in a multiple cropping system. This changing production pattern has created post-production problems that were not so obvious with the traditional, "non-improved" varieties, and traditional cultural practices. With the advent of multiple cropping, the offseason crop is therefore harvested in the wet season, where the combination of high moisture and the inability to adequately dry the crop by traditional sundrying methods have spawned the problem of rapid biological deterioration, manifested by discoloration and fermentation, before and during storage. In addition, the shortseason, photo-insensitive HYV's typically grown in a multiple copping system are easy shattering, and tend to have a wider range in kernel maturity than the traditional varieties at harvest, resulting in a higher percentage of high moisture, immature kernels or chalky kernels in storage. Therefore the introduced HYV's as a solution to the problem of food shortage for an evergrowing population has accentuated post production losses. These are likely to become increasingly important as production limits are reached due to limited availability of arable land, and the increase in production costs because of the increasing costs of required inputs, such as pesticides and fertilizers.

Apart from the increased strain on post production systems to adequately dry and store the increased production of paddy, it has also been repeatedly demonstrated that the HYV's developed by plant breeders are more susceptable to insect attack than the traditional, non-improved varieties, and that the increased yields per hectare being experienced in the field, can be lost due to their greater, inherent vulnerability to stored products insect pests in storage.

In small-scale storage systems where adequate pest control techniques are not employed, due to volume stored or rapid turnover of stocks, the advantages of varieties less susceptible to damage and loss inflicted by storage insects becomes very obvious. Haines (1982) reporting on unpublished work of Supriati in Indonesia, showed that among eight rice varieties grown in that country, the weight loss of paddy caused by an introduced infestation of Rhyzopertha dominica was more than seven times (7x) the loss on the most susceptible variety in the first two (2) months of storage, and had expanded to more than ten times (10x) in the third and fourth month of storage, when compared to the most resistant variety. Even in stores where pest control techniques are normally used, the utilization of the most resistant variety would lead to a reduction in the costs of whatever pest control strategy was employed. Duff (1979) found that in the Philippines, farm stored grain was generally in good condition because they gave a high priority to cleaning and drying that portion of the crop which was destined for home consumption. The storage problem is most definitely compounded with the marketable surplus. This is specifically with national government agencies maintaining buffer stocks in lieu of their price support/stabilization programmes, where stocks oftentimes enter long-term storage, and in spite of minimum quality standards, are buyers of the last resort.

PRINCIPLES AND TERMS USED IN INSECT RESISTANCE TO PLANTS:

The rate of increase of a pest population is influenced by a multitude of many interacting factors, one of which is the quality characteristics of the food medium on which the pest/pests are feeding. Many varieties of the same grain species appear to be less suitable than others for insect development, and are often described as being "resistant" (or in fact, less susceptible) to insect attack. Different definitions of resistance have been put forward. These include,

''relative amount of heritable qualities possessed by the plant which influence ultimate degree of damage done by the insect" (Painter, 1951)

"The collective heritable characteristics by which a plant species may reduce the probability of successful utilization of that plant as a host by an insect species" (Beck, 1965)

"Resistance is all those heritable traits of a plant, that lessen insect damage, while other plants of the same species and in the same environment receive greater damage. Resistance is therefore a relative phenomenom" (Owens, 1975)

It must be remembered that stored products insect pests are capable of inflicting serious damage to stored commodities, due to their very rapid capacity to increase in numbers, and to migrate and infest separate lots thus spreading and expanding the infestation. The use of resistant varieties, particularly in farmers and village cooperative stores, will more than likely extend the period that produce can be stored safely without the use of pesticides. When susceptible HYV's have been planted, rural storages maybe forced to utilize expensive pesticides that are often unavailable, or if so, in packaging and formulations that are not suitable for small-scale use.

Tolerance as a particular mechanism for resistance in actively growing crops in the field is related to endurance to insect attack and repair capabilities once pests are established. This component of resistance is therefore not applicable to grain in storage, since although seed respiration continues, individual kernels do not possess the capacity to "tolerate" damage by the above mechanism. Preferential feeding activities on the germ or starchy endosperm, irreversibly effect seed viability, dry matter loss and increases in moisture content that cannot be compensated for by the grain. Resistance to post-harvest insect attack is therefore attributed to the interrelated component factors of antibiosis and nonpreference.

An insect population will rapidly increase in numbers when 1) a high rate of egg laying is achieved, 2) growth and development is rapid, and 3) when the mortality rates are so low, that few insects die before reaching sexual maturity, and begin producing progeny.

The rate of population increase can therefore be adversely effected when utilizing a resistant variety that it;

1) causes a reduction in the rate of egglaying by;

AN D/OR

2) extends developmental periods by:

3) causes high mortality of the immature and developing stages of insects, therefore reducing the number of sexually reproductive adults emerging by;

In this context, antibiosis refers to plant characteristics that adversely effect insect mortality, size, and life history. The abnormal effects that may occur when a particular insect population feeds on an antibiotic plant (or seed) are;

  1. death of early stages of larvae (or nymphs) resulting in little or no insect infestation;
  2. death of pupae and late larval or nymphal stages reducing the size and potential of the next reproductive population;
  3. decreased fecundity reducing subsequent insect populations;
  4. reduction in the size of insects that emerge;
  5. reduction in weight of those insects surviving and emerging where both 4) and 5) have cumulative effects on insect feeding ability and increase the effects of environmental variation on the insect;
  6. decreased accumulation of insect food reserves affecting survival potential during periods of hibernation and aestivation (diapause);
  7. abnormal behavioural and physiological affects reducing the ability of the insect to continue feeding; and
  8. abnormal survival/longevity where immature survival may be unduly extended (a juvenile hormone effect). This exposes nymphs and larvae to natural enemies leading to fewer generations in a given time period; and, reduced adult life which limits time for mating and oviposition, therefore severely reducing fecundity.

Certain phenomena which are not heritable traits of the host plant to induce resistance (either single, or more preferably, multiple resistance) have been described by Horber (1983) as "pseudo resistance factors". Some have been utilized to great advantage in economic entomology, and play a substantive role in stored grain protection. These include;

  1. Evasion by potential hosts when insect numbers are low, such as when some cultivars mature early and evade field infestation (lack of "phenological coincidence").
  2. some individual kernels in a generally susceptible host variety may escape infestation as a purely random event. Insects in stored grain are generally not randomly distributed and ate located (and survive) in large numbers where conditions are most amenable and accessible. Grain outside of this area may therefore not become sensibly infested.
  3. Temporary resistance of a susceptible host maybe induced by treatment with oils, repellents or inhibiting materials. The possibility of substituting insecticides with repellents or feeding inhibitors, such as extracts of neem and black pepper, and the use of various vegetable oils has been covered elsewhere, while trypsin inhibitors have been investigated in wheat for their effect on Sitophitus oryzae by Su, et.al., (1974).

SOME CAUSES OF RESISTANCE:

PHYSICAL:

Much of the present knowledge on the resistance of crops to storage pests has been primarily concerned with establishing those factors that confer resistance. It is considered essential knowledge for plant breeders wishing to embark on breeding programmes to incorporate inherent varietal resistance to storage pests as an integral and desirable selection criteria for the suitability of varieties that are likely to be released for future productivity programmes.

1) Physical barriers:

The extreme value of a complete, well-fitting set of sheating leaves in maize to reduce pre-shelling infestation by Sitophilus spp. has been well recognized for many years. Tight husks are an important resistance mechanism for maize, wherever climatic conditions encourage field infestations.

In unhusked rice or paddy, the tightness of the glumes that cover each grain is of primary importance in reducing damage done by Rhizopertha dominica, Sitophilus oryzae, and Sitotroga cerealella, the success of these species, particularly Sitophilus spp., being dependent on the presence of hull defects. Rosotto (1966) screened 1700 varieties of rice collected worldwide for resistance to S. zeamis and found 80% were strongly resistant and directly related to the tightness (and integrity) of the husks surrounding the rice kernels. The varieties in which weevil progeny were produced had separated or gaping glumes, or had suffered some form of physical damage. This substantiated the findings of Breese (1960) where infestations did not develop in grain that possess intact husks. Feeding ability of the females and oviposition is directly related in Sitophilus spp., whereby if feeding is inhibited due to the inpenetrable barrier afforded by the husk, then oviposition will likewise be inhibited. Larvae are quite incapable of penetrating or feeding on sound intact paddy grain, even if eggs are deposited loosely amongst the grain. Eggs are normally laid outside the grain by females of Rhyzopertha dominica and Trogoderma granarium, and entry into it is achieved by the 1st instar larva, but in sound paddy, it is only the more well developed larvae that are capable of penetrating the husk. Larvae of Rhyzopertha dominica, as well as exploiting any splits or physical defects, can eventually gain entry via the inflorescence attachment (rachis) into the husk, and is more accessible when the kernels are not fully mature since it is softer and has less contraction of the pith, but survival is likely to be low. Larvae of Sitotroga cerealella are very adept seed penetrators, even when the husk is complete, but do not pose a serious threat in bulk stored paddy except for surface infestations, because of inability of the larva to penetrate deeply in the bulk, thus restricting its spread. Stored ear corn does not offer the same problem to S. cerealella larvae and is therefore better stored in bulk in shelled form. Many other factors other than genetics can affect the husk condition such as weather, fertilization rate, plant diseases and harvesting machinery. The degree of husk damage, amount of husked and broken kernels, immature, unfilled and shrivelled grains tends to be higher in combine harvested paddy than paddy harvested and threshed by traditional methods. The amount of physical damage to intack hulls and the high percentage of separated and gaping glumes that occurs in HYV's allows S. oryzae and R. dominica to occur frequently and in abundance in stored paddy in Southeast Asia.

Hussain, et.al., (1983) have also shown that S. oryzae has a strong competitive advantage over S. zeamais on paddy based on the averaged index of susceptibility in laboratory trials of 4 HYV's commonly grown in Indonesia (IR 36, IR 42, Cisidini and Cisidane) and confirms the species dominance observed in field surveys (Haines and Pranata, 1982). The most susceptible varieties in paddy form were those displaying widely divergent and gaping hulls.

Cogburn (1977) tested the effects. of R. domonica, S. oryzae and S. cerealella on weight loss and milling yield of six commercial rice varieties. After 3 generations, the lesser grain borer inflicted more weight loss of paddy than the "Angoumois grain moth," while the "rice weevil" inflicted the least weight loss. Milling analyses showed that the "rice weevil" again caused the least loss, but the grain moth caused a higher loss in total milling yield than the "lesser grain borer" even though both adults and larvae are voracious feeders. This was due to preferential areas of feeding and activity. This study underlines that the loss potential of rice is dependent on variety, storage time and species of insects present.

Rogers and Mills (1974) screened over 1500 sorghum varieties and also found tight glumes covering the kernels conferred almost total immunity to S. zeamais.

Naked barley varieties are also less inhibitory to S. oryzae yielding more progeny than barley with covered kernels. The seed pods of some legumes may also provide an efficient barrier to insect infestation, thus storage should be in unshelled form to take advantage of this relatively "cost-free" form of control.

The seed coat of food grains may also be sufficiently thick and tough so as to inhibit penetration to a degree, even though primary feeders are well adapted to chew into whole undamaged kernels. Certain characteristics of the cellular structure of the seed coats of some cowpea varieties (Vigna unguiculata) partially prevented entry of 1st instar larvae of Callosobruchus maculatus. In highly infestible commodities such as wheat, the absence of broken grain on which adult Rhyzopertha dominica can feed reduces the ovipositing rate to about 12% of the normal rate. The efficiency of penetration in whole grains of wheat is also strongly influenced by the presence of defects in the outer layers of the grain, so that varieties that possess strong, intact outer layers do exhibit a certain level of resistance to Rhyzopertha dominica, perhaps best reflected by the rate of population buildup.

Damaging the pericarp of maize (yellow corn) in some way increases its susceptibility to S. zeamais (Schoonhoven, et.al., 1976) Sitotroga cerealella develop very slowly on pellets composed of pure wheat endosperm, but when a small percentage of pericarp (bran) is added, the rate of development is enhanced, but then retarded and finally unsuccessful when the proportion of bran added was further increased, thus indicating that certain factor(s) in the pericarp offers resistance to entry in whole kernels.

The intack hull character is therefore not likely to be completely effective against all stored grain insect pests. Cogburn (1974) selected kernels of the commercial variety "Belle Patna" microscopically for intack hulls and subjected small samples to S. oryzae, R. dominica, and S. cerealella. S.oryzae adults starved to death and did not reproduce, while immature mortality was 91 % in R. dominica and 71% in S. cerealella. In a large commercial bulk of this variety, there would be an abundance of infestable kernels to allow survival and development of all three species. The rapid multiplication of a population of R. dominica in paddy is dependent on certain varietal characteristics such as the failure of the husk to close properly around the kernel which renders a small proportion of the bulk infestable to both S. oryzae and R. dominica. Also, rapid population growth is dependent on adequacy of food to maintain oviposition in the form of split husk grains, hulled grains, kernel fragments and immature paddy, and in the most part, is dependent on the methods of harvesting and threshing. Parboiled paddy when dried to 14% MC (w.b.) differs little in volume to that of raw paddy (12%), but the husk components once separated by the swelling of the kernel, do not close fully on drying, therefore allowing access by larvae of R. dominica and increasing its infestability. While parboiling allows larvae of R. dominica to gain access and attack the germ, the separated glumes also facilitates oviposition by S. oryzae, but is generally less infestable than raw polished rice because the endosperm has been toughened or hardened either peripherally or throughout, during the parboiling process.

2) Limited oviposition sites:

Nwanze, et.al. (1975) have found that cowpeas with smooth seed coats were more susceptable to Callosobruchus maculatus than varieties that possessed rough seed coats, the same separation of resistance being displayed by other legume seeds. Nwanze and Horber (1976) suggest that those bruchids that attach their eggs to the external surfaces of legume seeds lay fewer eggs on rough coated varieties than on smooth-coated varieties.

Members of the Bruchidae specialize on feeding on a wide variety of grain legumes with medium-sized, hard seeds. Horber (1983) reporting on work by Jansen (1969) claims that due to natural selection, grain legumes attacked by bruchids have adopted a common strategy of producing smaller seeds per unit of vegetative growth than those species not generally subject to infestation. Larger-seeded varieties of cowpeas (Vigna unguiculata) are better hosts to the cowpea weevil, Callosobruchus maculatus, while fewer progeny emerged from smaller cowpea seeds (Nwanze and Horber, 1975). Thickness of the seed coat in Bengal gram (Cicer arietinum) appeared to influence preference by the pulse beetle, Callosobruchus chinensis, rather than the size of the seeds (Gupta, 1970).

Resistance to the adzuki bean weevil or pulse beetle in chickpeas was attributed to the rough, nearly spiny seed coat, which effectively inhibited oviposition (Brewer and Horber, 1983). Broadbean (Vicia faba) was not preferred for oviposition by C. chinensis even though it possesses a relatively smooth seed coat, and was shown that most of the larvae died during the first instar due to inability to penetrate the thick seed coat. Brewer and Horber (1983) concluded that larval antibiosis due to mechanical, physical or biochemical factors, and ovipositional antexinosis (properties of the host plant that affect behaviour during search and host selection, such as in the resistant chickpea variety) are assumed to be present. Cowpea (Vigna unguiculata) and lentils (Lens culinarus) were preferred for oviposition, but few progeny emerged, possibly due to high levels of trypsin and chromotrypsin inhibitors in cowpeas.

C. chinensis and C. maculatus both develop much slower on lentils than other members of Leguminoseae. All mungbean (Vigna radiate), pidgeon pea (Cajanus cajan) and adzuki beans (Phaseolus angularus) were susceptible to C. chinensis in this study. Ovipositional preference is not however, an indication of the suitability of the host for insect development. The percentage reduction in germination of infested legumes is also not a good basis for assessing resistance. Germination reduction has been correlated with the number of emergence holes in the seeds, where with more holes, germination was reduced, slower and seedling vigour in those that did succeed in germinating was also reduced. (Southgate, 1979). However Brewer and Horber (1983) were not able to establish any definitive relationship between percent reduction in germination and numbers of insects or weight loss (naturally, a positive correlation existed between numbers of emerged weevils and grain weight loss in the preference for oviposition and feeding damage test!). In preferencial order for the growth and development of C. maculatus, Singh, et.al. (1980), infested green gram, cowpea, red gram, Bengal gram, black gram, pea and lentil, and reported that lentils increased the developmental period but retarded per cent emergence, size and weight of aduts, fecundity and longevity.

The structural qualities of seed pericarp in legumes have not been fully explored (Horber, 1983). These include the effects of glossiness, stiffness, permeability for gases and water vapour, impact and rupture strength which need to be substantiated with modern equipment to elucidate their protective role in preventing oviposition, penetration and adult emergence (from it).

With respect to cereals, Cogburn (1974) experimented with six commercial rice varieties grown in the United States that differed in their percentage of intact hulls. Female S. cerealella frequently deposits her eggs inside a grain with a broken or gaping hull. Breese (1960) states that the preferred oviposition site is a crevice, especially one in which the surfaces are not smooth, and it maybe for this reason oviposition does not normally occur on milled or polished rice. The husk, although difficult to penetrate, does in fact provide a better attachment for the entrance cocoon of the first instar larva than the smooth exterior of the milled rice kernel. Of the eggs deposited on any variety, the percentage inside glumes was directly proportional to the number of open glumes available, with the variety possessing the most open glumes receiving the most eggs, and the reverse being true for the variety possessing the least number of open glumes. However, the preference of the moth for the varieties for oviposition sites was not directly proportional to the open glumes available and tended to be more or less random between the two extremes.

As previously mentioned, feeding ability and oviposition are almost inseparable in Sitophilus spp., where if females are to oviposit in any grain, it is necessary for her to bore the hole in which the egg willy be laid. Females of R. dominica prefer to oviposit on rough surfaces and has a strong preference for crevices. Few eggs are laid loose according to Birch since many would be destroyed (and in fact, are) by the feeding and wandering activities of the adults. The preferred site in wheat are the cracks in damaged grain, or in the crease or under the loose testa in sound kernels. Unhulled rice is extremely rough, but when intact, may lack the type of crevice favoured by P. dominica for oviposition.

3) Seed hardness:

The hardness of seeds has been demonstrated, especially in cereals, as affecting the successful and rapid multiplication of insect pests. Hard, flinty maize varieties are often more resistant than soft, floury varieties, where those maize varieties that possess the "opaque-2" gene in their genotype are abnormally soft and reflects their high degree of susceptibility to attck by S. zeamais and S. cerealella. The good nutritional characteristics of these particular varieties must be combined and selected for hard kernels to enhance their storability.

Relative hardness of cereal grains has been measured various ways. One method is pearling a grain sample for a specified time and measuring the weight of the abraded material, the less weight loss inferring greater grain hardness. Russell and Rink (1965) found the relative hardness of various sorghum varieties was a dominant factor in affecting the oviposition and adult emergence (survival) rates of both S. oryzae and S. zeamais, where relatively soft varieties received as much as 2-6x more eggs than relatively hard varieties. Davey (1965) reported that the number of S. oryzaecompleting adult development form "mealy" (soft) varieties of sorghum could exceed the numbers emerging from "vitreous" (hard) varieties by a factor of 20 after only a single generation. A positive relationship exists between weight loss from sorghum samples by pearling and the number of eggs of S. zeamais laid, and the percentage survival of newlyhatched larvae to adulthood in S. cerealella (Rout, 1973).

Relative hardness has also been determined by the depth of impression made by a sharp object such as a diamond point under a constant weight. Hardness may however, not be the only factor affecting penetration, and the toughness and thickness of the pericarp may influence the impression made. A consistent difference was established between 5 isogenic lines of maize with genes for high Iysine content which were softer, and maize varieties with normal endosperm using the same determination for hardness as Rout (Schoonhoven, et.al., 1972). Dobie (1974) reviewed the susceptibility of 25 varieties of maize to S. zeamais utilizing his well known "Index of susceptibility" which takes into account both survival and adult emergence (number of F1 progeny or a measure of productivity) and the speed of development (average developmental period of these progeny). The hardness of the kernels, as estimated by the proportion of floury endosperm was related to susceptibility, and grain hardness was closely related to amylose content of the endosperm. Amylose content may also affect susceptibility via a different mechanism other than grain hardness, and susceptibility in those varieties tested was related to factors operating after oviposition, based on the number of egg plugs between susceptible and resistant varieties.

Rout, et.al. (1976) studied the relative susceptibility of 8 dehusked, high yielding paddy varieties to S. oryzae using the same demographic parameters of developmental tiome and numbers of F1 progeny emerging on each variety as Dobie, and found a positive relationship between grain hardness and resistance with the relatively more susceptable varieties producing heavier weevils. Hardness was estimated by alkali reaction and pressure exertion, where the pressure (in kg/cm) required to cause cracking was taken as a measure of relative harndess.

NUTRITIONAL FACTORS:

Maize with high Iysine content has received particular attention as a method of increasing protein quality for people in developing countries that rely on maize as a staple and where other sources of protein are deficient. The study of Schoonhoven, et.al. (1972) as previously mentioned did not demonstrate any correlation between high Iysine content in maize lines incorporating the "opaque-2" or "floury-2" genes, and resistance to attack by S. zeamais compared to normal endosperm maize of the same line. Significant differences in the emergence of weevil progeny were noted however, between high Iysine maize of different lines, or normal endosperm maize of different lines. Tests preformed by the International Maize and Wheat Improvement Centre in Mexico (CIMMYT) have indicated that as the level of tryptophan and Iysine is increased, the numbers and weights of emerging S. zeamais correspondingly increased as well. LeCato and Arbogast (1974) found 3 species of stored products insects multiplied faster on 2 hybrids of high Iysine content, but 6 other species did not. Rout, et.al. (1976) also found no consistent relationship between the starch and protein content of brown rice of different varieties and their relative susceptibility to S. oryzae in their study.

Amos et.al., (1988) studied the development of S. granarius, R. dominica, O. surinamensis and T. confusum on 23 varieties of wheat. Using step wise regression analysis on the data for each moisture content Since each variety was equilibrated to the experimental conditions of 26C and 60-70% RH) significantly influenced th productivity of S. oryzae, S. granarius and R. dominica.

High Iysine content cannot be concluded categorically as conferring resistance or susceptibility, since its effect on physical and chemical characteristics of the endosperm have not been elucidated.

CHEMICAL CAUSES OF RESISTANCE:

Toxins:

Most food crops do not contain toxic substances to insects, and if they are naturally present, they probably exist in concentrations that would not significantly affect insect or man. However, certain components that maybe in toxic levels to insects and rendered harmless to man by preparation and cooking are known in legumes or pulses and in some rootcrops, notably cassava.

Cowpeas (Vigna unguiculata) are generally very susceptible to attack by C. maculatus and cause substantial losses. The Grain Legume Improvement Programme of the International Institute of Tropical Agriculture (IITA) in Ibadan, Nigeria has the global mandate for improving cowpea production, and considerable effort has been devoted to developing resistant varieties in conjunction with the Tropical Development Research Institute (TDRI, formerly TPI) of the Overseas Development Administration (Dobie, 1981). One variety form Northern Nigeria displayed resistance to a strain of C. maculatus emanating from Brazil, which was important if cowpea production was going to be substantially increased in that country. Females of C. maculatus readily laid eggs on the seed surface, and larvae began feeding on the underlying cotyledons, but growth was extremely slow, and eventually most if not all the larvae died. Eighty to ninety-five percent (80-95%) of larvae completed their development on the control susceptible varieties, strongly suggesting the presence of a toxic or antibiotic compound(s). Saponins (triterpenoid glycosides of which 5 aglycones have been isolated in soybeans toxic to Callosobruchus spp.) and lectins (phytohaemag-glutinins found in beans, Phaseolus vulgaris (Jensen, et.al., 1976) were not found, but the resistant variety was shown to contain twice the level of a protease inhibitor (trypsin inhibitor) than the susceptible varieties, and was found to be inheritable, but its stability was unknown (Dobie, 1981). Trypsin is a peptidase enzyme splitting proteins at certain specific peptide links and is secreted as trypsinogen from the mammalian pancreas, occurring in the digestive juices of most animals. Peptides are compounds formed from two or more amino acids by the amino group (NH2) of one joining the carboxyl (COOH) group of the next forming peptide bonds (-HNCO-) with the elimination of water. Peptones are large fragments of peptides formed by the splitting of proteins by peptidase or protease enzymes. Trypsinogen inhibitors are also known in maize starch.

Most proteinaceous inhibitors have been , orated from the seeds of different members of Leguminosae. Refined extraction methods have isolated three proteinaceous fractions from soybean which strongly inhibit the growth of T. castaneum larvae; and more inhibitors have been isolated from wheat and lima beans (Applebaum and Birk, 1972). Certain cultivars of groundnuts and chickpeas possess higher concentrations of specific J. castaneum protease inhibitors, which would at least confer some partial resistance to damage in storage, and increasing their concentration via breeding programmes would not be detrimental to human and animal nutrition (Hober, 1983).

Apparently, L-canavanine, which may contribute from 8-10% of the seeds dry weight in some legumes of the family Lotoeideae, is toxic to insects and higher animals due to disruption of normal protein synthesis. L-canavanine indescriminately replaces Larginine in structural proteins which are physiologically incompetent.

Amalyase (or diastase) is a group of enzymes which split starch or glycogen variously to dextrin, maltose and glucose and are widely distributed in both plants and animals (such as in malt, pancreatic juices and in microorganisms) and can be purified by Affinity Chromatography.

A glycoprotein from red kidney bean (Phaseolus vulgaris) which inhibits mammalian amylase, has also been shown to inhibit amylase activity in insects such as the yellow mealworm Tenebrio molitor, the larvae of the mediterrancean flour moth, Ephestia kuhniella, and adults of the confused flour beetle, Tribolium confusum. (Powers and Culbertson, 1982; 1983). The rate at which the bean glycoprotein inhibits insect amylase and therefore their ability to breakdown starch to glucose, is dependent on pH, temperature and ionic strength.

Glycoprotein amylase inhibitors from Phaseolus vulgarus beans have been purified by either conventional techniques or affinity chromotography. Bean amylase inhibitors apparently inhibit mammalian but not microbial or plant amylases.

Bean amylase inhibitors have been suggested as a developed protective agent against insect attack. In the study of Powers and Culbertson (1983), Tenebrio molitor or the "yellow mealworm" was selected as the test insect since its amylase has been previously purified and characterized, and studied with its interaction with wheat amylase inhibitors. The rate of combination for the inhibitor and amylase at 30C and pH 5.4 (optimum for this enzyme) was calculated as a second-order rate constant of 2.7 x 105 per mole per second. At pH below 3.8, very rapid and irreversible loss of enzyme activity was found, and is similar to previous observations of the interaction of bean amylase inhibitor and porcine pancreatic a-amylase, where an increase in inhibition occurs below what is considered optimal for the enzymes pH.

Yetter, et.al. (1979) extracted a-amylase inhibitors from five hard winter wheat varieties and assayed against larval a -amylase of both S. oryaze and J. molitor, with correlation in some varieties between in-vivo inhibition (resistance) and in-vitro inhibition of larval a - amylase by extracted inhibitors. As probably with beam amylase inhibitors, it was concluded that a - amylase inhibitors in wheat could be involved in postharvest resistance to grain insects in storage.

Differing resistance responses in Phaseolus vulgarus beans to C. chinensis and Acanthoscelides obtectus, the "bean weevil," have been explained by the differential digestion of soluble heteropolysaccharides, which contain arabinose, xylose, rhamnose, glucose, and galactose in molar proportions approximating 9:2:1:1:4. The activity of heteropolysaccharides to C. chinensis (1% soluble fraction) is mainly due to its integral structure rather than component constituents, while insensitivity of A. obtectus ocurrs by effective digestion (hydrolysis) and release of arabinose at the larval midgut (pH of 6.7-6.8) resulting in biological inactivation of the polymer. According to Horber (1983), breeding for higher concentrations of this heteropolyssacharide in beans would be logical to protect them form both bruchid species.

S. cerealella was used as the test insect against 1000 varieties of rice obtained from the World Collection, of which around 20% displayed some level of resistance to moth infestation (Russell and Cogburn, 1977). In subsequent tests, 24 varieties were found to be promising, and three of these varieties were crossed with the very susceptible commercial variety "vista". All 3 crossed varieties displayed resistance through 4 generations, and the resistance was antibiotic, heritable and dominant.

Morallo-Rejesus and Dimaano (1984) compared the susceptibility of 20 varieties of milled rice to S zeamais and T. castaneum. Variable responses were found where some HYV's were more or less susceptible to one species or the other, the differences being attributed to some antibiotic effect on insect development and inhibited oviposition, or a combination of both, while other varietal characteristics such as protein content were not considered as significant contributory factors to resistance in this study.

LABORATORY METHODS FOR EVALUATING RESISTANCE/SUSCEPTIBILITY:

Mills (1976) has described the methods used to evaluate the resistance or susceptibility profile of different grain varieties being performed by the Department of Entomology at Kansas State University, Manhattan, Kansas, USA.

Samples of each variety, replicated 3-5 times, typically of 50 or 100 kernel size, are placed in small plastic boxes (48 x 48 x 18 mm) with brass inserts fitted into the lids to enable free air interchange. In "nochoice" tests using Sitophilus spp., 6 females and 3 males were commonly used as parental adults, left for 5-7 days for oviposition, and then removed. The relative resistance of each variety is ranked using the Duncans Multiple Range Analysis based on number of emerged progeny (F1) per sample. Resistance may also be conferred based on the speed at which immature development to adult emergence proceeds so an estimate based on mean developmental period for alI F1 progeny should be made. The Index of Susceptibility of Dobie (1974) is simply a ratio of the log of F1 progeny completing development x 100 divided by the mean of the development period where a higher ratio indicates greater resistance relative to other cultivars or varieties under evaluation. In some "no-choice" tests using R. dominica, S. cerealella or T. castaneum a certain number of eggs or newly hatched larvae were used (therefore removing any effects due to oviposition preference) and each sample of each variety evaluated for immature survival and adult emergence. Russell (1975) showed that varieties of paddy tested against S. cerealella maintain their respective rankings with regard to each other, irrespective of whether the infestation was initiated by adult moths or seeded with eggs. However, the standard error was over 2.5 x greater in analysis of the trials using adult infestations, therefore indicating that less variable results maybe achieved if eggs are used, apparently due to the unavoidable differences in fecundity of the chosen females and lack of any evidence to suggest any direct or deleterious effects on the adults by any of the varieties tested.

"Free-choice,' tests are probably more practical when large numbers of varieties are to be tested, and a larger number of adults are released in the same container, so it is not necessary to exactly determine the number of males and females (sex ratio) as in the "no-choice" tests because of the small sample size and small number of adults used. Free-choice can be conveniently used to eliminate obviously susceptible varieties, although in most cases, when "free-choice" and "no-choice" methods are compared, the rankings according to resistance tend to remain, although the number of progeny may be significantly larger in the "no-choice" tests.

"Free-choice" involves placing any number of samples (dependent on the size of the sample boxes) equidistant from the center of the circular chamber which has a small stoppered opening in the center of the lid, where an appropriate number of test insects can be introduced. Adults were removed after 5-7 days, the sample boxes removed, closed with screened lids, and held in the same constant environment for the emergence of progeny. The regime most commonly used is around 27 Cand 65-70% RH. All grain samples to be tested should be equilibrated in these layers in this atmosphere for at least 14 days prior to the commencement of the test and introduction of the artificial infestation. Various researchers have shown that the rankings of varieties under evaluation do not alter if the experiment is conducted under a range of different RH levels, even though, as expected, a greater number of progeny are produced at higher relative humidities.

Some research has indicated differences when adults being tested on a particular variety, were previously conditioned on that variety for successive generations prior to the screening test, or if eggs are used as in the case of S. cerealella, paper booklets are exposed to 30-50 adults bred from the appropriate variety and strips of the booklet pages containing the appropriate number of eggs are then seeded on replicates of conditioned rice of the variety in which the adults are bred. Russell (1975) determined that the averaged total emergence from eggs and adults of S. cerealella, over 6 generations on 9 varieties of rice generally increases with each proceeding generation, which could alter the value of any highly resistant variety reported in screening trials. There was greater variation between generations when varieties were "seeded" with adults as compared to egg trials, with the standard error being almost twice as great.

Although the yield of progeny is markedly parallel between generations using eggs or adults, the wide variation between varieties was due to failure of the developing larvae to enter the grain rather than differences in oviposition or survival of the immatures once the kernel had been penetrated.

THE INHERITANCE OF RESISTANCE:

Cogburn (1980) described varieties of rice that originally displayed resistance to S. cerealella that were crossed with "vista" an uncommonly susceptible commercial variety, and progeny evaluated through the F2 generation to establish the heritability of resistance.

When moth survival between 0-35% were plotted against the number of lines that fell in each percentile, survival on vista appeared as a normal curve ranging from 5-35%, while on the resistant variety, 92% of the lines fell in the 0-5% range. If these varieties were crossed, and resistance is determined by a single pair of genes, the cross should exhibit a 3:1 ratio. If many genes were involved (polygenic resistance) than the cross would have exhibited a normal distribution. However, the cross did exhibit data concentrated towards the resistant end of the scale, and therefore did not conform to either genetic convention. When the cross with vista was repeated with another resistant variety, almost identical results were obtained, although Cogburn stated that the meaning of his results at that time were not clearly evident.

Dobie (1981) has described experiments to establish the inheritance of resistance in cowpeas to C. maculatus. Give the varieties he described as A and B, the following crosses were made, the female parent being listed first in all cases.

PARENTS :
A
B

F1 crosses :
A B
B A

Back crosses:
(A B) A
A (A B)
(A B) B
B (A B)

F3 cross :
A B

Up to 25 seeds of each material were individually placed at random into glass tubes (5 x 2.5 cm) and a single female C. maculatus adult (0-48 hrs old) was introduced and left to oviposit. Emergence of F1 adults (and therefore, percentage survival since all eggs were counted) demonstrated a large maternal effect in inheritance within the F1 crosses. Resistant seeds come from the cross in which the female parent was the resistant variety, and was further demonstrated in the backcross where only the cross involving the resistant female parent produced resistant seed. A strong negative correlation was found between the percentage of trypsin inhibitor in the seed and the percentage survival of C. maculatus.

Singh and Batia (1978) studied the inheritance of resistance to S. oryzae in reciprocal F1, F2 and F3 generations of two wheat varieties, one of which was resistant, and the other susceptible. No-choice progeny test (3 fertilized female S. oryzae per 20 grain sample, 4 replicates, 7 day oviposition, F1 after 45 days) was used on grain samples of each generation, harvested from the same plot and in the same season, under uniform conditions of temperature, relative humidity and grain moisture. The following observations were made:

  1. The resistance to S. oryzae in crosses of these 2 varieties is heritable and genetically controlled;
  2. Resistance is polygenic in nature;
  3. "Additive" gene types were more important than "dominant" gene types;
  4. Transgressive segregation is evident in cresses involving these 2 varieties:
  5. Material effects did not have any controlling influence on resistance to S. oryzae;
  6. Heritability for resistance is quite high and offers wide scope for further refinement and improvement.

An analysis of variance showed that the variability of mean F1, emergence from each parent and from each reciprocal F1, cross were not significant ("F" value less than table value) and therefore non-genetic. However, in the segregating generations (the reciprocal crosses in the F2 and F3) were highly significant, therefore due to genetic causes and heritable (i.e., for rice weevil resistance).

The differences in mean F1, emergence of S. oryzae were not significantly different between each reciprocal cross in the F1, and F2 generation, and only significant at the 5% level in the F3 generation. The means were significantly different from the mid parental value (mean of the F1, weevil emergence of both parents), where the F1 and F2 generations tended towards the resistant parent (partial dominance). The F3 reciprocal crosses tended towards the susceptible parent with the F3 means being significantly higher than the F1, and F2 means.

Frequency distributions (based on the number of samples producing the same number of weevil progeny) showed continuous variation in the F2 and F3 generations forming unimodal curves. This indicated that rice weevil resistance was influenced polygenically (multiple gene system). They inferred that resistance in this study was mainly an additive gene effect with duplicate type of epistasis. In other studies, maize weevil resistance in maize was shown to be nonadditive. In this case either dominant or interactive effects were more important than additive types of gene effects, and maternal genotypes did play an important role (as in cowpea resistance to C. maculatus) in determining resistance.

DURABILITY OF RESISTANCE:

Horber (1983) states that resistance remains effective while the resistant varieties are extensively grown in environments favouring insect infestation.

Durability of resistance depends on the genetic control of the resistant factors, such as single gene of multiple gene resistance where each has a small additive effect on the resistant genotype. Polygenic resistance causing antibiosis or antixenosis is preferable (and more stable) than monogenic resistance mechanisms. Because resistance incorporating antibiosis or antixenosis are exerting a selective influence on insect populations, it is anticipated new biotypes will develop in conditions of prolonged use in isolated populations.

This has become apparent in resistant cultivars developed for pre-harvest pest and disease prevention, but has not as yet been reported in the literature for stored-products insects.

As mentioned previously, genetic uniformity is well recognized in connection with susceptibility to pest and diseases in the field. The lack of genetic diversity means that all major crops are at great risk to large losses after harvest due to the cosmopolitan nature of storage insects. Replacing tight-hulled varieties with loose gaping or thin hulls would be an example where losses would be accentuated. Genetic uniformity can be avoided by using different sources for resistance in the breeding stock.

Natural "gene-pools" carrying the necessary resistant codes are the evolutionary product involving spontaneous mutations, genetic recombinations and natural selection over several millenia. Once lost, they are impossible to replace. More extensive germ plasm collections carrying resistant biotypes from throughout the world, are essental for maintaining genetic diversity, or at least more emphasis on conservation via natural selection.

VULNERABILITY OF RESISTANT VARIETIES IN STORAGE:

As Horber (1983) states, "Nature is the master of safe packaging" whereby seeds are generally well protected and inherently resistant to ensure safe genetic transfer from one generation to the next.

If resistance is concentrated in the pericarp, resistance could be entirely lost if excessive mechanical damage occurs during harvesting or drying. An undamaged pericarp acts primarily as protective barrier against feeding and subsequent oviposition by female S zeamais in maize kernels.

Germination and fungal infection can also markedly reduce inherited grain resistance. S. zeamais is capable of penetrating tight husks in paddy when these were infected by moulds.

The harder the seed, the less the pericarp is open to penetration, so for full expression of resistance in the genotype, moisture absorption by the grain during storage or inadequate drying prior to storage, must be avoided. Grain hardness becomes an integral component of an IPM strategy, since it reduces breakage during storage and handling, reduces risk of high percentage of dust and brokers and the infestation by insects and infection by moulds in bulk storage, that would otherwise increase safety and health hazards.

PLANT BREEDING PROGRAMMES:

Crops that have been subjected to plant breeding to develop new and improved varieties have become increasingly important in the productivity programmes of developing countries. They have been developed with high yielding characteristics in mind. Usually they are resistant to one or several of the more common and destructive preharvest pests and diseases, combined with desirable agronomic characteristics such as a reduced growing period which enables multiple cropping systems to be realized. However, these HYV's and hybrids are generally very susceptible to attack by storage insects, which effectively reduces their impact in enhancing overall food utilization. The number of international and national breeding programmes that have been devised to improve the genetic characteristics of crops to post-harvest insect attack are very limited (Dobie, 1984). T.D.R.I. have done considerable work with maize. Collaborative work with IITA, (Ibadan, Nigeria) and the University of Durham (U.K.) has developed cowpea varieties (an inherently susceptible crop of immense value in terms of protein supplement) that display resistance to a range of Bruchid pests. The International Rice Research Institute has performed limited screening evaluations of the IR varieties, in conjunction with the Department of Entomology, University of the Philippines at Los Banos (UPLB).

Morallo-Rejesus, et.al. (1981; 1982) investigated that IR 24 was the most resistant paddy variety (among the 6 paddy varieties screened) to infestation by S. cerealella and R. dominica at 15 and 18 weeks storage. A further 23 IR varieties of paddy were screened against S. cerealella, and those that were found least damaged were; IR8, IR20, IR24, IR26, IR28, IR32, IR38, IR42, IR44, IR46, IR52 and IR54 based on comparative Indices of Susceptibility.

Morallo-Rejesus, et.al. (1981 + 1982) investigated the physiochemical properties of 15 varieties of brown rice to S. zeamais, T. castaneum and R. dominica, again using the Index of Susceptibility analysis. Susceptibility to S. zeamais was negatively correlated with amylose content, protein content, alkali spreading value, and significantly with grain hardness, while positive correlations were found with grain width and weight. The waxy varieties (high amylose content) combined with lowest hardness indices were consistently the most susceptible to S. zeamais, while IR24 (the lowest amylose content) was the most resistant (as well as IR 3800-10). Weight loss of bran to R. dominica was positively correlated to alkali spreading value and susceptibility to T. castaneum was positively correlated to amylose content, which underlines the fact that differing characteristics of the grain affect storage insects differently in their ability to feed, oviposit, survive, emerge and breed on different varieties.

Juliano (1981) in a review of rice grain properties and resistance to storage insects, also claims that studies on postharvest resistance is still essentially a neglected approach with regard to correlating varietal resistance with the grain's physiochemical properties. Research has shown that breeding for characteristics such as intact, tight hulls; high degree of grain hardness; high amylose content; high endosperm starch gelatinization temperature; low oil content; small and light weight grain proportions; and low moisture content, contribute to impeding the development of a variety of storage insects such as S. oryzae, R dominica, S. cerealella, T. confusum, P. interpunctella, T. castaneum and E. cautella. Juliano suggests that future research be conducted on the interrelationship between grain hardness and grain quality (such as protein content, amylose content, alkali spreading values and gel consistency) and that varietal resistance to the major storage insects be investigated using pairs of sister lines differing in the property under review.

Mills (1976) suggests that insects themselves can be used to select and increase resistance, a method traditionally used by farmers for centuries when selecting seed for planting. Exposing bulk seeds to insect attack and planting those that survived, showed a reduction of about 50% in emergence of S. zeamais from 50-kermel lots of sorghum after 2 years of continuous selection. A similar resistance to R. dominica was found when sorghum was replanted from seeds that initially survived unscathed during storage.

Cogburn (1980) also described attempts to intensify resistance in crosses of paddy varieties between two resistant varieties and "vista". Seeds from the three parents and from the two crosses were separated and duplicated, one lot being frozen for one year, while the other lot were infested. Each lot will be grown in a bulk planting and when harvested, the procedure is repeated. Simultaneously seed is tested for resistance to several species of storage insects, and the whole procedure is repeated for several years, which in theory, should select out susceptilbe seeds and leave only resistant seeds for planting.

Varieties of rice are likely to display variable resistance when grown under different geographical and agronomic conditions. This simply means that varieties that were shown to display resistance in screening tests might be susceptible under different circumstances of production, and therefore growing rice in different environments strongly affects their infestability. Resistant displayed in screening trials using small grain samples may also not be repeated when infested by the same test insects in a bulk storage situation. Mills (1976) mentioning work done by Diaz (1969) found that maize from tropical lowlands were more resistant to S. zeamais than from other areas, and this was explained by the fact that continuous weevil pressure in storage and field infestations naturally selected for resistance. This in part has been attributed to the resistance in local, indigenous varieties, being grown under traditional, nonimproved systems in developing countries as opposed to the recently introduced, improved varieties.

Release of resistant varieties may cause the selection of biotypes which in time will be able to feed and develop on the variety, such as C. maculatus on cowpea varieties with high trypsin inhibitor concentrations.

In order to anticipate such an eventuality, research must continue in order to identify alternative sources of resistance, such as resistance to oviposition, or resistance of seed pods to larvel penetration which can then be incorporated in the resistant variety by suitable breeding programmes.

USING RESISTANCE:

Although it is not necessary to know what causes resistance to use it, and this is the case in many situations, research must continue to more fully identify the mechanisms of resistance, and an interdisciplinary research programme comprising of physiologists, biochemists, geneticists, ecologists, entomologists and plant breeders is necessary. Total resistance against all storage insects is not possible, but any level of resistance is considered useful and virtually free, once it has been incorporated in agronomically and socially acceptable varieties. Most crop species that have been investigated have displayed a certain amount of variation in resistance to major storage pests. Breeding programmes should be designed to exploit such variation and produce resistant varieties, which will enable advances to be made in the reduction of losses, especially at subsistence farmer level, where investment capital in pest control is generally lacking.

Resistant varieties therefore does not require the introduction of technology which may not be suitable/inappropriate and therefore unacceptable for small scale farmers, and perhaps could also reduce the demands made on conventional chemical control techniques at all levels of crop storage.

CONCLUSIONS:

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