The demographic statistics of the ladybird Exochomus flaviventris Mader (Coleoptera: Coccinellidae) were investigated under laboratory conditions on the cassava mealybug, Phenacoccus manihoti Matile-Ferrero (Homoptera: Pseudococcidae), reared on four host plants with different levels of antibiotic resistance to the mealybug: two cassava (Manihot esculenta Crantz, Euphorbiaceae) varieties, Incoza and Zanaga, Faux-Caoutchouc (FC) (hybrid of M. esculenta × M. glaziovii Muel. Arg.) and water weed (Talinum triangulare Jack, Portulacacae). Preimaginal mortality, mean oviposition time and total fecundity of female ladybirds were strongly affected by the host plant even though there was no linkage with antibiotic resistance. The net reproduction rates R0 of the predator were 1.5,1.7 and 2.4 times higher on Zanaga than on water weed, Incoza and FC respectively, and the generation time (T) was significantly longer on Zanaga than on the other three plants. The intrinsic rate of increase rm was not modified by the antibiotic resistance of the host plants. With the two cassava varieties, Incoza and Zanaga, it was found that antibiosis significantly affected the life history parameters of the ladybird without modifying its fitness, and provided better cassava mealybug control.

A study was conducted to elucidate the effects of protein fractions isolated from maize (Zea mays L.) seeds on the development and survival of the Mexican bean weevil, Zabrotes subfasciatus (Boh.) (Coleoptera: Bruchidae). Maize seeds were fractionated into albumins, globulins, prolamins, glutelins and a residue and the bruchids reared on artificial seeds composed of susceptible background bean flour enriched with the different protein fractions. Survival (as indicated by LD50) and development (as indicated by WD50) of 20-day-old larvae of the bruchid were negatively affected by most of the fractions. The albumins and globulins were the most detrimental to Z. subfasciatus, significantly affecting both its survival and development.

The aim of the present study was to determine, by a field experiment, whether the seasonal variations in the levels of cyanide and rutin in the phloem, secondary compounds potentially implicated in cassava resistance to the cassava mealybug (Phenaccocus manihoti), could be influenced by different kinds of soil fertilisation (manure, NPK, KC1, lime, mulch). It was found that mean cyanide and rutin contents varied significantly with season, soil treatments and infestation. For each soil treatment, the highest contents of these compounds on uninfested plants was registered in the main dry season (Jul-Aug) for cyanide and at the end of the dry season (Jul-Aug) and beginning of the rainy season (Oct-Nov) for rutin. The infestation factor induced an increase in cyanide and rutin contents for 43 combinations of period and soil treatment among 48 in total. For cyanide content (phagostimulant function in mealybug), this increase appeared significant at the end of the rainy season (Apr-May) and in the main dry season (Jul-Aug) for control and KC1 fertilisation. For rutin content (an unfavourable substance for the insect) significant increase was induced by infestation in each period for manure and mulch and especially in the main dry season (Jul-Aug) and at the beginning of the rainy season (Oct-Nov) for other soil treatments. Concerning the seasonal variations in the level of rutin, these results showed that mulch and manure are the best fertilisers to increase cassava resistance against mealybugs.

Cassava varieties resistant to the cassava green mite, Mononychellus tanajoa (Bondar) ( Acari:Tetranychidae) have been reported in many parts of Africa where the pest has been introduced. However, the resistance mechanisms have not been identified. Laboratory studies on biology and feeding behaviour of cassava green mite were carried out at the International Institute of Tropical Agriculture (IITA), in order to determine whether this resistance is mainly due to pubescence.

Cassava pubescence was found to have no antibiotic effect; both pubescent and non-pubescent varieties were preferred by the mite for oviposition, food and shelter. In addition, pubescence does not afford a mechanical barrier to cassava against green mite infestation.

This study was conducted to identify the individual biochemical factors involved in the resistance observed in a few Mexican bean accessions against the bean weevil, Acanthoscelides obtectus (Say) (Coleoptera: Bruchidae). Soluble proteins from different bean genotypes were separated by fractionated acetone precipitation. Insects were reared on artifical seeds composed of a susceptible background bean flour enriched with the different protein fractions. At low doses (2% w/w), the 0–20% and 20–40% acetone fractions of the resistant bean accessions G 12954 and G 12880 inhibited larval growth of the bean weevil, but not of a related species, the Mexican bean weevil, Zabrotes subfasciatus (Boheman). The 0–20% fraction caused the highest mortality of first instar larvae. The 60–80% acetone fraction contains general resistance factors, like α-amylase and protease inhibitors, which when enriched in artifical seeds led to a strong depression of insect growth for resistant and susceptible bean accessions alike. These factors should also be taken into account in breeding for multigenic, durable resistance, in order to exploit the synergism between general and specific resistance factors.

Cowpea, Vigna unguiculata (L.) Walp., is susceptible to a wide spectrum of insect pests from seedling to harvest as well as in storage. Insecticide use has been the most commonly adopted method for the control of cowpea pests. However, due to the deleterious effects of insecticides on man, livestock and the environment, research over the years has been focused mainly on developing resistant/tolerant cultivars as a vital component in the integrated management of cowpea pests. This approach has, however, recorded varying degrees of success, due in part to the multiple and often overlapping pest complex attacking at different phenological growth stages of the crop.

In spite of the progress in the utilisation of insect-resistant cowpea varieties, the use of resistance as a control tactic has by no means been fully realised. There are many important cowpea insect pests for which resistant varieties are not yet available for commercial cultivation. Where resistant varieties are available for a given insect, these varieties may not be widely planted because of their unsuitability to local conditions.

In this paper, the status of host plant resistance research in cowpea is reviewed and some novel approaches aimed at solving its seemingly intractable pest problem are proposed, based on the state-of-the-art scientific and crop biotechnological development. The proposed framework for orientation of cowpea resistance research should ensure yield stability and sustainability for the resource-poor, smallscale farmers in Africa.

The effect of a 2-month infestation by the cassava mealybug (Phenacoccus manihoti Mat. Ferr.) on the metabolism of nitrogen (amino acids) and carbon (carbohydrates), leaf area and total dry weight of five cassava varieties (Manihot esculenta Crantz), faux-caoutchouc (a hybrid of M. esculenta and M. glaziovii Muell, Arg.), poinsettia (Euphorbia pulcherrima Wild) and talinum (Talinum triangulare Jack) was studied. Free amino acid and free sugar contents as well as relative free amino acid composition in the leaf extracts, although found to be very different from one plant to another, were not significantly modified by P. manihoti infestation, except for the total amino acid contents of the cassava variety 30M7. Variations in one particular amino acid induced by mealybug infestation were not linked to the antibiotic resistance of these plants. Infestation by the cassava mealybug did not modify the total dry weight but reduced the total leaf area although this reduction was only significant in cassava varieties 59M2, 30M7 and M'pembe, and in faux-caoutchouc. Within the genus Manihot, the reduction in leaf area was strongly correlated (r= -0.878, P≤0.05) to the degree of antibiotic resistance and was coupled to an increase in the ratio of sugars to amino acids, suggesting a similarity between the effects of water stress and those of mealybug infestation.

A total of 2256 germplasm accessions, 3000 advanced generation breeding lines and 150 released varieties of rice were screened under artificial infestation in the greenhouse. Among these, 15 new germplasm accessions, 50 breeding lines derived from 16 crosses and 15 released varieties were resistant. Antibiosis was the predominant resistance mechanism; antixenosis affecting oviposition was noted in 33 resistant varieties. Biochemical composition of a set of resistant varieties along with susceptible checks, revealed that nitrogen content of seven test varieties was significantly higher than that of the susceptible checks, while in other six varieties it was lower than the checks. Similarly, eight of the resistant varieties had considerably higher percentage of polyphenolic compounds in basal stem portion, but seven other cultivars showed significantly lower concentration as compared to susceptible checks. Thus, it was apparent that there was no clear trend among the resistant cultivars as a group in relation to quantitative differences of chemical components analysed.

Antixenotic and antibiotic components of resistance to the cassava mealybug, Phenacoccus manihoti Matt. Ferr. were evaluated in different varieties of cassava (Manihot esculenta Crantz), in Poinsettia (Euphorbia pulcherrima Wild), Talinum (Talinum triangulare Jack) and Faux-caoutchouc (hybrid of M. esculenta and M. glaziovii Mull. Arg.). Resistance, was estimated in the field on 25 varieties of cassava by means of varietal screening. Although we were unable to identify varieties ofcassava totally resistant to P. manihoti, there was evidence of partial resistances. Thus the Incoza variety is the most resistant, followed by the Moudouma and Zanaga varieties. On the other hand we found very susceptible varieties, such as Dikonda, Kataoli, 3M8 and 1M20. Laboratory evaluations of the antibiotic component of resistance, made by estimating the intrinsic capacity for increase rc, showed that the host-plantshave a considerable effect on the multiplying capacity of the mealybug. Indeed rc ranges from 0.038 (Poinsettia) to 0.160 (Ganfo cassava variety), i.e. a ratio of 1 to 4 between the two extreme values. If the cassava varieties alone are considered, the percentage of maximum variation is 20% between the Incoza (rc = 0.133) and Ganfo (rc = 0.160) varieties. The Incoza cassava variety is the most resistant, in terms of both antixenosis and antibiosis. Classification of the other cassava varieties, from the most to the least resistant, differs according to the resistant component under consideration. Poinsettia and Talinum both have a very strong antixenotic component, but their antibiotic component differs (rc = 0.038 and 0.150 respectively). Our results suggest that the resistance mechanisms of the host-plants of P. manihoti intervening in the fixation of the pest (antixenosis), are different from those acting on the development of the mealybug (antibiosis). We established significant correlations between the size of the mealybugs and their demographic characteristics for all the plants studied: the duration of the prereproductive period is shorter and the net reproduction rate is higher when the mealybugs are large-sized. These results are discussed in the context of an integrated monitoring programme.

The biology of Maruca lestulatis Geyer was studied on different parts of 18 cowpea cultivars to determine the role of antibiosis in their levels of resistance. M. testulalis larvae were successfully reared from first instar to adult stage on floral buds, flowers and sliced pods, but not on stems, terminal shoots and intact (unsliced) pods, where there was complete larval mortality. The mean pupal weight ranged from 43.5–54.5 mg on floral buds, 38.5–58.6 mg on flowers and 42.7–58.6 mg on sliced pods, with highly significant differences between (P ≺ 0.01) resistant and susceptible cultivars on each part. Growth indices showed that sliced pods were the most suitable for larval growth and development, followed by flowers and floral buds.

The levels of resistance found were inadequate for solving the M. testulalis problem in cowpea. Selection and breeding for cowpea ideotypes (architypes), to combine such resistance with morphological, biochemical and biophysical traits, could enhance the low levels of resistance in the cowpea crop and ultimately lead to the effective management of this pest. The correlations between growth and development parameters showed that larval period had the most influence on M, testulalis survival and population build-up in the succeeding generations.

Experiments were conducted in 1989 to evaluate converted sorghum accessions for preflowering extended panicle and soft-dough stage panicle feeding resistance to fall armyworm, Spodoptera frugiperda (J. E. Smith), by incorporating plant tissue into artificial insect diet. Spodoptera frugiperda biological parameters measured were larval and pupal weights, mean duration of the larval stage, time to adult eclosion, survivorship, fecundity, net reproductive rate, intrinsic rate of increase, and relative fitness. The converted lines IS 2246C, IS 2403C, IS 2825C, IS 7007C, IS 8337C, IS 12592C, IS 12612C, IS 12657C, IS 12666C, and IS 12681C were more resistant to both preflowering and soft-dough stage panicle feeding by S. frugiperda than the resistant check NK Savanna 5. The genotypes IS 1340C, IS 2553C, IS 2569C, IS 6911C, IS 7498C, IS 12219C, IS 12617C, IS 12662C, and IS 12664C were more resistant than the resistant check to S. frugiperda feeding in the soft-dough stage, but not in the preflowering stage of panicle development. Plant antibiosis was the major mechanism of resistance in these lines at both stages of the panicle development.

Earias vittella (Fabricius) feeds on some malvaceous plant species, but has never been reported to attack Thespesia populnea Cav., even though this plant belongs to the same family. The factors responsible for the failure of E. vittella to establish on T. populnea were, therefore, studied. When whole fruits or fruit pericarps of this plant were provided as larval food, larvae failed to develop and none of them pupated. However, on developing seeds of these fruits, larval growth and development were normal and comparable to those of larvae reared on developing seeds of the most favoured host plant, Abelmoschus esculentus Moench. Ether extracts from fruit pericarps exhibited antifeedant and antibiosis activity against this insect. Ether as well as ethanol extracts affected oviposition to some extent. In a choice situation, significantly less eggs were laid on treated than on untreated okra fruits. However, in a no-choice situation most of the eggs were laid on treated okra fruits.

As part of an extensive and systematic programme to find genetic sources of resistance to Aphis craccivora, a major pest of cowpea, Vigna unguiculata, a total of 181 accessions from the germpiasm collection at the International Institute of Tropical Agriculture were screened in glasshouse tests using a simple visual damage scale. Twenty-four accessions showed considerably reduced damage. To confirm these results on IS selected accessions, aphid population growth over 10 days was recorded. Zero aphids were found on three accessions (310,408-P2 and 801); another six accessions had populations of less than 30, compared with 200–400 on susceptible varieties. The other accessions showed only partial antibiosis and/or tolerance.

The potential of plant genetics in the overall context of integrated pest control could be viewed simply as the potential of plant resistance in pest management. Genetic resistance in plants is one of the most effective and economic means of controlling plant pests. Resistant plants are the first line defence against pests. Successes in breeding for pest resistance depend upon the sound knowledge of the plant genes controlling resistance to pests.

Recent report on the existence of two nonatlelic and independent genes, Rac1 and Rac2 in cowpea cultivars ICV 10 and TVu 310, and ICV 11 and ICV 12, respectively, from the ICIPE has broadened the scope of cowpea breeding against aphid biotypes. Use of biotechnology as a plant breeding tool in transferring resistance genes into agronomically suitable cultivars appears to hold good promise for the future.

Four soybean genotypes, PI 227687, PI 229358, Davis and Hardee were tested for their antixenosis and antibiosis to the soybean hairy caterpillar, Spilarctia casignata Koll. PI 227687 and PI 229358 showed greater antixenosis and strong antibiosis (high larval and pupal mortalities, reduced pupal weight, and a low growth index). Davis and Hardee were the favoured hosts as indicated by high larval preference based on increased feeding and low larval and pupal mortalities, increased pupal weight and a high growth index.

The spotted stem borer, Chilo partellus (Swinhoe) is an economically important pest in many sorghum growing areas of Asia and Africa. A chronological progress made in Indian and other programmes on natural and artificial screening techniques is reviewed. Numerous germpiasm accessions were evaluated and resistant sources identified and documented for various parameters of resistance such as foliar damage, “dead hearts” and stem tunnelling.

Studies on the mechanisms of resistance showed predominance of tolerance, variable degrees of antibiosis and antixenosis as only a factor for oviposition. The morphological characters and biochemical factors associated with resistance are discussed. The nature of resistance is polygenic and partially dominant over susceptibility. The inheritance patterns of resistance to foliar damage and/or “dead hearts”, and stem (stalk and peduncle) tunnelling damage are different and independent. Pedigree breeding with single and multiple crosses has been useful to transfer resistance in more economic background. Continuous screening for stem tunnelling over several generations improves the level of resistance. Recurrent selection was suggested alternatively with and without infestation for resistance and yield, respectively to increase the level of tolerance. The resistant sources utilized in developing commercially released high yielding varieties, hybrids and derivatives with low to moderate levels of resistance are listed. Efforts to develop cultivars with levels of resistance higher than presently grown cultivars and combining cross resistance to stem borers and shootfly are under way. Current progress has been reviewed and future strategies are suggested.

The biology of Chilo partellus (Swinhoe) was studied on two resistant, IS 2205, IS 5469 and two susceptible, HC 136, ICSV1 sorghum genotypes. Significant differences were observed between resistant and susceptible genotypes in regards to larval mortality in whorl, larval period, larval weight, larval length, per cent pupation, pupal weight, fecundity per female and total life span of this pest. Growth index was also influenced by the resistant genotypes. It has been concluded that high mortality of larvae on resistant lines is due to antibiosis.

The correlations between the characters of the pest biology have been worked out and larval stage was the most important parameter influencing the total pest survival and population build-up in the succeeding generations.

The field experiments were conducted during the summer of 1985 to determine the degree of winter wheat resistance to the grain aphid Sitobion avenae. Asta, Grana, and Saga cultivars were generally more resistant than Dana, Liwilla, and Emika. The resistance mechanism involved both antixenosis and antibiosis. Grana was least preferred during aphid migration. Asta, Saga, and, to a lesser extent, Grana had an antibiotic effect on growth and development of the apterous aphids (longer prereproductive period, lower fecundity and lower values of the intrinsic rate of increase, rm, during the first days of reproduction). Moreover, var. Asta showed a “secondary antixenosis” during colonization of ears due to the presence of awns.

The midge, Contarinia sorghicola Coquillett, is a major pest of grain sorghum in many areas of Asia, Africa, Australia, Europe and America. Literature documenting the status of research on the biology, behaviour, and field and cage screening techniques is reviewed. Numerous germplasm entries have been evaluated and many resistant sources are identified from various midge screening programmes. A high degree of resistance was found in nine working groups viz., Caudatum, Durra, Caudatum/Nigricans-Zerazera, Caudatum/Kafir (Hegari), Nigricans, Caffrorum-Darso, Durra-Nigricans, Durra-Nandyal and Caudatum-Bicolor. The identified resistant sources and the midge resistant varieties released from the breeding programmes are listed. Studies on the mechanisms of resistance showed predominance of antixenosis for oviposition over adult attraction and variable degrees of antibiosis. The morphological characters and biochemical factors associated with resistance have been explored. Short and compact glumes which remain closed during anthesis conferred resistance. The tannin content in the ripening sorghum grain is related to resistance against the larva. The inheritance of resistance showed quantitative nature. The open and closed glume character expression indicated its dominance and recessiveness, respectively. Short glume character can be used as an useful genetic marker. Pedigree and modified pedigree breeding systems were suggested to transfer high level of midge resistance into agronomically acceptable types for selection and hybridization.

The shootfly, Atherigona soccata Rondani, is a major seedling pest of sorghum in many areas of tropical Asia, Africa and Mediterranean Europe. A chronological review of the progress made in various programmes on the screening techniques (natural and artificial) with artificial diets, sources, stability and mechanisms of resistance, bases (physico-morphological and biochemical factors) of resistance, inheritance and breeding for resistance is made. Many resistant varieties have been identified and sources of resistance utilized in breeding programmes to transfer resistance in high yielding background are listed. In addition, the glossy and trichome traits identified is sorghum cultivars are also reported. Glossy leaf surface and trichomes are frequently associated with shootfly resistance.