The whitefly, Bemisia tabaci (Gennadius) is documented as a major pest on soybean. It was reported that whitefly response towards its hosts and their cultivars varies, and is mediated through various host-related factors. Considering the significance of leaf morphological characteristics in influencing the host–whitefly responses, the present investigation was conducted in screen-house conditions to study the prevailing variations in leaf morphological characteristics of soybean genotypes and their role in governing the adult whitefly attractiveness and oviposition preference. In the multiple-choice test, the whitefly population (eggs, nymphs and adults) was found to be lowest in moderately resistant genotypes (SL 1028 and SL 1074) compared to highly susceptible (DS 3105) and susceptible genotypes (SL 688, SL 958 and SL 1113). The foliar trichomes were characterized using scanning electron microscopy and leaf area, leaf lamina thickness and leaf shape data were acquired using standard procedures. To determine the factors involved in the resistance/susceptible responses towards whitefly, Pearson correlation was applied between the morphological characteristics and the whitefly population. The results show that the leaf area, trichome density, trichome length and trichome angle showed a significant positive correlation with the whitefly population, whereas leaf lamina thickness was negatively correlated. Thus, for developing whitefly-resistant germplasm, breeders should choose genotypes having narrow and thick leaves with sparse, short and flat trichomes.

The citrus red mite, Panonychus citri McGregor, is a key pest of different citrus species in various parts of the world. Considering the key role of resistant host plants in integrated pest management strategies, we evaluated the effects of five citrus species including grapefruit (Citrus paradisi), lime (Citrus aurantifolia), tangerine (Citrus reticulata), orange (Citrus sinensis), and sour orange (Citrus aurantium) on life table parameters of P. citri under laboratory conditions (25 ± 1°C, 65 ± 5% RH, 16:8 L:D). In addition, biochemical traits of the citrus plant species were evaluated in order to understand any possible relationship between important life history parameters with biochemical metabolites of citrus plant leaves. Phytochemicals were determined in leaf extract of citrus plant species. Various citrus species had significant effects on life history and demographical parameters of P. citri. The longest pre-adult time was observed on grapefruit (16.52 ± 0.43 days). Higher fecundity rate was on orange (15.05 ± 2.41 eggs) and tangerine (14.60 ± 3.07 eggs) and the lowest was on grapefruit (7.21 ± 2.00 eggs). The highest intrinsic rate of increase (r) was recorded as 0.071 (day−1) on tangerine, and the lowest value of this parameter was obtained on grapefruit (0.016 day−1). Significant correlations were observed between life history parameters with biochemical metabolites (carbohydrate, phenolic compounds, anthocyanin, and flavonoid). The results revealed that grapefruit was a relatively resistant host plant and tangerine was the most suitable host plant for feeding of P. citri. Our findings could be helpful for sustainable management of P. citri in citrus orchards.

Common bean (Phaseolus vulgaris L.) is amongst the most important grain legume crops in Africa in general, and Ethiopia in particular. The Mexican been weevil (Zabrotes subfasciatus Boheman) heavily attacks the grain of common bean. A total of 300 common bean entries were subjected to a ‘no-choice’ test at Melkassa Agricultural Research Centre, Ethiopia, using a randomized complete block design with three replications, to evaluate for resistance to the Mexican bean weevil. Data on insect and seed traits were collected and a significant level (P<0.01) of variation in all parameters measured was observed amongst genotypes. Relative resistance was recorded in landraces, improved genotypes and breeding lines, but the resistant genotypes, RAZ-11, RAZ-36, RAZ-2, RAZ-44, RAZ-120, RAZ-40 and MAZ-203, showed consistently complete resistance, with zero index of susceptibility value. Two other promising entries were also identified from the breeding lines (SCR-11) and landrace collections (NC-16) of Ethiopia. Stratified ranking diagrams showed that accessions from different eco-geographical origins in Ethiopia and those with different colours showed different patterns of response to infestation. The Ethiopian bean breeding programme should take up the resistant genotypes for a comprehensive yield trial at the national level and direct release them as commercial varieties. The incorporation of bean weevil resistance genes into adapted varieties through backcross breeding techniques, supported with marker assisted selection, seems to be the best strategy not only in terms of time saving but also in terms of effectiveness and efficiency.

Lepidopteran stemborers, mainly the African stemborer Busseola fusca Fuller and spotted stemborer Chilopartellus Swinhoe, are among the economically important pests of sorghum in Kenya. Identification of sources of resistance is valuable for the development of an integrated pest management strategy to minimize losses. This study evaluated 27 genotypes with artificial infestations (mostly for tolerance) in two different agroecological zones. Data on damage from leaf feeding, dead heart, exit holes, and stem tunnelling; plant height; days to 50% flowering; total grain yield; and hundred-grain mass were collected. Significant differences (P< 0.001) were observed in all the traits evaluated, except leaf damage, under infestation by C. partellus. Classification into different categories of resistance revealed genetic variation. Genotypes resistant/moderately resistant to both pests were as follows: ICSA 464>ICSB 464>ICSB 467>ICSA 472>ICSB 472>ICSA 473>ICSA 474>IESV 91131 DL>Macia. The relative contributions of the damage parameters to grain yield loss were partitioned into direct and indirect relationships by path coefficient analysis with grain yield as the resultant variable. The results revealed that damage from exit holes, dead heart, leaf feeding and stem tunnelling had a negative indirect effect on grain yield. The identified genotypes can be used to enhance resistance to both pests in susceptible cultivars that farmers prefer.

It i.s difficult to compare genotypic resistance to insects across seasons and locations because of the variation in the onset and severity of insect infestation. Therefore, in this study, we used the no-choice cage technique and detached leaf and artificial diet incorporation assays for evaluating chickpea genotypes for resistance to the beet armyworm Spodopteraexigua (Hubner). The results indicated that the no-choice cage technique was not useful for evaluating chickpea genotypes for resistance to S. exigua. In the detached leaf assay, leaf feeding by S. exigua larvae was significantly lower in ICC 12 475 and RIL 20 genotypes at the vegetative stage and in ICC 10 393, ICC 12 475, KAK 2, RIL 20 and RIL 25 genotypes at the flowering stage, while larval weight gain was lower in insects reared on EC 583264, ICC 10 393, ICC 12 475 and RIL 20 genotypes at the vegetative stage; and in those reared on ICC 10 393, ICC 12 475, EC 583264, ICCL 86 111, KAK 2, RIL 20 and RIL 25 genotypes at the flowering stage in plants raised under greenhouse conditions. In plants raised under field conditions, the EC 583260, ICC 12 475, ICCL 86 111, ICCV 10, KAK 2, RIL 20 and RIL 25 genotypes in the November sowing and the KAK 2, ICC 3137, ICCL 86 111 and RIL 25 genotypes in the December sowing suffered low leaf damage at the vegetative stage; and EC 58 320, EC 583264, ICC 12 745 and RIL 25 genotypes in the November sowing and the EC 583264, ICC 3137, ICC 12 475, 1CCL 86 111, KAK 2, RIL 20 and RIL 25 genotypes in the December sowing suffered low leaf damage at the flowering stage, while low larval weights were recorded in insects reared on the ICC 12 475, EC 583264, ICCL 86 111 and RIL 25 genotypes at the flowering stage. In the diet incorporation assay, the survival of S. exigua larvae reared on diets with leaf powder of the ICC 12 475, ICC 10 393 and RIL 25 genotypes was significantly lower, while a significant reduction in larval weights was recorded in those reared on diets with leaf powder of the ICC 10 393, ICC 12 475, ICCL 86 111, KAK 2, RIL 25 and ICC 3137 genotypes. The fecundity of insects was also reduced in insects reared on diets with leaf powder of the RIL 25, RIL 20, ICCV 10, ICCL 86 111, ICC 12 475, ICC 3137, KAK 2 and ICC 10 393 genotypes. The results suggest that detached leaf assay could be used for large-scale screening of chickpea genotypes for resistance to S. exigua, while the diet incorporation assay could be used to gain additional information on the antibiosis mechanism of resistance to this insect.

The black currant-lettuce aphid, Nasonovia ribisnigri, is an important pest of cultivated lettuce, Lactuca sativa. Since 1982, the control of this aphid on lettuce is largely based on host plant resistance, conferred by the Nr gene, introgressed from Lactuca virosa. The resistance mechanism remains to be identified. N. ribisnigri populations virulent on the Nr-based resistance in lettuce have emerged in several locations in Europe since 2007. The objective of this study was to investigate the resistance mechanism mediated by the Nr gene in lettuce by detailed studies of aphid feeding behaviour and performance. Both avirulent (Nr:0) and virulent (Nr:1) biotypes of N. ribisnigri were studied on five resistant and two susceptible near isogenic lines (NILs). In addition, survival and colony development were quantified. Nr:0 aphids showed a strong decrease in sieve element ingestion and took longer to accept a sieve element on resistant NILs compared with susceptible NILs, and no aphids survived on the resistant NIL. Nr:1 aphids fed and performed equally well on the resistant and susceptible NILs. The resistance mechanism against Nr:0 aphids encoded by the Nr gene seems to be located in the phloem, although we also observed differences in feeding behaviour during the pathway phase to the phloem. Nr:1 aphids were highly virulent to the resistance conferred by the Nr gene. The consequences of the appearance of Nr:1 aphids for control of N. ribisnigri are discussed.

The resistance or susceptibility of sorghum genotypes to damage by the spotted stemborer Chilo partellus (Swinhoe) is commonly measured in terms of leaf damage, deadheart formation and stem tunnelling. Effect of the host plant on the insect survival and development (antibiosis) and the response of the plant to insect damage (tolerance or recovery resistance) are also important parameters for measuring host plant resistance to stemborers. We compared ten selection indices commonly used to select genotypes for tolerance/resistance to various stresses, namely functional plant loss index (FPLI), antibiosis index (ABI), mean productivity index (MPI), mean relative performance (MRP), relative efficiency index (REI), tolerance index (TOL), Fischer's and Maurer's stress susceptibility index (FMSSI), Fernandez stress tolerance index (FSTI), geometric mean productivity (GMP) and Schneider's stress severity index (SSSI). The results suggest that the indices based on grain yield reduction can be combined with those associated with grain yield response and potential under borer-infested and uninfested conditions, since each index assesses different biological mechanisms (such as tolerance, adaptation and/or productivity). Among these, FPLI can be used to differentiate the sorghum genotypes for different components of resistance by taking into account both foliar damage and deadheart formation. GMP, REI and FSTI provided a better discrimination of the stemborer-tolerant genotypes under borer-infested conditions, and were better predictors of grain yield performance than TOL, MPI, MRP, ABI, SSSI, FMSSI and FPLI. A strong association between GMP and REI indicated that both these indices could be used to select for low grain yield loss and high productivity. However, the selection of genotypes with high grain yield potential and adaptation to borer infestation may be achieved by combining selection indices related to the mean grain yield performance under borer-infested and uninfested conditions (GMP and REI), and low levels of grain yield loss under borer infestation.

Use of host plant resistance for the management of the banana weevil Cosmopolites sordidus (Germar) requires development of rapid cultivar screening methods. A previous study showed prolonged development of larvae feeding on excised plant material of resistant cultivars and suggested partial breakdown of resistance in such plant material. In the current study, development and survivorship of larvae feeding on potted plants of banana cultivars with differing levels of resistance were evaluated. Weevil performance was evaluated on one susceptible (Atwalira), one intermediate (Kabula) and four resistant cultivars (FHIA 17, Kayinja, Kisubi and Yangambi Km5). Differences in development of larvae feeding on the susceptible Atwalira and intermediate/resistant cultivars became apparent at 15 days after infestation of plants. Differences in survivorship of larvae feeding on different cultivars were not statistically significant. Results of this study show that experiments using potted plants can be used to screen for resistance to banana weevil based on the development of larvae. The influence of previous feeding damage to banana plants on larval development and survivorship in a susceptible (Atwalira), an intermediate (Kabula) and a resistant cultivar (Kisubi) was also examined. While no effects on larval development rate were apparent, survivorship was reduced in previously attacked plants for the intermediate cultivar, indicating that resistance may be induced by previous weevil attack of plants.

Resistance mechanisms to the pod-sucking bug (PSB) Clavigralla tomentosicollis Stäl, in cowpea genotype IT86D-716, were studied in field and laboratory. Non-preference was evaluated on three cowpea varieties (IT86D-716, Moussa local and KVx396-4-5-2D) through measuring differences in population parameters and extent of damage under field and laboratory conditions. Antibiosis was evaluated based on development parameters of C. tomentosicollis when reared on pods of the aforementioned varieties. The variety IT86D-716 consistently exhibited both non-preference and antibiosis resistance mechanisms to C. tomentosicollis. A chemical analysis of the pods was conducted to identify compounds conferring antibiosis. Several compounds including cyanogenic heterosides, flavonoids, tannins and trypsin inhibitors were present in pods of IT86D-716. Evidence suggests that antibiosis due to these compounds may contribute to the resistance to PSB. In the case of antibiosis, 5-day long exposures were adequate to observe nymphal mortality. Based on this finding, a fast, reliable and low-cost screening of pods can be carried out in the laboratory for identification of lines that are C. tomentosicollis resistant.

Host plant resistance is an important tool in the management of the banana weevil Cosmopolites sordidus (Germar). Although ovipositing females do not discriminate between resistant and susceptible cultivars, plants of resistant cultivars have smaller larval populations and sustain less damage in the field. These observations suggest that lower damage levels observed in resistant cultivars reflect larval success. This study was carried out to evaluate laboratory screening of banana cultivars as a rapid screening method for resistance to the banana weevil by determining the influence of cultivars on weevil survivorship, development duration and adult weight. Larvae were raised on corm pieces of two susceptible, three intermediate and four resistant cultivars, resistance categories having been determined from a previous field screening trial. The developmental period was prolonged in resistant cultivars. However, cultivars had less influence on survivorship and adult weight. Moreover, the ranking of survivorship did not correlate with resistance levels observed in the field. The implications of these findings for laboratory screening of cultivars for resistance to C. sordidus are highlighted.

The spotted pod borer Maruca vitrata (Geyer) is known for its economic importance throughout its geographical distribution because of its destructive nature to reproductive parts of several grain legume crops including pigeonpea. In view of the importance of the pest, the present study was carried out on the association of different morpho-chemical traits with resistance/susceptibility to M. vitrata at the International Crops Research Institute for the Semi-Arid Tropics, Patancheru, India. Trichome length and density, sugars, proteins and phenols were found to be associated with resistance to M. vitrata in short-duration pigeonpea genotypes. Pod damage by M. vitrata on different short-duration pigeonpea genotypes in the field ranged from 5.8 to 68%. Laboratory studies showed less consumption of food and reduced larval and pupal weights of M. vitrata when reared on the resistant genotypes ICPL 98003 and ICPL 98008 indicating antibiosis effects of the genotypes. Trichome density on upper and lower surfaces of the leaf (390 and 452/9 mm2), and length (3.5 mm) and trichome density (442/9 mm2) and length (5.9 mm) on pods were found positively correlated with the resistant genotype ICPL 98003. High sugar content in flowers (22%) and pods (10.6%) was responsible for the susceptibility of ICPL 88034, while high phenol concentration in flowers (6.5%) and pods (9.3%) in ICPL 98003 was responsible for resistance. Protein content in pods was significantly higher (25.5%) in susceptible ICPL 88034 when compared with resistant ICPL 98003 (16.5%). Based on these results, ICPL 98003 and ICPL 98008 were categorized as highly resistant and ICPL 98012 as moderately resistant. This paper discusses the physico-chemical traits associated with resistance to M. vitrata in short-duration pigeonpea genotypes.

Sorghum is one of the most important cereal crops in the semi-arid tropics. Several insect pests damage it. The possibility of identifying genotypes with multiple resistance to these pests and transferring the relevant genes into high-yielding varieties and hybrids has been considered as an attractive approach to reducing yield losses. A set of 12 cytoplasmic male-sterile and maintainer lines, 12 restorer lines and their F1 hybrids were evaluated for resistance to sorghum shoot fly Atherigona soccata Rondani, spotted stemborer Chilo partellus (Swinhoe) and sugarcane aphid Melanaphis sacchari (Zehntner) under field conditions. There were significant differences among the genotypes tested. A total of 50% of male-sterile lines, 41.7% maintainers, 58.3% restorers and 35.4% of the hybrids tested showed moderate to high level of resistance to the three pests. The male-sterile and restorer lines showing resistance to different insects can be exploited for developing hybrids with multiple insect resistance for cultivation by the resource-poor farmers in the semi-arid tropics.

Glasshouse studies were conducted to determine mechanisms of resistance of five genotypes of lentil Lens culinaris (Medikus) previously identified as resistant and moderately resistant to the pea aphid Acyrthosiphon pisum (Harris). A susceptible standard, ILL 8127, was also included. French Indigo had high levels of antibiosis, demonstrated by reduced reproductive period, fecundity and longevity, while Spanish Brown had high levels of antixenotic and tolerance components of resistance. Based on overall resistance, French Indigo was the most resistant genotype followed by Spanish Brown. These genotypes could be a source for resistance to A. pisum. All five genotypes had varying levels of antibiosis, antixenosis and tolerance components of resistance compared with ILL 8127.

Various insect pests, of which stemborers are the most widely distributed and damaging, affect about 30 out of the 35 million hectares planted with maize in developing countries. Chemical control, biological control; cultural methods and host plant resistance constitute the four general approaches to stemborer control. The use of stemborer-resistant maize increases farming efficiency by both reducing yield losses from stemborer damage and reducing or eliminating the cost of insecticides and other inputs. In the past, CIMMYT followed conventional breeding methods to develop germplasm resistant to stemborers and molecular technology, quantitative trait loci (QTL) in marker-assisted selection (MAS) to select for improved stemborer resistance in elite lines. More recently, CIMMYT has developed the capacity to produce transgenic maize with resistance factor(s) derived from genes that encode delta-endotoxins derived from the soil bacterium Bacillus thuringiensis (Bt). A sub-tropical source population with multiple borer resistance (MBR population) was developed by recombination and recurrent selection under infestation with four stemborer species. Marker-assisted selection is being used in two African countries to promote the transfer of resistance into elite and adapted germplasm. The Insect Resistant Maize for Africa (IRMA) project is pursuing the transfer of Bt-based resistance to adapted maize germplasm, initially in Kenya, but later to other interested African countries. CIMMYT's varietal release strategy is to pyramid Bt genes into maize populations with existing multigenic pest resistance, in order to enhance both the levels and durability of plant resistance to maize pests. This paper discusses the various approaches used at CIMMYT to develop stemborer-resistant maize germplasm.

Lepidopteran stemborers are major limiting factors in the production of maize and sorghum in Africa and Asia. Host-plant resistance and biological control are important components of an IPM programme and can effectively maintain pest populations below economic threshold levels, especially in low-input subsistence farming systems. When damaged by herbivores, plants produce allelochemicals which can attract natural enemies of the pest. These volatiles occur systematically throughout the plant and are different from those emitted in response to mechanical damage. We report evidence of plant volatiles-mediated differential reaction of parasitoid activity to sorghum genotypes. These volatiles could have a positive impact on the efficacy of stemborer parasitoids, leading to increased host finding or increased searching efficiency, thereby enhancing the suppression of stemborer populations. In a single-season case study, parasitoid activity was higher in stemborer-resistant genotypes than in susceptible ones and varied with crop age. Knowledge of such interactions should be explored and intensified by plant breeders and chemical ecologiste aiming at producing plant materials possessing appreciable levels of resistance to stemborers and able to encourage natural enemy activity in cereal-based ecosystems.

Studies were conducted from 1993–1995 at Samanko, Mali, on rearing and pheromone trapping of Busseola fusca Fuller (Lepidoptera: Noctuidae), and on the insecticidal effect of extracts of Jatropha curcas (Euphorbiaceae) on B. fusca and Sesamia calamistis Hampson (Lepidoptera: Noctuidae). A technique for mass rearingof B.fusca larvae on a chickpea and sorghum leaf-based meridic diet was developed, requiring about 10 g of diet per larva. An average of 7.3 successive generations were obtained per year. Pupal and larval mortalities were less than 15%. Oviposition was obtained on artificial substrates, with a fecundity of 300–500 eggs per female. Laboratory tests were conducted with diets in which extracts of nuts of J. curcas had been added. A 100% larvicidal effect was obtained with B. fusca and S. calamistis at 0.01% and 1% crude oil concentration respectively, and with 0.025% of the phorbol ester fraction on S. cahmistis. Trapping experiments using traps made of aluminum trays baited with synthetic pheromone showed that B. fusca male moths flights could be satisfactorily monitored. An improvement of traps and/or pheromone effectiveness is, however, necessary before their use as a control method.

The effect on the larval development of Helicoverpa armigera of caffeoylquinic acids and their structural units, caffeic acid and quinic acid, was evaluated. 5-Caffeoylquinic acid (5CQA) (syn. chlorogenic acid) significantly retarded larval development and increased the number of days to pupation at a concentration of 9 mM in an artificial diet, although final pupal weights were not severely affected. Caffeic acid and quinic acid also inhibited larval development although their effect was less marked. In all cases the effect of the compounds was dose-dependent. A mixture containing 5CQA, 3-caffeoylquinic acid (3CQA) and the novel compound, 1-caffeoyl-4-deosyquinic acid (ICdQA), which were extracted from the wild groundnut species, Arachis paraguariensis (Chod et Hassl.) was also evaluated at a concentration equivalent to that found in the plant (9 mM). This mixture was a potent inhibitor of larval development and more potent than 5CQA alone at the same concentration. The potential role of these substances in host plant resistance of groundnuts and their interaction with other components of pest management strategies are discussed.

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.

Phagostimulant activities of sucrose, sitosterol, stigmasterol, campesterol, and soybean leaf extractables were evaluated towards the third-instar larvae of the cabbage looper, Trichoplusia ni Hubner (Lepidoptera: Noctuidae) on elderberry-pith discs under laboratory conditions. There was a linear relationship between sucrose dosage and larval feeding on the pith discs. Water fraction from PI 227687 soybean leaf extractables at dosages > 400 μg/disc inhibited feeding by T. ni larvae. Ethyl acetate fraction showed phagostimulant properties comparable to sucrose. Sterols (5 μg/disc) and soybean leaf extractables (40 μg/disc) in combination with sucrose (400 μg/disc) showed synergistic effect as phagostimulants. Implications of genotypic differences in sterol content in relation to host plant resistance are discussed.

Field experiments at the Msekera Regional Research Station during 1988–1990 and at Masumba (Luangwa Valley) Sub-station during 1989–1990 showed that six groundnut genotypes—ICG.485, ICG.2271, ICG-2306, ICG.5041, ICG.5045 and HyQ (cg) S-10 were distinctly less susceptible to the leafhopper, Empoasca dolichi (Paoli). ICG.2271 also appeared to combine low susceptibility to early leaf spot, Cercospora arachidicola Hori and good yield potential. This genotype as well as ICG.5041 and ICG.5045 also are known to be resistant to thrips and/or termites. The scope for their utilization in exploitation of leafhopper resistance in groundnut improvement programme in Zambia is indicated.