Alfalfa (Medicago sativa L.) hosts several species of aphid, Acyrthosiphon pisum (Harris), Aphis craccivora Koch and Therioaphis trifolii (Monell). The preference of the aphids of alfalfa plants for dense assemblies or individual plants, as well as for healthy or infested plants, was investigated in the field as in the laboratory. Years of field research have revealed the specific preferences of all three species of aphid. A. pisum and T. trifolii are most commonly found in alfalfa crops, while A. craccivora is mostly found on alfalfa weeds. Also, a single species of aphid alone is usually present on a plant. In order to determine the reason for this clear preference and to establish whether at the very beginning, i.e. at the stage of choosing a host, aphid species distance themselves from each other, we tested the effect of the volatiles of healthy and infested plants on their attractiveness to aphids. A. craccivora is repelled by the volatiles of dense crops and plants previously infested with one of the other two species. A. pisum and T. trifolii choose a dense assembly of plants, repelled by the volatiles of plants previously infested with A. craccivora. A. pisum displays the weakest competitive traits, and A. craccivora the strongest. This research showed that competition between aphid species does not occur only when they find themselves on the same plant at the same time, fighting for resources, but also in the choice of plant, in order to avoid later competition.

In the present study, we aimed to assess the suitability of predatory parameters, namely voracity, consumption index, conversion efficiency and growth rate, as measures of prey suitability. For this purpose, fourth-instar larvae and 10-day-old adult females of four co-occurring ladybirds, namely Coccinella septempunctata (L.) (C7), C. transversalis F., Cheilomenes sexmaculata (F.) and Propylea dissecta (Mulsant), fed on two essential species of aphid prey, Aphis craccivora Koch and Lipaphis erysimi (Kaltenbach), were observed. The results revealed that, with the exception of C7, the other three ladybirds performed better on A. craccivora. Previous studies based on reproductive and developmental attributes have also shown that A. craccivora is a more suitable prey for these three ladybirds, indicating a strong correlation and affirming the viability of using these measures as a mark of prey suitability. In a similar manner, fourth instars and adult females of C7 performed better on L. erysimi over A. craccivora. This detectable difference in consumption and utilization of aphid prey species by the four ladybirds indicates that these predatory parameters can be used as tools for assessing prey suitability in ladybirds.

Adult pale morphs of an aphidophagous lady beetle, Propylea dissecta (Mulsant) (Coleoptera: Coccinellidae), were reared on a non-insect diet for periods ranging from 0 to 30 days after eclosion. The prey-deprived females, when provided with the cowpea aphid, Aphis craccivora Koch (Homoptera: Aphididae), laid fertile eggs, indicating that they had retained their reproductive potential. The oviposition period, post-oviposition period, fecundity, viability of eggs and longevity of females decreased significantly, whereas the pre-oviposition period and developmental period of progeny increased significantly with an increase in the duration of prey deprivation. There was no significant effect on the mean reproductive rate (16.14 ± 0.53 to 18.03 ± 0.67 eggs per day) at various prey deprivation levels, although the reproductive output and oviposition duration decreased. These findings indicate that lady beetle females can withstand the stress of prey non-availability, by switching to alternative food sources and reproducing when essential prey becomes available again.

Comparative field studies were conducted during the rainy and dry seasons at the Mbita Point Field Station of ICIPE in southwestern Kenya, to investigate the pattern and rate of dispersal of Aphis craccivora on aphid-resistant (ICV-12) and aphid-susceptible (ICV-1) cowpea cultivars in relation to key weather factors. The effects of the dispersal trends on crop performance and aphid population dynamics were analysed. Treatments consisted of initial aphid releases at the north, south, west, east and centre of test plots and uninfested controls maintained on plants for 22 days. Parameters recorded included: pattern (direction of spread of aphids) and rate (number of aphid-infested plants at a given time) in test plots; crop growth and yields and associated factors such as incidence of sooty mould and plant mortality, and aphid density and associated factors, including the incidence of natural enemy species, particularly coccinellids. Dispersal was fastest when releases were made in the west, north and centre of plots, and resulted in adveVse effects on ICV-1 growth and yields. Infestations of ICV-12 did not significantly affect crop performance. There was an apparent direct, positive relationship between wind direction and pattern of spread of aphid infestations, but this pattern was more apparent during the rainy season, when wind speeds were higher, than during the dry season. The incidence of natural enemy species correlated with the spread of aphid infestations, while the abundance of coccinellids correlated with aphid density.

Field studies were conducted to characterise the effects of infestations by adult and nymphal stages of cowpea aphid, Aphis craccivora Koch, on the growth and yield of cowpea, Vigna unguiculata (L.) Walp. Seedling, flowering, and podding stage plants of aphid-resistant (cv. ICV-12) and aphid-susceptible (cv. ICV-1) cowpea cultivars were used in the studies. Four treatments (consisting of infestations with adult and nymphal aphids, caged controls and uncaged controls) were administered on plants for 22 days post-treatment. Eight parameters of crop success were measured: extended leaf heights (ELH); plant mortality; incidence of sooty mould; incidence and abundance of natural enemy species; crop growth parameters (net assimilation rate, [NAR] in g/ dm2/day, and crop growth rate [CGR] in g/dm2 land surface/day); and plant yields (seeds per pod, weight per seed). Data were analysed using analysis of variance (ANOVA), orthogonal contrasts and 95% confidence intervals (C.I.). There were no significant (P > 0.05) differences between adult and nymphal infestations or between caged and uncaged controls, so the respective sets of data were combined for comparisons of aphid infestations with control treatments. Infestations caused severe plant stunting and other growth deformities, drastic yield reductions, higher plant mortality, greater incidence of natural enemies and abundance of Cheilomenes spp. on cv. ICV-1 than on cv. ICV-12, and on infested and uninfested plants. Aphid infestations did not significantly affect the incidence of sooty mould on plants of cv. ICV-12 or cv. ICV-1.

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.

Adult (2–20 hr old) viviparous Aphis craccivora were treated with 0.5 to 4.0 μg of precocene II. Treatment led to dose-related reduction in the progeny number, survival and life span of the aphid, but wing development was not affected. With 2.0 μg/aphid or more of precocene II, all surviving offsprings shared precocious metamorphosis, but even with 0.5 μg/aphid, about 20% progeny showed such metamorphosis with 1.0 and 1.5 μg/aphid. Early nymphs tend to undergo precocious metamorphosis earlier, in the second moult.

Trials for cold storage of the mummies of Aphis craccivora Koch parasitized by Trioxys indicus Subba Rao & Sharma were made. Newly formed mummies having fourth instar larvae of the parasitoid were put either at 20–25°C or at 12–15°C, 10 hr photoperiod for 24–72 hr before storage inside the refrigerators (4–8°C) for 10–40 days. A higher proportion of adult emergence was usually associated with the group of mummies passed at 12–15°C before storage and practically good emergence was recorded even if the mummies were stored for a month at 4°C if kept at 12–15°C for 72 hr prior to storage.

The density responsiveness of larvae and adults of Cheilomenes lunata (Fabricius) preying on Aphis craccivora Koch was investigated under laboratory conditions where temperature and relative humidity were uncontrolled and fluctuated between 24–30°C and 43–90% respectively. The feeding rates of C. lunata were positively correlated with the population density of prey. The number of prey consumed daily by each predator stage tested, increased more steeply at lower, than at higher prey densities, exhibiting thus the Type II functional response. Fecundity of C. lunata females also increased to a plateau with increasing prey density.

Field screening of Lablab niger germplasm for 3 years revealed that two cultivars, PLS-24 and PLS-16-1 consistently displayed resistance to pod borer, Adisura atkinsoni Moore larval boring and egg laying. Observations at different intervals of plant growth permitted a comprehensive evaluation of cultivar response to the pod borer complex.

Preliminary tests indicated pod colour, pubescence and fragrance to be associated with the borer resistance in L. niger. However, none of the characters per se imparted resistance or susceptibility, partly or fully. Cumulative effect of the aforesaid characters appeared to impart borer resistance to Adisura. Pod wall did not form a physical barrier to Adisura larval entry. Positive relationship existed with amount of chloroplast and susceptibility, and starch grains and resistance in four cultivars studied for anatomical details. Anthocyanin pigment in the pod wall was associated with resistance in L. niger to Adisura.

Based on field and laboratory evaluations of Lablab niger cultivars to the bean aphid, Aphis craccivora damage, the “local” and H-2, rated as highly susceptible to borers, reacted as resistant and PLS-24 and PLS-16-1 rated as resistant to borers reacted as highly susceptible to the aphid attack. It is therefore imperative that promising Lablab niger cultivars be evaluated for resistance to both the sucking and chewing insect pests.

Under artificial infestation conditions the resistant cultivars exhibited susceptibility to larval boring by Adisura, but confirmed resistance to ovipositional response. Non-preference for oviposition was the most important factor in imparting resistance to the borer damage in L. niger. Factors inducing host preference for larva and for adult were different and independent. PLS-24 and PLS-16-1 contained significant degrees of antibiosis as demonstrated by reduced larval survival, larval and pupal weights, prolonged larval duration and altered sex ratio.

Cultivars that sustained low levels of seed yield loss at high infestation levels were identified as tolerant. MS-9579 and IC-661-1 exhibited low degree of tolerance. Resistant cultivars proved non-tolerant even to slight infestations (ca 10%) received under field conditions.

Cowpea, until recently the major food legume in Ghana (in terms of area under cultivation) has been witnessing a decline in production at the expense of groundnuts. Among the factors leading to the decline are the havocs caused by insect pests and diseases with the former being of major importance.

The paper presents some of the systematic screening and testing procedures of both exotic and indigenous materials with the aim of evolving varieties that are not only high yielding but also resistant to a broad spectrum of these prevalent insect pests.