Field trials were carried out for two seasons on the effect of insecticides on insect pests, damage and yield of three cowpea cultivars.

During the first season, the preflowering stage of crop growth was found to be infested by Aphis spp, Empoasca spp, Ootheca spp and Epilachna spp, the flowering stage by Megalurothrips sjostedti, Heliothis armigera, Astylus atromaculatus, Coryna kersteni and Mylabris amplectens and the postflowering stage by H. armigera and a number of coreid and pentatomid bugs. Fenthion gave the least protection against pod damage by H. armigera. There were significant increases in the number of harvestable pods per plant with insecticides. There were no significant differences in hull weight and shelling percentage. Grain yield was increased by 96.2 and 109.4% after treatment with dichlorvos and cypermethrin, respectively.

In the second season, cypermethrin and monocrotophos were most effective. Cypermethrin reduced the number of pod-sucking bugs by 58%, H. armigera by 93%, while monocrotophos reduced Empoasca spp by 39% and M. sjostedti by 79%. Cypermethrin reduced damage by pod-sucking bugs by 28%, and by H. armigera by 95%. Monocrotophos reduced damage by pod-sucking bugs by 14%, and by H. armigera by 80%. Significant increases in the number of harvestable pods per plant were obtained with monocrotophos and cypermethrin treatments. There were no significant differences between the number of seeds per pod, and the weight of 1000 seed-grains. Grain yield was increased by 55.4% after both cypermethrin and monocrotophos treatment. The application of these insecticides were financially beneficial but the gain was reduced when six sprays were applied. Cypermethrin and monocrotophos are promising insecticides for controlling insect pests on cowpeas.

The sugar-cane borer (SCB), Diatraea saccharalis (F.), and the ratoon stunting disease (RSD) bacterium independently caused significant reductions in stalk weight, cane tonnage and sugar yield. A three-fold difference was found in the comparison of per cent loss of cane tonnage due to RSD alone with that due to the disease in combination with SCB damage in first ratoon sugar-cane. Variance analysis failed to detect interaction between damage by SCB and that by RSD. The data analysed were obtained from an experiment of six replication which evaluated the effects of SCB damage and RSD on sugar-cane yield in Louisiana. We used the randomized complete-block experimental design with split-plot arrangement of treatments. Three schedules of insecticide application to the growing cane, against SCB were the main plots whilst the subplots comprised of RSD inoculation vs. no inoculation of sugar-cane plants grown from seed canes with light vs. severe SCB damage.

Continuous trapping of aphids for 13 months in 1985–1986 in a citrus orchard at Kikambala, Kenya coast at an altitude of 15 m indicated that Toxoptera citricida was present in citrus throughout the year. There was a marked seasonal periodicity of flight of alate aphids as shown by two distinct peaks in population density constituting about 72% of the total catch of aphids for the 13 month period. Effect of climatic factors on population levels of aphids is illustrated. Rainfall was found to have an indirect effect on aphid population increase. The negative binomial distribution fitted very well the spatial distribution of the aphids in the field and provided an explanation of the patchy spread of tristeza disease in the citrus orchard at Kikambala. The possibility of reducing the rate of spread of citrus tristeza virus in the field is discussed.

Three neem products—an emulsifiable concentrate of neem oil (NOEC), neem kernel water extract (NKWE) and neem leaf water extract (NLWE)—were applied on field cassava for protection against damage by the grasshopper, Zonocerus variegatus (L.). Three conventional insecticides viz. lindane 0.3%, cypermethrin 0.006% and monocrotophos 0.24% were also compared. All the neem products protected cassava for varying periods. Efficacies of NOEC and NKWE were superior to insecticides. From the results, neem products recommended to check the grasshopper damage were as follows: NOEC 0.5–2% to be applied every 8 days, NOEC 3–4% to be applied every 10 days, NKWE 7–10% to be applied every 12 days, NLWE 50% to be applied every 6 days.

Observations on the growth and development of Chilo partellus (Swinhoe) larvae under laboratory and field conditions indicate sexual dimorphism as well as a general developmental polymorphism. Larvae of both sexes went through fifth, sixth or seventh instars before pupation. The mean developmental period, larval head capsule width, and the number of instars before pupation as well as pupal weight were higher in the female compared to the male larvae. These observations suggest differences in the physiological state of fifth-instar larvae in a population; some of them might be in the penultimate, while others might be in the ultimate instar.

Third instar larvae of Spodoptera litura (Fab.) treated with polyhedrosis virus at dosages above 3.2 × 107 polyhedra inclusion bodies (PIB)/ml produced over 90% mortality. The LC50 was 2.3 × 104 PIB/ml. The estimated median lethal time (LT50) was 7.4 and 6 days at the lowest and highest dosages, respectively.

Polyhedra were observed in the epidermal cells, tracheal matrix, muscles and all nuclei of tissues of the virus treated larva 48–72 hr after treatment.

Persistence of four insecticides (prothiofos, Bacillus thuringiensis Berliner, fenveralate and methamidophos) were evaluated against the larvae of Plutella xylostella (L.), under field conditions. The effective biological action of the chemicals against the larvae varied with different instars. The insecticides were biologically effective for 6 to 8 days after treatment against first and second instars, and their index of persistent toxicity was higher than that for third and fourth instars. They were effective against third and fourth instars only for one day after spraying. Prothiofos and B. thuringiensis were more toxic to all the instars than methamidophos or fenveralate.

Gravid Lucilia treated topically with ivermectin (0.05μg/g, which represented approx. 4% of the 48 hr LD50) laid fewer viable egg batches (less than 50% of controls) and more sterile ones, when compared to untreated (P = 0.019) and acetone-treated flies (P = 0.028). This effect was also seen when mature third instar larvae were dosed with 7.6 μ g ivermectin/g (P = 0.0075 and 0.016 respectively); number of viable egg batches produced were only 25% of the number laid by the progeny of untreated larvae.

Dosage with fenthion stimulated an increase in the number of viable egg batches laid when incorporated in the original larval feeding medium at 0.2 μg/g liver; fewer viable egg batches were laid by the flies which developed from mature larvae treated with 7.6 μ g fenthion/g. Acetone solvent alone had a mild stimulatory effect on topically dosed flies.

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.

Non-parametric statistics were used to describe the spatial distribution of eggs laid by Zabrotes subfasciatus Boh. (Coleoptera, Bruchidae) inside pods of wild Phaseolus lunatus and cultivated P. vulgaris. Eggs tended to be uniformly distributed among seeds inside the pods of P. lunatus whereas egg distribution patterns were random or aggregative inside P. vulgaris pods. These differences were explained in terms of the internal pod morphology of the two host plants. It was concluded that this bruchid normally oviposits uniformly among accessible seeds inside the pods of its host plants in the wild.

Juvenile hormone analogues (JHAs), JHA I (6,7′-epoxy-3′,7′-dimethyl-dec-2′-enyl 6-ethyl-3-pyridyl ether), JHA II (3′,7′-dimethyl-undec-2,6′-dinyl 6-ethyl-3-pyridyl ether), MV-678 (2-methoxy-9-[4-isopropyl phenyl]-2, 6-dimethylnonane), and R-20458 (4-ethylphenyl-6, 7′-epoxy-geranyl ether) in acetone solution were applied to insect diets at 10ppm (v/v), and their effects on four species of stored products beetles observed. The larvae of the flour beetles, Tribolium confusum Jacquelin duVal, and Tribolium castaneum (Herbst), reared in media treated with JHAs I and II failed to pupate. At the end of the 6-week observation period, these larvae had not yet pupated. MV-678 and R-20458 produced ecdysial failures, lethal morphological abnormalities and larval/pupal and pupal/adult intermediates. Only a few apparently normal adult flour beetles emerged from these treatments. The lesser borer grain beetle, Rhizopertha dominica (Fabricus), completely failed to reproduce in a medium treated with JHA I. JHA II, MV-678 and R-20458 affected the reproduction of the two grain beetles, R. dominica and Sitophilus oryzae (L.), to varying extent, with MV-678 being the least potent JHA.

Coffee canopy damage studies with the yellow-headed borer, Dirphya nigricornis Olivier were conducted on mature plants. Single borer infestation of the pest in coffee plantations were established. Age-induced destruction of the coffee canopy was evidenced by the withering of branches, stems and nodes and were correlated to larval developmental days 0–260. Larval frass excretion was immense being 16,214.91 and 12,076.90 g for female and male beetles, respectively and represented direct physical damage by the pest to plants. Analysis of accumulated data of physical measurement on coffee yield components in the three categories of canopy partitions relative to height of the stem above the ground showed that damage to plants by the pest adversely affected them. Generalized non-linear equations relating berries, pinheads, flowers and leaves (P = 0.05 to P = 0.001) to heights of canopy partition above ground level are described. The physical model developed approximates the responses of the canopy to infestation by the beetle, and shows that these were either depressed or enhanced depending on whether the yield components measured were in the bottom, mid or top partitions of the canopy. Threshold damage constants for leaves (0.34), berries (0.41), pinheads (1.54) and flowerbuds (1.07) which caused variability were established.

Ultrastructural studies were carried out on biochemically confirmed Leishmania aethiopica in the oesophagus of naturally-infected, wild-caught sandflies, Phlebotomus pedifer. The parasites with an infection rate of 2.8% were either free in thelumen or attached to the cuticular lining by hemidesmosomes. Previously undescribed pseudopodia-like projections which seemed to enhance the anchorage of L. aethiopica in the oesophagus were observed.

Laboratory studies with Megalotomus parvus (Hemiptera: Alydidae), feeding on seeds or pods of legumes, and on different parts of the soybean plant, Glycine max, demonstrated that nymphal survivorship, developmental time and body weight at adult emergence, and adult reproductive parameters, survivorship, longevity and body weight changes, exhibited differences depending on the food source utilized. In general, mature soybean seeds (R8 stage) were the most suitable food. The greatest nymph and adult survivorship and the greatest adult longevity were obtained from this food source. Body weight at certain ages and fecundity of females was also greater on mature soybean seeds than on the other foods. Immature soybean seeds for nymphs, and soybean pods for adults, were the second most suitable food, followed by green beans, Phaseolus vulgaris and immature lupin, Lupinus luteus seeds (for nymphs) and lupin pods (for adults); nymph and adult performance was drastically reduced on lupin seeds'pods and no nymphs survived to adult on soybean stems or leaves. On all foods tested, duration of the nymphal stages differed significantly, with the first stage being the shortest, and the fifth the longest. Females achieved significantly greater body weight than males on all foods, except on lupin pods. Results of these laboratory studies and those of field observations suggest that mature soybean seeds are the most important food source in the seasonal phenology of M. parvus.

A field evaluation of nine chemicals was conducted at Cotton Research Station Kibos for their efficacy against bollworm and cotton stainer species attacking cotton in Kenya. Based on the yield and percentage stained seed cotton, Decis'Triazophos, pp 321 Karate and Baythroid and Decis were the most effective insecticides in 1985 and 1986 respectively. However, there was no consistency in results of performance of insecticides tested against cotton insects in the 2 years. The apparent poor performance of most insecticides tested against cotton insects was attributed to fixed insecticide spray schedule which resulted in superfluous and often mistimed chemical applications. It is therefore necessary to establish damaging threshold(s) of major cotton pests in this country so that chemicals can be efficiently used to control them with minimum adverse effects on our environment.

Eighteen cowpea varieties were evaluated for their resistance to cowpea weevil, Callosobruchus rhodesianus (Pic) infestation. The experiments were conducted at 27°C and 65–70% r.h. The varieties showed a differential response to infestation by C. rhodesianus with respect to oviposition, egg hatch, adult emergence, adult survival and development period. The intrinsic rates of increase and the indices of susceptibility of the varieties were estimated. The varieties were categorized into three groups of susceptibility (low, medium and high) to C. rhodesianus infestation based on the adult survival data. IT 82D-1064 and IT 81D-1032 were the least susceptible varieties.