The effects of egg clutch size on development and survivorship of the larger grain borer, Prostephanus truncatus (Horn) (Col.: Bostrichidae) on maize were measured in the laboratory using single grains of the white maize variety, TZSR-W, at 30 ± 2°C and 70 ± 5% RH. Varying numbers of P. truncatus eggs (1, 2, 4, 8, or 16) were introduced into a hole drilled into a single maize grain. Destructive sampling was used at regular intervals to obtain data on immature developmental parameters, the location of immatures within the grain and the weight and sex of any emerged adults. First instars fed mainly on the floury endosperm tissue whereas the second and third instars preferred the germ tissue. Mortality due to competition was highest in first instars. At high initial densities (> 4 per kernel), some P. truncatus larvae reduced competition by moving out of the grain. A maximum of 6 adults emerged from a single grain. The mean number of adults that emerged per grain for initial egg densities of 8 and 16 were 3.3 and 3.5, respectively. Prostephanus truncatus adult weight at emergence was not significantly influenced by initial egg density except in the case of clutch size 16. The sex ratio of emerged adults was also unaffected by competition, and was always 1:1. Complete developmental period within grains ranged between 28 and 32 days.

Larvae of Cirina forda (Westwood) (Lepidoptera: Saturniidae), a popular food insect in the southern parts of Nigeria were raised in the laboratory on cut foliage as well as on a growing Vittelaria paradoxa tree at the University of Ilorin, Nigeria. Comparative observations were made on the insect's biology and behaviour in both rearing systems in order to ascertain if the larvae could be reared in captivity on cut foliage. Egg eclosión began 24–48 hours after egg-laying and lasted 10 days. First instars initially aggregated after egg hatch, then migrated in single file to feed after approximately 12 hours after emergence. The first and second instars fed as an aggregate, while the third and subsequent instars fed separately. The first two instars did not exhibit any defensive response when disturbed, but the subsequent four instars emitted a green fluid which may act as a repellant to potential predators. High mortalities were suffered by the first and the penultimate instars. Results have shown that Cirina larvae could be easily reared artificially on V. paradoxa to serve as source of food for humans.

A laboratory study of the dietary preferences and duration of food plants in the gut of Zonocerus variegatus was carried out using three plant species, namely Manihot esculenta, Vernonia amygdalina and Chromolaena odorata. Whole or cut up plants were offered to the grasshopper in the experiments. Overall, whole V. amygdalina was the preferred diet of Z. variegatus, while the insect ate cassava pieces better than whole plant. Chromolaena odorata was the least preferred diet of the three. The duration of the food plant within the insects' gut was shortest for V. amygdalina, which was consumed all at once soon after being offered to the insects. The insects consumed cassava in two successive sessions and C. odorata in three. The total time spent eating was similar for V. amydalina and cassava, which were significantly shorter than that spent eating C. odorata. Evidently, V. amygdalina could serve as a good bait plant for Z. variegatus. However, its status as a human food plant precludes its use as a bait followed by chemical control; it may be used with, for example, fungal biocides such as Metarhizium.

The dispersal characteristics of a population of 124 marked adult individuals (62 males and 62 females) of the armoured ground cricket (AGC), Acantliophis speiseri Brancsik (Orthoptera: Tettigoniidae, Hetrodinae), released at the centre of a circular study site, were investigated using the capture-mark-release-recapture method for five successive days. The 110 m-diameter study site comprised five grass-trap annuii (each 1 m wide) spaced at 10-m intervals from the centre.

The frequency curve of numbers derived from recapture data for day 1 departed from normal (Kurtosis = 2.7), indicating that the released AGC population was heterogeneous in its dispersing behaviour. A comparison of mean numbers of recaptures from grass-traps per quarter portion of the study site revealed no drift in the dispersal of the population (P > 0.05), though differences in the variances of the recapture frequencies indicated a tendency towards non-random movement in the AGC population.

The population covered a mean distance of 24.47 m in the first 24 h following release and the rate of dispersal changed daily in the five-day study. The implications of the dispersal characteristics observed in this study to future design of control strategies for the pest in Zambia are discussed.

Investigations were conducted in six farm fields (1.5 to 3 ha each) in Burkina Faso with objectives to (1) study adult Smicronyx spp. (Coleoptera: Curculionidae) population dynamics during Striga hermonthica (Scrophulariaceae) seed-pod production, (2) assess the impact of different population levels of Smicronyx spp. on St. hermonthica capsule production and, (3) estimate Smicronyx female reproductive success in seed-pod galls. A Univac portable sampler was used to sample adult Smicronyx spp. and St. hermonthica densities were assessed using aim2 metal quadrat. Fifty St. hermonthica shoots per field were sampled each week. Before the emergence of St. hermonthica, Smicronyx adults were found on nearby Striga éspera (Willd) Benth. fields. Striga hermonthica shoots produced on average ca. 24 seed capsules each, while each female Smicronyx galled from 12 to 32 seed pods. Seed-pod galling alone was not sufficiently high to be considered as a successful reduction factor for St. hermonthica seed production in natural conditions. Smicronyx spp. adult population levels decreased during the St. hermonthica capsule production period. Increased population levels of Smicronyx spp. resulted in higher percentages of St. hermonthica seed-pod galls, suggesting that a ugmentative techniques, if used as part of an integrated control programme could be a worthwhile strategy.

The circadian activity of adults of Smicronyx guineanus Voss (Coleóptera: Curculionidae) was studied in semi-field conditions. Pairs of weevils were observed on a 24-h cycle starting from 0800 to the following day at 0600 hrs for 21 days in cages containing Striga hermonthica (Del.) Benth (Scrophulariaceae) plants parasitising sorghum. Each female and male was observed every second hour. Occurrences of the various behaviours exhibited by the weevil were recorded. In decreasing frequency, the weevils were observed to be standing still, feeding, mating, walking, moving and foraging. In addition, the difference in time spent on the different parts of the host plant—classified into four groups: A- stem, leaves and bud; B- corolla; C- calyx; and D- sorghum, petiole, soil, capsule and cage—was highly significant (P = 0.0001,10 couples/days, n = 21 days). The weevil adults spent 46.8% of their time on the Striga inflorescence (bud, corolla and calyx). Striga plants were also stratified with respect to distance from the base and categories were designated as upper, central and lower parts. Smicronyx adults spent 85.2% of their time in the upper stratum, 9.8% in the central stratum and 5% of their occurrences were in the lower third of the Striga shoot. They were more active during daytime, suggesting that the best period of day to sample the weevils in the field is from 0700 to 1100 or from 1600 to 1800 hrs.

Diet change in the cotton stainers, Dysdercus cardinalis and D.fasciatus, occasioned by their migration from Gossypium hirsutum (cotton) to alternate hosts Sterculia rhynchocarpa and Adansonia digitata, was overall beneficial to their bionomics. Only the host Abutilon mauritianum appeared to possess antibiotic effects against the fecundity, fertility and longevity of the stainers. The occurrence in tropical natural ecosystems of both suitable and unsuitable hosts appears to constitute a vital and a viable stainer population-regulatory mechanism. Overall, D. cardinalis exhibited superior reproductive parameters on a variety of diets than did D. fasciatus. The latter apparently suffered profound negative effects on its bionomics when feeding on certain hosts.

S2 line and S2 test-cross recurrent selection procedures were initiated to improve the resistance of a maize (Zea mays L.) population to the spotted stemborer, Chilo partellus (Swinhoe) (Lepidoptera: Pyralidae). Thirty-five selections developed from the population were evaluated for resistance to the stemborer in western Kenya. Resistance was rated according to leaf feeding (LF), stem tunnelling (ST) and deadheart formation (DH) following artificial infestation with first-instar C. partellus. Larval establishment and growth were also assessed. The reduction in foliar damage ratings, percentage stem length tunnelled, and percentage of plants showing deadheart after one cycle of recurrent selection were 4.38 %, 74.80 % and 13.92 %, respectively. A very low larval establishment on all the selections was attributed to their resistance to C. partellus attack. Results from this study indicated low levels of larval growth and development. It is concluded that antibiosis was the type of resistance mechanism against C. partellus in the maize populations investigated and that more cycles of selection are required to significantly shift the gene frequencies for resistance.

A method of formulating granules from an acetone extract fraction of the fruit of Melia volkensii, a Kenyan tree known to possess insecticidal properties, was developed. The granules were tested for toxicity to third and fourth instars of Aedes aegypti L. Some acute physiological effects were observed within 24 hours of adding the granules to water as larval food. A mortality of 95% was achieved on various developmental stages of the mosquito after 12 days, with only 5% adult emergence being recorded.

Laboratory studies were conducted to assess the toxicity and growth-inhibitory activity against Aedes aegypti larvae, of the crude acetone extracts of Polygonum senegalense (Meissn.) leaves, its flavonoid component 2',6'-dihydroxy-4'-methoxydihydrochalcone and the internal tissue flavanol quercetin. The first two showed significantly high insecticidal and growth-inhibitory activity even at low concentrations, whereas quercetin was toxic only at concentrations above 7μg/ml.

Out of 85 Bacillus thuringiensis strains isolated from Nigerian soils, 18 were larvicidal to Aedes aegypti, Culex quinquefasciatus and Anopheles gambiae. At 10−6 dilution of the final whole culture (FWC), the 11 most active strains, code-named GSC1, GSC3, GNA13, GNA14, OGL8, BUS4, BUS5, OBG1, OBG4, OBG8 and BAR3 produced mortality of 11.7–61.7% and 3.3–48.3% in Ae. aegypti and in C. quinquefasciatus fourth instars respectively. At 0.5 × 10−5 dilution of the FWC, the B.t. strains caused mortality in An. gambiae larvae ranging from 31.7 to 94.1%. All the 18 mosquitocidal isolates belonged to the subspecies israelensis, i.e. serotype 14. Microscopic examination revealed all the isolates to be typically Bacillus, with irregular or amorphous crystals of varying sizes.

The parasitoids Dinarmus basalis Rond. (Hymenoptera: Pteromalidae) and Eupelmus vuilleti Crwf. (Hymenoptera: Eupelmidae) develop mainly on relatively old larvae and pupae of Callosobruchus maculatus (F.) (Coleoptera: Bruchidae) found in cowpea seeds. Therefore, it is possible to consider biological control against C. maculatus using these natural enemies. The intrinsic rates of increase for C. maculatus and the two parasitoids were determined during the dry season in the sub-humid area of Bobo-Dioulasso, Burkina Faso, from January to July 1995. Functional response was also analysed for D. basalis and E. vuilleti. At the beginning of the dry season, the cowpea weevil and both its natural enemy species had limited intrinsic rates of increase, attributed to the relatively low temperatures and low relative humidity thatprevail during this period. In March, when temperature and relative humidity began to rise, conditions were optimum for their reproduction and development. Dinarmus basalis showed the higher intrinsic rates of increase and parasitism through the study period. Analysis of the functional response revealed that fecundity and potential for parasitism of both parasitoids were density-related and varied with climatic conditions. However, D. basalis showed a more efficient response. This study has shown that the variations of climatic conditions such as temperature and relative humidity during the dry season significantly influence the development and reproduction of C. maculatus and its parasitoids. Dinarmus basalis appeared to be the better biological agent in controlling C. maculatus.

A questionnaire survey of 100 smallscale cotton growers was conducted in two cotton-growing provinces of Mozambique to assess the constraints faced by the farmers in relation to the timing of insecticide application. The results showed that majority of the farmers started spraying too early (four weeks after germination) and were unaware that such sprays reduced the population of natural enemies of insect pests and might not lead to increased yields. The farmers applied an average of three sprays in the cotton growing season, about half of them according to pest observance and 41 % by following instructions of extension officers, but were constrained by the unavailability of insecticides and sprayers. Another constraint was lack of information on the timing of insecticide application. More than half of the farmers had suffered mild insecticide poisoning and needed more effective training on the judicious and safer use of insecticides, integrated with non-chemical pest management practices.

Screen house studies on the incidence and yield losses in three cowpea (Vigna unguiculata) varieties, TV × 3236, Ife Brown and Ife Bimpe due to the leaf beetle Ootheca mutabilis Sahl were carried out. Artificial infestation was done at two growth stages of cowpea: the 5-leaf stage (2 weeks after emergence) and 4 weeks after emergence. Variety TV×3236 was found to be the least susceptible, while var. Ife Bimpe was the most susceptible to leaf beetle damage. Young plants were more susceptible than older ones.

Flowering was delayed in plants infested with beetles and the extent of defoliation had a pronounced effect on the yield of cowpea.

The percentage grain yield loss ranged from 19.48% to 59.77% depending on level of beetle infestation, variety and age of the plant at infestation.

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.