Scanning electron microscopic examination of the egg shell and associated structures of Eocanthecona furcellata (Wolff.) (Hemiptera: Pentatomidae) reveals a distinct pattern of structural elements, viz. microsculpture of the chorion, pseudoperculum, egg burster and aeromicropylar processes. The egg shell has a highly decorated chorion with a structural difference at both the anterior and posterior ends. A reticulate type of microsculpture with varying sizes of spine-like projections measures 4.02 ± 0.06 to 8.12 ± 0.05 μm and forms an asteroid-like shape at the anterior pole, while the surface at the posterior end has elongated striations over the chorion measuring 72.00 ± 0.074 μm. The pseudoperculum of the egg shell is a circular lid measuring 518 ± 11.43 μm in diameter. During hatching the line of rupture follows the micropylar ring, and this sometimes extends beyond the ring of micropylar processes. An inverted T-shaped black egg burster measures 202 ± 4.63 μm in length, is well developed and can be seen prior to hatching within a thin and transparent egg shell. It is a median sclerotised part of embryonic cuticle. The aeromicropylar processes are situated at the anterior pole of the barrel-shaped eggs. There are 26–28 aeromicropylar processes arranged in a circle at almost equidistance and each micropylar process is slightly curved opposite to the pseudoperculum lid with a bulbous end at the apex. The aeromicropylar processes are 25.40 ± 0.083 μm in length and 3.02 ± 0.07 μm in width. Each micropylar process has a central canal for the passage of sperm whereas the rest of its body is porous, and allows respiratory interchange.

The diversity of visitors to flowers of the grass Brachiaria ruziziensis R. Germ and Evrard is very low in Ngaoundéré, Cameroon. Bees (Hymenoptera) and syrphids (Diptera) were observed gathering pollen, and each insect group had its own strategy depending on the prevailing environmental conditions. Syrphids are early-morning flower visitors and take time to feed on wet pollen. They visit flowers from 0630 h when the relative humidity is higher than 80% and temperature lower than 25°C. Bees gather dry pollen from 0930 h when the relative humidity is <50% and temperature>30°C, and carry it to the nests for feeding.

Ant–acacia mutualisms are conspicuous biotic associations in Savannah and neotropical ecosystems; however, the effects of tree size and forest structure on ant behaviour and tree traits are rarely examined. We tested two hypotheses related to these effects: (1) ant responses to disturbance are influenced by tree size and forest basal area; and (2) tree traits important to ants are predictable by tree size and forest basal area. We investigated these hypotheses in a dry tropical forest (Ometepe Island, Nicaragua) with the myrmecophytic Collins acacia (Vachellia collinsii Saff.) and the ant Pseudomyrmex spinicola (Emery 1890). We measured trees from three size classes and three basal area classes and quantified resources that are important for ants, including food resources (nectaries and Beltian bodies) and domiciles (thorns), as well as a measure of potential tree reproductive fitness (seedpods). We also evaluated ant responses to experimental disturbances. Three important findings emerged: (1) on average, 1140–1173% more ants responded to experimental disturbances of large trees than small- or intermediate-sized trees, respectively; (2) forest basal area did not affect ant responses to disturbance; and (3) neither tree size nor forest basal area was correlated with branch-level mean numbers of nectaries, food bodies or thorns. Our studies support the hypothesis that tree size is an important factor regarding ant behavioural responses to disturbance, but not forest basal area. Our work suggests that future studies of ant behaviour on myrmecophytes should consider tree size.

The incidence and distribution of the sorghum head bug, Eurystylus oldi Poppius (Heteroptera: Miridae), and other panicle pests of sorghum in research stations and farmers' fields in West and Central Africa (WCA) were assessed from 1985 to 1994. Maximum head bug abundance was observed during the dough stage. Head bug abundance was greater on improved cultivars, while the local guineense sorghums with long glumes were less susceptible, both at the research stations and the farmers' fields. Grain damage was moderate to severe, although farmers were mostly unaware of this insect and its damage potential, because it remains hidden inside the panicle. Sorghum midge (Stenodiplosis sorghicola Coq.) damage was very high on farmer's fields in Burkina Faso, Niger, and Nigeria. The shootfly Atherigona soccata Rond., stemborer Busseola fusca Fuller, grasshoppers, spittle bug Locris rubens Erichson, Campylomma spp., Creontiades pallidus Ramb., Agonoscelis spp., head caterpillars Helicoverpa armigera Hub., Pyroderces simplex Wlsm. and Eublemma gayneri Roths., were other insect pests damaging sorghum in the areas surveyed. It is proposed that future efforts in sorghum improvement for WCA should focus on developing cultivars with resistance to sorghum head bugs.

Between 1979 and 1982, H. hampei spread itself from a small area in the lowlands of central Jamaica, to the entire coffee growing region (alt. 15–1603 m) of the island. The levels of infestations in the 390 farms surveyed ranged from less than 5–100%; in plains it was about 45% but only about 3.4% at altitudes of over 1000 m. On a parish basis the infestation ranged from 2.8 to 19.17% (mean 9.45%) in June 1982, before the spraying with thiodan EC35 began.

In November 1982, 8 weeks after the last spray of the three-spray cycle, the infestation levels ranged between 1 and 20.21% (mean 9.56%). In fact, it had increased from about 6.7% in 1979 to 9.56% in 1982, despite the multi-million dollar spraying programme over 4 years. The shortcomings of the chemical control strategy are discussed and an integrated approach involving chemical and cultural practices is advocated.

No insect parasite or predator of H. hampei were found in Jamaica; the fungus Beauvaria bassiana was however, found to have inflicted 6–40% mortality in the adult borer.

In West Africa, Callosobruchus maculatus Fabricius mainly develops on cowpea (Vigna unguiculata (L.) Walpers) and sometimes on Bambara groundnut (Vigna subterranea (L.) Verdcourt). A survey on the development of a C. maculatus strain selected on cowpea was undertaken during eight generations on two local varieties of Bambara to determine their level of acceptability and suitability for the pest and its evolution over time and successive generations. When C. maculatus females were in contact with seeds from both Bambara varieties for the first time, they laid fewer eggs on these hosts in comparison with cowpea. However, females that have developed inside Bambara cv. Local Beige seeds deposited more eggs on their hosts of origin from the first generation onwards. The same result was obtained on Bambara cv. Local Striped seeds from the F4 generation onwards. Egg viability was not affected by host variability. On the contrary, the larval survival was significantly reduced on Bambara seeds. Larval survival and intrinsic rates of natural increase were improved on both Bambara varieties with successive generations in comparison to cowpea. Our results demonstrate that in a non-choice situation, C. maculatus is able to develop on both Bambara cv. and even increase its reproduction potential over successive generations, regardless of the climatic variations. The results help to better understand why C. maculatus is becoming a serious concern on Bambara in West Africa where the pest is frequently in contact with this occasional host.

Lufenuron, N–[2, 5–dichloro–4–(1, 1, 2, 3, 3, 3– hexafluoropropoxy) –phenylaminocarbonyl]–2, 6–difluorobenzamide, a novel acylurea insect growth regulator, exerted significant sublethal effects in larvae, pupae and adults of the diamondback moth, Plutella xylostella (Linnaeus) (Lepidoptera: Plutellidae). Second, third and fourth instars were treated at their respective LCM using a standard 'leaf–disc dip' method of bioassay. Sizes and weights of pupae from the treated lots were significantly reduced but larval and pupal durations remained unaffected. All the pupae formed from larvae treated as fourth instars were deformed and produced no adults. Only 38 and 14% of adults emerged from the pupae formed following treatment at the second and third instar, respectively, as compared to 88–92% adult emergence in the controls. Apart from significant reduction in body length, some adults from the treatments had fringed and twisted wings which prevented normal flight. IGR treatment also resulted in pronounced reduction in fecundity (up to 10.2 eggs per female in comparison with 135.8 per female in the control) and oviposition periods (up to 3.2 days in comparison to 7.8 days in the control). Field trials to investigate the management of P. xylostella using lufenuron are recommended.

Several species of Chilo are serious pests (stem borers) of sorghum, maize, rice and sugar-cane. Studies on genetics of resistance to Chilo species have been carried out only In a few cases. In maize and sorghum resistance to C. partellus, measured in terms of leaf-feeding, dead hearts and stem-tunnelling is polygenic. Both additive and non-additive gene effects are important in the inheritance of resistance to C. partellus. However, the importance of additive and non-additive gene effects in determining resistance, varies with the resistance parameter in two crops. Resistance to leaf-feeding and stem-tunnelling is governed predominantly by additive genes, while both additive and non-additive genes are important in the inheritance of dead hearts. The non-additive gene action, however, is of higher magnitude and more important in the inheritance of resistance to dead hearts in maize than that in sorghum. Recurrent selection using S1, S2 lines would be efficient in increasing levels of resistance both in sorghum and maize. Tolerance should be considered as an important factor in sorghum breeding for borer resistance.

The level of resistance in rice to C. suppressalis is low to moderate and appears to be under polygenic control. A recurrent selection programme using male sterile lines could be effective in improving the level of resistance. No information is available on genetics of resistance to Chilo spp. in sugar-cane.

Laboratory experiments were conducted to investigate the effect of diet on the biology and prédation capacity of the phytoseiid mite Amblyseius zaheri Yousef & El-Brolossy. The predatory mite was able to develop and reproduce when fed on nymphs of the twospotted spider mite Tetranychus urticae Koch, nymphs of the citrus brown mite Eutetranychus orientalis Klein, motile stages of the eriophyid mite Eriophyes dioscoridis Soliman & Abou-Awad, eggs of the scale insect Parlatorio ziziphus Lucas, nymphs of the whitefly Bemisia tabaci (Genn.) and pollen grains of date palm, Phoenix dactylifera. The predatory mite's development was fastest on a diet of nymphs of Er. dioscoridis. All of the diets led to a female-biased progeny, which was most pronounced in predators fed on date palm pollen. The results are discussed in light of the biological control capacity of the phytoseiid mite.

The first report on the entomopathogenic fungi in the tropics dates back to 1853. Despite their variety, little attention has been focused on their incidence on agricultural pests. Studies over the last 20 years in the tropics have revealed that entomogenous fungi (Deuteromycetes and Entomophthorales) may play an important role in natural control of arthropod populations. Therefore, the use of insect pathogenic fungi as pest control agents is promising.

An important consideration in developing an entomopathogenic fungus as a mycoinsecticide is the selection of the correct and well-adapted strain. New techniques in biotechnology will help in developing methodology for mass production and formulations. Other factors which can be considered, and which may be determinants on the potential of insect pathogenic fungi include the environmental conditions and cultural techniques (e.g. intercropping, planting density, irrigation). The cultural techniques can result in change of the environment favourable to the disease.

The Asian tiger mosquito, Aedes albopictus (Skuse) and the yellow fever mosquito, Aedes aegypti (Linnaeus) are medically important species that vector several arboviruses. Globally, populations of both species experience (and are sensitive to) photoperiodic variations. The present study aims to test if photoperiod regimes affect the population performance of Ae. albopictus and Ae. aegypti. Since mosquitoes have sex-specific strategies to maximize fitness, we also tested the hypothesis that Ae. albopictus and Ae. aegypti would exhibit differences in the male and female response (sexually dimorphic response) to various photoperiod treatments. We reared cohorts of first instar larvae to adulthood in three photoperiod treatments: short day (10 h light), control (12 h light) and long day (14 h light). We measured and compared survival to adulthood, population growth, development time of males and females, and wing length across treatments. Although we detected no effects of photoperiod on the population performance of both species, we found evidence of a sexual dimorphic response to photoperiod in Ae. albopictus, but not in Ae. aegypti, with Ae. albopictus females being more sensitive to variations in photoperiod. The observed differences between sexes of Ae. albopictus are consistent with sex-specific developmental constraints. The absence of a sexually dimorphic response to photoperiod in Ae. aegypti can be attributed to different strategies evolved in this species to prepare for unfavourable conditions associated with shorter day length. We discuss the ecological and medical implications of our findings.

Chilo partellus (Swinhoe) was first recorded in South Africa in 1958 and became a major pest of maize and grain sorghum. It has invaded the main maize producing areas in the Highveld Region, gradually competitively displacing the indigenous Busseola fusca (Fuller). Between 1980 and 1993 two egg, seven larval and two pupal parasitoids were introduced for biological control of Ch. partellus. Cotesia flavipes Cameron (Braconidae), Paratheresia claripalpis van der Wulp (Tachinidae), Tetrastichus howardi (Olliff) (Eulophidae) and Xanthopimpla stemmator Thunberg (Ichneumonidae) were released and recovered from the field. Recoveries were made up to a few weeks after releases but parasitoids failed to survive the winter and establish. The harsh winter conditions in the release sites, the long larval diapause period of Ch. partellus, lack of synchronisation between the development of the parasitoids and that of their host larvae, and absence of alternative host plants were identified as the main obstacles for the parasitoids establishment. It is concluded that the prospects of achieving any degree of biological control of stemborers in the Highveld Region with parasitoids from the tropics are poor. A parasitoid from a temperate zone of the USA, Macrocentrus grandii Goidanich (Braconidae), was introduced recently into South Africa and is currently reared on Ch. partellus in quarantine. Macrocentrus grandii stays dormant in its diapausing host, Ostrinia nubilalis (Hubner), during winter to emerge in spring. Because of its ability to diapause in winter it is hoped that after releases, M. grandii will establish on Ch. partellus in the Highveld Region of South Africa.

Larval establishment and feeding site selection of Eldana saccharina Walker were studied on maize (Katumani Composite B) and sorghum (Serena). These parameters were defined as the location within the plant of the larva 7 days after hatching; and feeding activity was measured by the extent of stalk tunnelling. In naturally infested maize, more larvae settled in the basal section of the plants between second and fifth node/internode, and in sorghum they settled between fourth and seventh node/internode. In the artificially infested plants of both crops, larvae settled in the middle section of the plant mainly between fourth and seventh node/internode. The neonate larvae first settled in the leaf sheath until they were third instars when they entered the stalks. However, in sorghum stalk entry was limited by stalk wall toughness and waxes on the stalks.

The extent of stalk tunnelling indicated that in maize larval feeding is highest in the internodes whereas in sorghum it is in the peduncles.

The effect of fluoride on the food and dietary water intake and utilisation were studied in final instars of silkworm, Bombyx mori L. race NB4D2 fed ad libitum with mulberry leaves treated with sodium fluoride at concentrations of 25 ppm, 50 ppm and 75 ppm. The larval duration increased by two days while food consumption, assimilation rate, assimilation efficiency, feeding and metabolic rates increased significantly in all the treated larvae. On the other hand, the amount of faeces produced, food conversion, conversion rate and conversion efficiencies decreased significantly in all the treated batches when compared to controls. The water intake and utilisation parameters increased significantly while rate of water loss through faeces, water retained in the body and retention efficiency decreased in all the fluoride-treated silkworms. The factors responsible for these changes in treated batches are discussed in relation to the effect of fluoride on cocoon and shell weights.

Differences in the susceptibility or resistance of different plants to an insect are reflected in the magnitude of its population established on them and the resulting damage. The population of an insect on a plant is determined by an interaction between its responses and the plant's characters. As explained previously, the following six main types of responses, operating in as many stages, are involved in the establishment of an insect on a plant: (1) orientation, determining the insect's arrival on or avoidance of a plant in response to its attractant or repellent stimuli; (2) feeding responses, determining the quantity of food ingested from the plant; (3) metabolic responses involving the utilisation of the ingested food and determining the insect's nutrition; (4) development of the insect, if in the larval stage, determined by the quantitative food-intake and nutrition; (5) egg-production in the adult stage, determined by the quantitative food-intake and nutrition; and (6) oviposition.

The first, second and last of the above mentioned responses are behavioural which determine the initial selection or rejection of a plant by an insect. In order, therefore, to understand the principles governing the susceptibility/resistance of plants to an insect species, it is necessary to compare the above mentioned responses to susceptible and resistant plants and, next, to examine the role of the plant characters in determining these responses.

On the basis of the existing literature on the subject, the role of different behavioural responses of insects to plants and their characters in determining their susceptibility or resistance to their insect pests has been considered in this paper.

Lipids were extracted from the plasma and gonads of fourth and fifth instars and adults of Dysdercus koenigii (Heteroptera: Pyrrhocoridae). Thin layer chromatography of the extracts revealed fractions of free fatty acids (FFA), monoglycerides (MG), diglycerides (DG), triglycerides (TG), phospholipids (PL), free cholesterol (FC) and esterified cholesterol (EC). There were distinct differences in the lipid profiles within sexes for both tissues during development. Phospholipid from female plasma showed significant fluctuation during development, while that from males remained constant. In female gonads, PL levels increased significantly from the fouth instar to the fifth instar. Monoglycerides were detected only in adult male plasma and adult female gonads, at very low levels. Plasma DG in males was found to increase significantly during development. Maximum gonadal DG was recorded in male fourth instars and female fifth instars. Triglycerides in female plasma declined from the fourth instar to the adult stage. In contrast, TG in gonads increased significantly in the fifth instar of both sexes. Plasma FFA of both sexes fluctuated during development. Free cholesterol was detected only in adult male plasma. Plasma EC increased from the fourth instar to the adult stage in males, and it was highest in gonads of fifth instars of both sexes.

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 effects of established phenomena of plant resistance on the levels of tolerance to malathion of an organophosphate resistant strain of Myzus persicae were investigated using topical application insecticide bioassay. Tolerance was significantly decreased when aphids were reared on the moderately aphid-resistant Brussels sprout variety Early Half Tall or on sprout plants given reduced nitrogen fertilization. Differences in insecticide tolerance were found to occur only at older plant ages. It is argued that the differences (less than 50% in LC50 terms) reported are of practical importance, and attention is drawn to much greater differences (5-fold) obtained in experiments at different times of the year.

In the major breeding areas of tropical migratory locust, Locusta migratoria migratorioides (R. & F.) in Mali, Chad, Sudan and Madagascar, man has played a vital role in maintaining locust numbers above the levels found in undisturbed environments, through his agricultural and pastoral activities. Recent rapid agricultural development, particularly broad-acre cultivation of mono-cotyledonous summer crops in areas climatically suitable for migratory locust, has caused population upsurges in new areas, which have occasionally led to outbreaks and plagues. Some outbreaks appear to have been a reaction to the initial disturbance and have never recurred while others recur when meteorological conditions are favourable. Continued agricultural development in tropical and sub-tropical areas of the Old World will bring the risk of further upsurges of tropical migratory locust.