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The effects of mixed cropping systems containing maize, sorghum, millet and beans on infestations of cereals by lepidopteran stemborers and on associated parasitoids, as well as on yields and land equivalent ratios (LER) were assessed during four consecutive rainy seasons at two sites in the semi-arid eastern region of Kenya. Systems containing the non-host bean were more efficient in reducing pest densities than those with millet or sorghum only. Higher parasitism in diversified systems compared to monocrops was due to density-dependent effects rather than superior suitability of such systems to parasitoids. The maize-bean system, which had the highest proportion of bean plants, had LERs >1.65 while most other systems had LERs <1. It is concluded that mixed cropping with several cereal species has little advantages in terms of yield loss abatement due to stemborers and land use efficiency. However, including the drought-tolerant crops such as sorghum and millet in the system stabilizes food security in drought-prone areas such as eastern Kenya.

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Orius sauteri (Poppius) (Heteroptera: Anthocoridae) is a polyphagous predator used as a biological control agent of palm thrips, Thrips palmi (Karny) (Thysanoptera: Thripidae). We studied O. sauteri' s searching efficiency, time allocation on a leaf, leaving tendency, and attacking of prey. Approximately 78% of the encountered prey was eaten. Searching for prey was concentrated for 86% of the time on the lower leaf side, where palm thrips are usually found. Patch residence times on empty leaves were different from those on leaves with T. palmi larvae. Walking activity was not affected by the thrips density, and walking took place during 64% of the total search time. The leaving tendency of O. sauteri was affected by the time from patch entry and the presence or absence of palm thrips, but not by the thrips density. If prey were present, the leaving tendency decreased (multiplication factor 0.327), resulting in longer giving-up times than when no prey was present. The fact that the leaving tendency increases when patch exploitation lasts longer suggests that not much time is wasted on patches where encounters with prey are scarce.

In this study we examined the relationship between clutch size and parasitoid development of Muscidifurax raptorellus (Hymenoptera: Pteromalidae), a gregarious idiobiont attacking pupae of the housefly, Musca domestica (Diptera: Muscidae). Host quality was controlled in the experiments by presenting female parasitoids with hosts of similar size and age. This is the first study to monitor the development of a gregarious idiobiont parasitoid throughout the course of parasitism. Most female wasps laid clutches of one to four eggs per host, although some hosts contained eight or more parasitoid larvae. In both sexes, parasitoids completed development more rapidly, but emerging adult wasp size decreased as parasitoid load increased. Furthermore, the size variability of eclosing parasitoid siblings of the same sex increased with clutch size. Irrespective of clutch size, parasitoids began feeding and growing rapidly soon after eclosion from the egg and this continued until pupation. However, parasitoids in hosts containing five or more parasitoid larvae pupated one day earlier than hosts containing one to four larvae. The results are discussed in relation to adaptive patterns of host utilization by gregarious idiobiont and koinobiont parasitoids.

Laboratory and field trials were conducted to evaluate the effect of plant species (maize, sorghum), plant age (young, middle, old) and four different nitrogen fertilization levels (N0-N3) on the bionomics of the invasive crambid Chilo partellus and the performance of its braconid larval parasitoid Cotesia flavipes. Plant N varied significantly between N0 and N1-N3, but the differences among the latter were not significant. Intrinsic rates of increase and net-reproductive rates of C. partellus followed the same trends: they were lowest with N0 and similar among the other treatments. On maize only, mortality of C. partellus and parasitism by C. flavipes tended to decrease with age of the plant while the percentage of borers reaching adulthood (i.e. pupation) increased. Borer mortality and parasitism was lower and pupation higher on sorghum than on maize. On both host plants, percent dry matter content of frass, which could affect ingress of the parasitoid into the borer tunnel, did not vary with nitrogen level but varied with age of the host plants: on maize, it was highest on young plants and on sorghum on old plants. Tunnels were shorter on young maize and sorghum plants; longer tunnels on older plants indicated compensatory feeding by the larva as a result of lower nutritive value of the food source. Consequently, larval weight was lower on older than younger plants. The level of nitrogen fertilization had no effect on food conversion efficiency of C. partellus. Nitrogen did not affect number of C. flavipes progeny while egg load of progeny increased significantly with nitrogen level, on both plant species. Differences in egg load between sorghum and maize were mostly not significant. It was concluded that on depleted soils only, an increase in nitrogen via mulching, rotation with a leguminous crop or fertilization would increase survival of C. partellus on both maize and sorghum and an increase in acreage of maize and in application of nitrogen fertilizer in an area would also increase the parasitism of C. flavipes.

Abstract. The spatial synchrony and the temporal auto-correlationship of the exotic stemborer Chilo partellus (Swinhoe) (Lepidoptera: Crambidae) and the indigenous Sesamia calamistis Hampson (Lepidoptera: Noctuidae) and Chilo orichalcociliellus (Strand) (Lepidoptera: Crambidae), and crosscorrelationship with their indigenous and introduced larval parasitoids Cotesia sesamiae (Cameron) and Cotesia fl avipes Cameron (Hymenoptera: Braconidae) was studied using 3-year data collected in coastal Kenya. An autoregressive model was used to study the effect of climatic stochasticity or population density-dependent factors on stemborer populations. It appeared that rainfall did have a direct impact on stemborers in the south coast and an indirect one in the north coast. Spatial nonparametric correlation functions (SNCF) and cross-correlation functions (SNCCF) were applied for spatial synchrony analysis. The regional synchrony of Ch. partellus and S. calamistis decreased and that of Ch. orichalcociliellus increased after the introduction of Co. fl avipes. The positive crosscorrelation coeffi cient between stemborers and parasitoids suggests a synchrony between the pest and its natural enemy. , ont été étudiés à partir de données collectées sur trois ans dans la région côtière du Kenya. Un modèle d'auto régression a été utilisé afi n d'étudier l'effet stochastique du climat ou les facteurs dépendants de la densité de population sur les foreurs. Il est apparu que la pluie avait un impact direct au sud ou indirect au nord de la région étudiée sur les populations de foreurs. Des fonctions non-paramétriques de corrélation spatiale et croisée ont été appliquées pour l'analyse du degré de synchronie spatiale. Le degré de synchronie régionale des populations de Ch. partellus et de celles de S. calamistis a diminué alors qu'il a augmenté pour celles de Ch. orichalcociliellus après l'introduction de Co. fl avipes. Le coeffi cient positif d'auto corrélation entre les populations de foreurs et celles des parasitoïdes suggère un certain degré de synchronie entre les insectes ravageurs et leurs ennemis naturels.

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