Studies were conducted at Mtwapa, in the coastal area of Kenya, during the long and short rains of 1999 and 2000 to evaluate interaction of mortality factors in lowering Chilo partellus (Swinhoe) populations in diverse agroecosystems. Treatments consisted of hedgerows of leucaena only, gliricidia only, alternating rows of leucaena and gliricidia, and three plots without trees. During the cropping seasons, maize was planted between the hedgerows while a row of cowpea was planted between the rows of maize. One of the plots without trees was planted to an intercrop of maize and cowpea. The remaining two plots without trees were planted to maize alone, one of which was mulched. The treatments were laid out in a randomized complete block design with four replications. There was no evidence of enhanced mortality in the more diverse systems indicating that the vegetation structure and cultural treatments studied did not enhance activity levels of C. partellus mortality factors. The highest mortality occurred in the early and medium larval stages in all treatments. This mortality was attributable to ‘disappearance’, which represented mortality from sources other than parasitism. The total real generation mortality showed that from the initial cohort of first and second instar larvae, less than 10% of C. partellus survived to adult stage under field conditions in all the treatments. Mortality by Cotesia sesamiae Cameron and Cotesia flavipes (Cameron) was very minimal, indicating the parasitoids were not important mortality factors of C. partellus under the cropping systems studied. Their effects were also not influenced by the cropping systems studied.

Busseola fusca Fuller is one of the major lepidopteran stemborers of maize and sorghum in Africa. Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) is an indigenous, gregarious larval endoparasitoid that attacks mid- to late-instar stemborer larvae, which is reported to be one of the most important larval parasitoids of B. fusca in many countries in sub-Saharan Africa. Previous work has shown that two biotypes of C. sesamiae occur in Kenya which express differential abilities to develop in B. fusca. A biotype from western Kenya successfully develops in B. fusca, while a biotype from the eastern coastal area does not. We treated fourth-instars of B. fusca with calyx fluid from C. sesamiae from western Kenya (Kitale) before offering them to C. sesamiae from the Coast (Mombasa) for oviposition, and found that the eggs were not encapsulated. This suggests that factors in the calyx fluid of C. sesamiae from the western Kenya biotype were responsible for suppressing the immune system of B. fusca. We speculate that polydnaviruses in the calyx fluid were involved in disarming the host immune system, and that the polydnaviruses in the two biotypes are genetically different.

This paper addresses the competitive interaction between Cotesia sesamiae, a gregarious larval endoparasitoid that is native to Africa, and Cotesia flavipes, an introduced parasitoid from southern Pakistan which has recently been established in East Africa for biological control of the exotic stemborer, Chilo partellus. The two parasitoids are morphologically similar, and they both attack medium-sized to large larval stages of gramineous lepidopteran stemborers. Based on previous work which suggested that Cotesia flavipes was competitively superior to Co. sesamiae, we speculated that Co. sesamiae may be locally extirpated in some areas of Africa. However, evidence from the field does not support this prediction. Recent field surveys in areas where both parasitoids occur, revealed that Co. sesamiae was still present, and densities were not declining. The interaction of the two parasitoids, as well as the influence of this interaction on host regulation, are discussed.

Studies to determine the relative abundance and distribution of stemborer species and their associated parasitoids in maize and sorghum were carried out during the 1995–1996 and 1999–2000 cropping seasons on Unguja and Pemba, the two main islands of Zanzibar. Three stemborer species were found on both islands in maize and sorghum, with the exotic species Chilo partellus (Swinhoe) (Crambidae) being the most abundant and widely distributed species, accounting for 75.3 % of stemborer attack. It was followed by Sesamia calamistis Hampson (Noctuidae) and Ch. orichalcociliellus Strand (Crambidae). Stemborer abundance was significantly higher in Pemba (1.47 ± 0.20) than Unguja (0.85 ± 0.05). Overall stemborer density in Zanzibar, during the period of study, was 1 03 ± 0.08 stemborers per plant. Stemborer infestation was significantly higher during the short rainy season than long rainy season but it did not vary between maize and sorghum or coral rag and plantation areas. The indigenous parasitoids recorded included ten larval parasitoids: Bassus sp., Chelonus sp., Cotesia sp., Cotesia sesamiae (Cameron), Cotesia ruficrus (Haliday), Dolichogenidea sp., Dolichogenidea aethiopica Wilkinson, Dolichogenidea polaszeki Walker, Megaselia sp. and an unidentified Tachinidae. Among these, the braconid Co. sesamiae was the most common parasitoid, attacking all the stemborer species and recorded from 85.2 % of parasitised larvae. However, parasitism was low— 3.9 % on Unguja and 1.9 % on Pemba. In addition, the efficiency of Co. sesamiae was reduced by two hyperparasitoids—Aphagnomus fijiensis Ferrière and Elasmus sp. Seven pupal parasitoids were recorded, namely Dentichasmias busseolae Heinrich, Brachymeria sp. Westwood, Brachymeria olethria Waterston, Pediobius furvus Gahan, Psilochalcis soudanensis Steffan, Syzeuctus ruberrimus Benoit and an unidentified Chalcididae. The differences in species diversity between the two islands are discussed in the context of the equilibrium theory of biogeography and the geographical size. Classical biological control of the introduced stemborer Ch. partellus using the exotic parasitoid Co. flavipes to supplement the indigenous parasitoids in population regulation is proposed as a potentially useful option for Zanzibar.

The stemborers Busseola fusca (Fuller) and Chilo partellus (Swinhoe) cause the highest economic damage to maize and sorghum in Zimbabwe. A number of studies on stemborer bioecology and management in Zimbabwe have been conducted, but most are still incomplete. The more important of these are studies on life history, incidence and distribution patterns, host plants, yield losses, chemical control, cultural control, host plant resistance, the use of sex pheromones, and biological control. These are discussed here in detail, and areas where further research is needed are pointed out. As part of an integrated strategy for managing stemborers in Zimbabwe, an exotic braconid larval parasitoid, Cotesia flavipes Cameron, has been released in the lowveld area and prospects for its establishment appear to be good. However, this establishment might be curtailed mainly by the inability of Co. flavipes to adapt to the seasonal carryover mechanisms utilised by the indigenous Cotesia sesamiae (Cameron).

Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) is an indigenous, gregarious, larval endo/parasitoid which is widely distributed in Africa and attacks mid- to late instars of stemborers. There is renewed interest in using Co. sesamiae for biological control to reduce the problems caused by stemborers. However, Wolbachia sp., a bacterial symbiont of some populations of Co. sesamiae, poses some threat to the successful use of the parasitoid for stemborer control. This bacterium's common effect is cytoplasmic incompatibility between infected males and uninfected females. Polymerase chain reaction (PCR) tests were carried out on parasitoid samples collected from 5 locations in Kenya and 8 other countries in eastern Africa. Wolbachia presence was detected in Co. sesamiae populations from Coast and Machakos in Kenya, and from samples collected from 6 other eastern and southern African countries. No infection was detected from western Kenya, Uganda and Malawi. Cross mating among 3 infected populations (North Coast, South Coast and Machakos) and 2 uninfected populations (Kitale and Kuja) was done to establish which populations were reproductively compatible. Biological parameters, such as brood size and percent females were compared among the different crosses. One-way incompatibility, indicative of Wolbachia infection, was observed from the crosses between males of either Mombasa or Machakos arid females from Kitale. Selection of compatible Co. sesamiae populations is critical to successful augmentation biological control programmes.

The biosystematics of species in the Cotesia flavipes complex is reviewed with reference to studies conducted on their morphology, morphometrics, enzyme electrophoresis and hybridisation. A survey of 5 morphological characters scored from 17 populations indicated that the scuto-scutellar sulcus and the rugosity on the propodeum could be used to separate the species. The shape of the male genitalia was valid for separating the populations of C. flavipes from those of C. sesamiae and C. chilonis. Canonical variate analysis separated the complex into three distinct clusters with populations from Africa together, populations from Asia and the neotropics forming a second cluster, and material from China and Japan forming the third cluster. Electrophoretic studies showed that esterase, glucosephosphate isomerase, hexokinase, sorbital dehydrogenase and phosphogluconate dehydrogenase, among others, had fixed alelles and could be used to distinguish the species. Cladistic analysis of the allozyme data indicated that allopatric populations currently included in C. flavipes may not be a monophyletic group. Interspecific crosses revealed that males of C. sesamiae could mate with females of C. chilonis and produce viable females.

Progeny production and sex ratios of the parasitoids Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae) and its exotic congener Cotesia flavipes Cameron (Hymenoptera: Braconidae) emerging from three species of field-collected stemborers were compared. There were no significant differences in numbers of progeny of the two parasitoids from the African stemborers, Chilo orichalcociliellus (Strand) (Lepidoptera: Pyralidae) and Sesamia calamistis Hampson (Lepidoptera: Noctuidae). Cotesia flavipes produced more progeny than C. sesamiae on large larvae of its coevolved Asian host, Chilo partellus (Swinhoe). No significant differences in the number of parasitoid progeny were observed for different-sized Ch. orichalcociliellus or S. calamistis larvae. These data suggest that under field conditions, Co. flavipes may be superior to Co. sesamiae as a parasitoid of its co-evolved host, Ch. partellus.

Difficulties of identification of natural enemies of cereal stemborers for ‘redistribution’ in Africa are discussed. Tritrophic level studies on wild and cultivated habitats of borers and beneficial species are needed to judge the impact of a natural enemy species in the cropping system. Areas with low pest pressure which are climatically favorable for pest development may be selected for study. Based on the results of various countrywide surveys to map the relative importance of Sesamia calamistis Hampson, Eldana saccharina (Walker) and Busseola frisca (Fuller) in western Africa, recommendations are given for sites for tritrophic level studies. It is hypothesised that because maize is not always present in the field and because of its high susceptibility, natural biological control has to come from wild habitats. This emphasises the importance of the knowledge on the wild host-plant range. Survey results complemented with oviposition and life-table studies in the laboratory showed that, rather than being reservoirs for pests, most wild grass species act as trap plants causing mortalities of 100%. A comparison of light trap catches with pupae found on wild hosts and the scarcity of known wild hosts in areas with high pest pressure suggest gaps in our knowledge of the range of host plant species. Based on comparison of known natural enemy complexes in East and West Africa, the scelionid egg parasitoid Telenomus isis and an East African strain of the braconid larval parasitoid Cotesia sesamiae (Cameron) are proposed for redistribution against B. fusca and S. calamistis, respectively. Telenomus isis has never been reported from East Africa whereas G sesamiae is common in East and southern Africa and scarce in western Africa, suggesting that G sesamiae is probably not adapted to S. calamistis and B. fusca in this region.