Sex determination in the order Hymenoptera is based on haplodiploid arrhenotoky; in which males develop from unfertilized eggs and are haploid, whereas females develop from fertilized eggs and are diploid. However, some hymenopteran species produce diploid males through a mechanism known as single-locus complementary sex determination (sl-CSD). In these species, heterozygous individuals at a single sex locus develop into females, whereas hemizygotes (haploids) and homozygotes (diploids) develop into males. Inbreeding leads to homozygosity and consequent production of diploid males. We investigated the presence of sl-CSD in the braconid Cotesia flavipes using a series of inbreeding crosses among five isofemale lines. Sex ratio (proportion of females) did not differ among the within-line crosses, between-line crosses and crosses carried out between isofemale lines and a mixed (outbred) colony. Brood size of within-line and between-line crosses did not differ. Culturing populations for 25 generations did not result in changes to more male-biased sex ratios. We conclude that CSD does not exist in C. flavipes. The implications of absence of CSD in C. flavipes are discussed in context of mass rearing for classical biological control programmes.

Diploid males are expected to occur in populations of some Hymenoptera because of the single-locus complementary sex determination (sl-CSD) mechanism. If sl-CSD is found in a species that produces gregarious broods and sib mates at a high frequency, a fraction of these sib-matings (matched mating) produces diploid male offspring. We developed models to predict the matched mating frequency in populations with different frequencies of sib mating and egg fertilization. The predictions of these models are used to determine if we can use the distributions of brood sex ratio and brood size to determine if a species has sl-CSD. The models show that sl-CSD can be detected from these brood sex ratios if the diploid male offspring survives. We applied our models to Cotesia sesamiae and its exotic congener Cotesia flavipes (Hymenoptera: Braconidae), to determine if sl-CSD occurs in these species. Parasitoids were reared from stemborer larvae sampled from maize fields in Kenya between 1992 and 1999. We found no evidence for the presence of sl-CSD with survival of diploid males in both braconid species, but we cannot exclude the possibility that sl-CSD with diploid male mortality takes place in these species.

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 irtDst abundant and widely distributed species, accounting for 75.3 % of stemborer attack. It was followed by Sesamia calamistis Hampson (Noctuidae) and Ch. oriclialcociliellus 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 i adigenous parasitoids recorded included ten larval parasitoids: Bassus sp., Cltelonus sp., Cotesia sp., Cotesia sesamiae (Cameron), Cotesia ruficrus (Haliday),;Dolichogenidea sp., Dolichugcnidea 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 Unguj-i and 1.9 % on Pemba. In addition, the efficiency of Co. sesamiae was reduced by two hyperparasitoids-Aphagnomus fijiensis Ferriere 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. 335 336 E. I. NIYIBIGIRA et al. Hampson (Noctuidae) et Ch. orichalcocilielhis Strand (Crambidae). L'abondance des borers etait plus eleve dans l'ile de Pemba (1.47 ± 0.20) que dans celle d'Unguja (0.85 ± 0.05). En general la densite des borers a Zanzibar pendant la periode d'etude etait de 1.03 ± 0.08 foreurs par plant. L'infestation par les borers etaient particulierement eleve pendant la courte saison des pluies, mais n'a pas variee entre le mais, le sorgho, ou entre les plantations et les banes de coraux. Dix parasitoids locaux ont ete collectes parmi lesquels, Bassus sp., Chelonns sp., Cotesia sp., Cotesia sesamiae (Cameron), Cotesia ruficrus (Haliday), Dolichogenidea sp., Dolichogenidea aethiopica Wilkinson, Dolichogenidea polaszeki Walker, Megaselia sp., et un Tachini...

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