Genetic variability in Cotesia Davipes and its importance in biological control of lepidopteran stemborers

Abstract

Lepidoperan stemborers are a major constraint to increasing the production of maize and sorghum under subsistence farming conditions in sub-Saharan Africa. Classical biological control is considered as the most cost-effective form of pest management but it has not attained the desired success rate. It has been postulated that one major reason for the failures in classical biological control is related to the genetic diversity of released_ individuals of natural enemies. The aim of this study was to examine the importance of genetic variability in the establishment and performance of the parasitoid Cotesia flavipes (Hymenoptera: Braconidae) as a classical biological control agent of lepidopteran stemborers. Field surveys carried out on Unguja and Pemba islands of Zanzibar showed that the introduced stemborer Chilo partellus (Crambidae) was the most abundant and widespread species in the stemborer complex. Although a number of indigenous natural enemies were recorded, they had a low impact on stemborer populations and hence, classical biological control was considered as an option. The endoparasitoid C. flavipes, an old association natural enemy of C. partellus, was collected from central India and imported in Kenya for laboratory and field studies. Olfactometer studies to examine the effect of duration of laboratory rearing on the parasitoid's responses to host- and host plant-associated odours showed that the overall behavioural response was stable over many generations regardless of the genetic diversity of the population. A reduced genetic variability at the sex locus of some Hymenoptera can result in biocontrol failures due to the production of diploid males from fertilized eggs as a consequence of single locus complementary sex determination mechanism (sl-CSD). In species with sl-CSD, inbreeding that may occur during rearing and release in the field will result in the production of diploid males and the associated reduction in population growth rate. Models were developed to predict the frequency of matched matings in populations with different frequencies of sib mating and egg fertilization. The models showed that sl-CSD could be detected from brood sex ratios if the diploid ~ale offspring survives. Analysis of sex ratios from field-collected data showed that sex ratio was highly female-biased. Sex ratio frequency followed a unimodal distribution instead bimodal distribution that is expected if sl-CSD with diploid male survival exists in this parasitoid species. However, the results suggested that the presence of sl-CSD with diploid male mortality could not be excluded. Inbreeding crosses to deduce further evidence for the existence of sl-CSD in C. flavipes revealed that brood sizes resulting from matched matings were not smaller than those from crosses among unmatched matings, suggesting that sl-CSD with diploid male mortality did not occur in crosses involving matched matings. Moreover, inbreeding of populations for several generations did not result in male-biased sex ratios. The importance of genetic variability to the colonization and establishment of C. flavipes was investigated through the release o f three genetically impoverished populations (isofemale lines) and one genetically diverse (mixed) population on two islands of Zanzibar. The mixed population showed higher colonization of stemborers than one of the isofemale lines but it was not different from the other two isofemale lines. This suggests that genetic variability may not have been an important factor in the colonization of C. flavipes, possibly due the similarity in climate of the area of origin in central India and the release areas in Zanzibar. Suggestions for future research to improve the biological control of lepidopteran stemborers in maize and sorghum are presented.