Parasitization of Selected African Stemborers By Cotesia Flavipes Cameron and Cotesia Sesamiae (Cameron) (Hymenoptera:Braconidae) with Emphasis on Host Selection and Host Suitability.

Abstract

Chilo partellus (Swinhoe) is a major pest of maize and sorghum in East Africa and indigenous natural enemies have been unable to maintain the pest population at a level acceptable to farmers. In Kenya, a classical biological control programme was initiated in an attempt to increase natural suppression of Chilo partellus and other stemborers. To do this, a natural enemy of Chilo partellus, Cotesia flavipes Cameron (Hymenoptera: Braconidae), has been introduced into Kenya from Pakistan since 1991. This work investigated the host and host habitat location, the acceptability and suitability of Chilo partellus and indigenous stemborer hosts for the development of Cotesia flavipes and a local natural enemy, Cotesia sesamiae (Cameron) (Hymenoptera: Braconidae), as well as the semiochemicals involved in host finding. The host selection process of the larval parasitoids Cotesia flavipes and Cotesia sesamiae was studied in the laboratory. Female parasitoids were attracted to odours from uninfested maize, sorghum and napier grass in a Y-tube olfactometer. In a dual choice test, the three plant species infested with Chilo partellus, Chilo orichalcocillellus Strand, Busseola fusca (Fuller) or Sesamia calamistis Hampson larvae were more attractive than uninfested plants. Cotesia flavipes and Cotesia sesamiae did not show preference for any of the stemborer species under study in dual choice tests. Odours from frass, produced by the four stemborer species fed on maize, sorghum, and napier grass were attractive to both parasitoid species. Cotesia flavipes was more attracted tofrass produced by Chilo species than frass produced by B. fusca in dual choice tests. No preference was observed in other combinations. Cotesia sesamiae was equally attracted to all types of frass. Attractive volatiles from infested plants were collected using activated charcoal traps and were subjected to gas chromatography –electroantennography (GC-EAD) and gas chromatography-mass spectrometry (GC-MS). Twelve GC-EAD active peaks were observed. Three of the peaks were identified as anisole, (E)-β- farnesene, and (Z)-3-hexenyl acetate. Other chemicals identified from maize infested with Chilo partellus included myrcene, 2-heptanone, 4, 8 -dimethyl-1, 3, 7- nonatriene, (Z)-2-hexenal, (Z)-3-hexen-l-ol, cyclosativen, cedrene and a-copaene. GC comparison of infested and uninfested maize seedlings showed a quantitative and qualitative difference in the volatile composition. Anisole, (E)-β- farnesene and 4, 8-dimethyl-l, 3, 7-nonatriene were absent in the volatile collection of uninfested maize. The chemical composition of the headspace of artificially damaged maize seedlings treated with larval regurgitant was similar to that of infested maize seedlings. Behavioural bioassays showed that Cotesia flavipes was attracted to volatile extracts of infested and uninfested maize and that they were attracted to synthetic anisole, (E)-β-farnesene, (Z)-3-hexenyl acetate. Chilo partellus, Chilo orichalcocillellus, B. fusca and S. calamistis, were exposed tofemale of Cotesia flavipes and Cotesia sesamiae to assess their acceptability for oviposition and suitability for the development of the parasitoids. There were no differences in the acceptability of the four hosts exposed to Cotesia flavipes. In contrast, Cotesia sesamiae preferred S. calamistis larvae (92%), followed by the two Chilo species. Busseola fusca larvae were least attacked (48.8%) by Cotesia sesamiae. The suitability of the four hosts also varied with the parasitoid species. In B. fusca both parasitoid species did not develop and egg encapsulation was observed. Chilo partellus, Chilo orichalcocilielius and S. calamistis were suitable hosts for the development of Cotesia flavipes. However, a higher mortality of immature parasitoids was observed in S. calamistis as compared to Chilo partellus. No differences were found in the mean number of progeny per female, or the sex ratio. The most suitable host for Cotesia sesamiae was S. calamistis, followed by the two Chilo species, which were equally suitable. When third, fourth, fifth and sixth instars of Chilo partellus were exposed to Cotesia flavipes females, third instars were less suitable than fourth, fifthor sixth instars as measured by immature parasitoid mortality. Total haemocyte counts increased in the blood counts in parasitized larvae two to six days after parasitization, suggesting an active immune interaction between the suitable host, Chilo partellus, and the invading parasitoids.