Biology and ecology of Apanteles taragamae, a larval parasitoid of the cowpea pod borer Maruca vitrata

Abstract

Maruca vitrata Fabricius is a key insect pest of cowpea in West Africa. Larvae of this moth can cause up to 80% of yield losses. The first classical biological control programme against M. vitrata started in 2005 with the introduction of Apanteles taragamae Viereck from Taiwan into Benin by the International Institute of Tropical Agriculture (IITA), Benin station.Thorough knowledge on the bioecology of A. taragamae is a prerequisite for implementing such programme. The work described in the present thesis evaluated the biological potential of this larval parasitoid to fill to the gap of information on its biology and ecology. Special emphasis was put on the main factors that determine the effectiveness/suitablility of biological control candidates, such as reproductive capacity, functional response, climatic adaptability, host foraging capacity, and non-target effects. The results revealed that two-day-old larvae were the most suitable host age, giving the highest percentage parasitism, lifetime fecundity and proportion of females. Larvae older than three days were not successfully parasitized. The percentage parasitism of two-day-old larvae was positively correlated with host density, indicating a good functional response of A. taragamae. Between 20 and 30 °C, the curve that described the relationship between the intrinsic rate of natural increase and the temperature for A. taragamae was above that of M. vitrata, suggesting that the parasitoid can faster build up its population than its host. The parasitoid showed its ability to use volatiles produced by cowpea flowers and host caterpillars when foraging. A host plant odour experience enhanced the capacity of the parasitoid to find uninfested flowers. The growth of non-parasitized or A. taragamae-parasitized larvae was slower and with reduced proportion of female wasps on some host plants compared to those reared on artificial diet. With regard to the non-target effects, the physiological host range and competitive ability of A. taragamae were assessed. None of the following lepidopteran species, Eldana saccharinaWalker, Chilo partellus (Swinhoe), Mussidia nigrivenella Ragonot, Cryptophlebia leucotreta (Meyrick), Sylepta derogata FabriciusandCorcyra cephalonica Stainto,was successfully parasitized by A. taragamae, suggesting its specificity for M. vitrata in Benin. In no-choice competition with the egg-larval parasitoid Phanerotoma leucobasis, A. taragamae outcompeted the latter. All the above attributes suggest that A. taragamae should be a suitable biocontrol agent against M. vitrata. A cage release strategy involved the host plant Sesbania cannabina, which was artificially infested with M. vitrata, and inoculated with adults of A. taragamae. The parasitoid was released in seven selected locations in Benin but the first recovery studies did not yet yield any information on its establishment after the first generation. </p