Behavior and performance of a specialist and a generalist parasitoid of bruchids on wild and cultivated beans

The performance of parasitoids is directly related to the quality of the herbivore's host plant. When plant quality is altered, for instance through plant domestication, insects associated with cultivated plants will have to adapt to these changes. This study investigated host acceptance and performance of a generalist and a specialist parasitoid species that attack bruchid beetles on wild as well as cultivated seeds (beans) of Phaseolus vulgaris. Both parasitoids are candidate biological control agents against bruchid pests. Wasps reared from field-collected wild P. vulgaris seeds were used to create three strains of each parasitoid species: two on P. vulgaris (wild and cultivated seeds), and one on Vigna unguiculata as a control. After 12 generations in the laboratory, the strains were subjected to transplant experiments on the different seed types and parasitism and performance of the parasitoids was studied. We found significant differences in the motivation to parasitize, host acceptance, and in performance of both parasitoid species correlated with the seed type they were reared on. The specialist Stenocorse bruchivora (Hymenoptera: Braconidae) was selective, parasitizing fewer hosts and mainly in the seed type on which they performed best, resulting in offspring with similar sizes and development times. In contrast, the generalist Dinarmus basalis (Hymenoptera: Pteromalidae) was far less selective, parasitizing almost all hosts offered, and its performance was more variable and strongly affected by seed type. Only the sex allocation by D. basalis was consistent with sex ratio theory, indicating that this wasp is also able to assess host quality, but readily accepts inferior hosts, mainly for male offspring. These differences in behavior and performance are discussed in the context of the different selective pressures that may operate on both parasitoid species. (C) 2004 Elsevier Inc. All rights reserved.