The role of chemical cues in host finding and acceptance by Callosobruchus chinensis

We studied the response of female Callosobruchus chinensis to chemical cues emitted by cowpea seeds at different stages of bruchid infestation (uninfested, egg carrying, L1-, and L4-infested). Olfactory attractiveness was determined in Y-tube olfactometer assays by testing individual seed categories against either clean air or uninfested seeds. Oviposition preferences between uninfested and infested seeds were determined in petri-dish choice-experiments. The olfactometer assays revealed that weevils discriminate between seeds containing different stages of developing bruchids on the basis of olfactory cues. While odors from uninfested and egg-carrying seeds acted as attractants, odors from L1- and L4-infested seeds failed to induce a positive response by the bruchids. When given a choice between uninfested and infested seeds in the olfactometer, weevils preferred uninfested seeds over L1- and L4-infested seeds, but failed to distinguish between uninfested and egg-carrying seeds. In the oviposition experiment as well, bruchids showed a distinct preference for uninfested seeds when offered in combination with L1- and L4-infested seeds. This experiment further showed a reduced acceptance of egg carrying seeds. Our results indicate that C. chinensis females use chemical information during both host searching and host acceptance. Volatiles from uninfested or egg carrying seeds act as attractants, while deterrence increases as development of bruchid immature stages progresses.