Modifications of the chemical profile of hosts after parasitism allow parasitoid females to assess the time elapsed since the first attack.

In solitary parasitoids, only one adult can emerge from a given host. In some of these species, when several eggs are laid on the same host, supernumerary individuals are eliminated by lethal larval fights. In the solitary parasitoid Anisopteromalus calandrae, the probability of a second larva winning the fight depends on the time elapsed since the first oviposition. The older the first egg is at the moment a second egg is laid, the less chance the second egg has of winning the competition. As a consequence, females of this species lay their eggs preferentially on recently parasitized hosts rather than on hosts parasitized by an egg about to hatch. Anisopteromalus calandrae females parasitize bruchid larvae located in cowpea seeds. In a series of choice test experiments using an artificial seed system, we demonstrated that the cue that allows parasitoid females to differentiate between hosts parasitized for different lengths of time comes from the host and not from the artificial seed or the previously laid egg. This cue is perceived at short range, indicating that the chemicals involved are probably partly volatile. Interestingly, although parasitism stops host development, cuticular profiles continue to evolve, but in a different way from those of unparasitized hosts. This difference in the host's cuticular profile after parasitism, therefore, probably underlies the parasitoid female's discrimination.