Behavioural and physiological responses to conflicting oviposition stimuli in a synovigenic parasitoid

The behavioural and physiological responses of the synovigenic parasitoid, Dinarmus basalis (Rond.) (Hymenoptera: Pteromalidae), were investigated whilst the wasps were submitted to conflicting oviposition stimuli: an environment which induced them to restrict egg-laying and a physiological factor, egg-load pressure, which induced them to lay eggs. When individual females depleted patches containing hosts parasitized 24 h or 72 h beforehand, they laid preferentially on unparasitized hosts and the number of eggs laid decreased significantly. Although they remained for several days in such patches, egg-load pressure did not lead them to modify their strategy (i.e. to accept all the available hosts including the already parasitized ones). In this synovigenic species, the oviposition constraint resulted in the retention of oocytes. Dissection and histological observation showed that the unlaid eggs are resorbed, a reproductive regulatory process which has seldom previously been reported in the presence of unparasitized hosts. This physiological response was induced by the mere presence of the parasitized hosts rather than their density. The females’ response was also influenced by the presence of conspecifics. They still avoided laying on parasitized hosts but the number of eggs laid daily became comparable to that observed in patches containing only unparasitized hosts. Thus, D. basalis females adapted their strategy in response to their environment, in which the most significant factor seemed to be the quality of the hosts provided, and their physiology. The originality and the importance of these results are discussed.