Presence of conspecifics triggers host searching behavior in an egg parasitoid wasp

Parasitoids and predators compete for host or prey species. The efficiency of obtaining prey or host items is reduced by intraspecific competition. As the optimal search behavior depends on the intensity of competition, it is important for the parasitoid or predator to obtain information on this intensity. Previous studies have shown that parasitoids can obtain information regarding competition from encounters with already parasitized hosts. They then change their host searching behavior accordingly. However, whether parasitoids obtain the information directly from observing the presence of conspecifics remains unclear. We used T iphodytes gerriphagus ( M archal) ( H ymenoptera: S celionidae), the solitary egg parasitoid of water striders, for testing the effect of density of conspecifics on host searching behavior. Females of T . gerriphagus dive into the water to search for hosts and sometimes they dive without hosts present. Thus, we investigated whether T . gerriphagus changed underwater activities in response to the density of conspecifics in the absence of hosts. Four densities (1, 2, 4, and 8 female T . gerriphagus ) were investigated. Females in competitive situations (groups of 2, 4, and 8 females) displayed host searching behavior, but the solitary females did not. This indicates that the presence of conspecifics triggers host searching behavior and that T . gerriphagus females obtain information on competition directly from conspecifics and use it for modifying their behavior.