Mate finding via a trail sex pheromone by a parasitoid wasp

In field observations and laboratory experiments, we found that virgin females of the solitary parasitoid Aphelinus asychis did not emit a volatile sex pheromone to attract males, contrary to what has been reported in many other parasitoid species. Instead, we found that virgin females deposited a sex pheromone on the substrate to which males responded by intensively searching on and near the marked area. Males did not respond to leaves exposed to mated females or to other males. In patches of 64 wheat leaves, males were dispersed from a central release point, and more males were subsequently observed on leaves exposed to virgin females than on unexposed leaves. The pheromone faded to inactivity in less than 24 h. To examine whether the trail pheromone would be sufficient for mate finding by males in the field, we modeled random movement of males among plant stems where the trail pheromone was the only cue males used to find females. The probability that females encountered at least one male in their lifetime increased with male density and time after female emergence. Given the range of densities of A. asychis in barley and wheat fields near Montpellier, France, the model generated an encounter probability sufficient to explain the survival of established populations. The model also suggested that difficulty in finding mates at low density might be a problem for invading populations.