Influence of cornicle droplet secretions of the cabbage aphid, Brevicoryne brassicae, on parasitism behavior of naïve and experienced Diaeretiella rapae

Insects have evolved amazing methods of defense to ward off enemies. Many aphids release cornicle secretions when attacked by predators and parasitoids. These secretions contain an alarm pheromone that alerts other colony members of danger, thereby providing indirect fitness benefits to the releaser. In addition, contact with cornicle secretions could also threaten an attacker and could provide direct fitness to the releaser. However, cornicle secretions may also be recruited as a kairomonal cue by aphid natural enemies. In this study, we investigated the effect of the cornicle droplet volatiles of the cabbage aphid, Brevicoryne brassicae (L.), on the host-searching behavior of naive and experienced female Diaeretiella rapae (M’Intosh) parasitoids in olfactometer studies. In addition, we evaluated the role of B. brassicae cornicle droplets on the oviposition preference of the parasitoid in a two-choice bioassay. Naive females did not exhibit any preference between volatiles from aphids secreting cornicle droplets over non-secreting aphids, while experienced parasitoids exploited the secretions in their host location. Experienced females were also able to choose volatiles from both secreting and non-secreting aphids over clean air, while this ability was not observed in naive females. Although secretion of cornicle droplets did not influence the percentage of first attack in either naive or experienced females, the success of attack (i.e. resulting in a larva) was significantly different between secreting and non-secreting aphids in the case of experienced parasitoids.