Electroantennogram and behavioural responses of different forms of the bird cherry-oat aphid, Rhopalosiphum padi, to sex pheromone and a plant volatile.

Winged (alate) virginoparae were induced in the laboratory by crowding the bird cherry-oat aphid, Rhopalosiphum padi (L.) under long-day conditions. Males and gynoparae (the winged female form that produces sexual females) were induced by short days. Electroantennogram (EAG) and behavioural responses were investigated in each of the three forms to two aphid sex pheromone components, (-)-(4aS,7S,7aR)-nepetalactol and (+)-(4aS,7S,7aR)-nepetalactone, and benzaldehyde, a volatile which is released by the winter host plant. All three compounds elicited EAG responses with the males showing the highest sensitivity to each compound. Both the nepetalactol and the nepetalactone elicited larger EAG responses in gynoparae than in the winged virginoparae but antennae from virginoparae were more responsive to benzaldehyde. Although the nepetalactone is not a sex pheromone component in R. padi the EAG responses were similar to those evoked by the nepetalactol, the sex pheromone, in all three aphid forms. In a linear-track olfactometer, significantly more male R. padi moved into air containing nepetalactol, nepetalactone or benzaldehyde than into a simultaneous choice of clean air (i.e. attraction) but nepetalactol was more attractive than nepetalactone. Males, however, showed no response to a mixture of nepetalactol and nepetalactone. Gynoparae were attracted only to the nepetalactol but were less sensitive than the males and showed no response to the nepetalactone or benzaldehyde. In contrast, alate virginoparae showed no behavioural responses to any of the compounds. The present study supports the idea that the male R. padi utilise both sex pheromone and benzaldehyde for mate/host-plant location in autumn. It also demonstrates, for the first time, polyphenic differences in the olfactory responses at the peripheral level between the two female forms. Such differences impact on the life-cycle strategy where winged virginoparae move between graminaceous summer host plants while gynoparae move from the summer hosts to the bird cherry, winter host. The latter move appears to be assisted by the sex pheromone released by sexual females, already present on that host, acting as an aggregation pheromone.