Subversion of alarm communication: Do plants habituate aphids to their ownalarm signals?

When attacked by a predator, pea aphids, Acyrthosiphon pisum, emit an alarm pheromone, (E)-beta -farnesene, which causes nearby conspecifics to disper se from the area. However, herbivore-damaged plants also emit (E)-beta -far nesene. We hypothesized that plants release farnesene to habituate aphids, i.e., to disrupt their alarm-pheromone responses, perhaps to reduce herbivo ry by increasing parasitoid or predator efficacy. Thus, we addressed two qu estions: (1) Do aphids habituate to (E)-beta -farnesene, and (2) Are they h abituated at levels produced by aphid-infested plants? On an artificial die t devoid of farnesene, aphids were exposed to 10 ng/cm(3) of (E)-beta -farn esene or a hexane control over 24 h. Habituation was achieved, as dropping responses to 50 ng/cm(3) of (E)-beta -farnesene decreased after exposure. W e then exposed aphids to 0.8 ng/cm(3) of (E)-beta -farnesene, a pheromone c oncentration emitted by plants, or a hexane control for 24 h. Their reactio n to 38 ng/cm(3) of (E)-beta -farnesene, the maximum pheromone concentratio n found in aphid-cornicle droplets, was not significantly different before and after exposure. Thus, our hypothesis that plants emit farnesene to disr upt aphid alarm communication remains unsupported.