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

Abstract

When attacked by a predator, pea aphids, Acyrthosiphon pisum, emit an alarm pheromone, (E)-β-farnesene, which causes nearby conspecifics to disperse from the area. However, herbivore-damaged plants also emit (E)-β-farnesene. We hypothesized that plants release farnesene to habituate aphids, i.e., to disrupt their alarm-pheromone responses, perhaps to reduce herbivory by increasing parasitoid or predator efficacy. Thus, we addressed two questions: (1) Do aphids habituate to (E)-β-farnesene, and (2) Are they habituated at levels produced by aphid-infested plants? On an artificial diet devoid of farnesene, aphids were exposed to 10 ng/cm3 of (E)-β-farnesene or a hexane control over 24 h. Habituation was achieved, as dropping responses to 50 ng/cm3 of (E)-β-farnesene decreased after exposure. We then exposed aphids to 0.8 ng/cm3 of (E)-β-farnesene, a pheromone concentration emitted by plants, or a hexane control for 24 h. Their reaction to 38 ng/cm3 of (E)-β-farnesene, the maximum pheromone concentration found in aphid-cornicle droplets, was not significantly different before and after exposure. Thus, our hypothesis that plants emit farnesene to disrupt aphid alarm communication remains unsupported.