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.
Density dependence of the alarm pheromone effect in pea aphids, Acyrthosiphon pisum (Sternorrhyncha: Aphididae)
