Soybean Aphid Response to their Alarm Pheromone E-ß-Farnesene (EBF)

Abstract

When attacked by natural enemies some insect pests, including many aphid species, alert neighboring conspecifics with alarm pheromones. Cornicle secretions with pheromones benefit the attacked aphid but are costly to produce, while alarm pheromone benefits probably fall largely on alerted conspecifics. Given these variable benefits, the likelihood of a secretion may change depending on aphid density. Thus, we first hypothesized that the common alarm pheromone in aphids, E-ß-farnesene (EBF), was present in soybean aphid (Aphis glycines Matsumura) cornicle secretions and would elicit an alarm response in aphids exposed to it. Second, since aphids other than the secretor also benefit from cornicle secretions, we hypothesized that the likelihood of secretion would increase concurrently with the density of neighboring clonal conspecifics. Third, because alarm reaction behavior (e.g. feeding cessation) is probably costly, we hypothesized that alarm reaction behavior would decrease as conspecific density (i.e. alternative prey for an attacking natural enemy) increased. We found that soybean aphids 1) produce cornicle secretions using EBF as an alarm pheromone, 2) are less likely to release cornicle secretions when alone than in a small group (~10 individuals), but that the rate of secretion does not increase further with additional conspecific density, and 3) also exhibit alarm reaction behavior in response to cornicle secretions independent of aphid density. We show that soybean aphids can use their cornicle secretions to warn their neighbors of probable attack by natural enemies, but that both secretion and alarm reaction behavior does not change as density of nearby conspecifics rises above a few individuals.