Soybean aphid (Aphis glycines) alarm pheromone production and response

Insect pests of agriculture can cause severe economic damage, but their populations are controlled in part by natural enemies such as predators. When attacked by predators some insect pests, including many aphid species, alert neighboring conspecifcs with alarm pheromones. Aphid alarm pheromone (typically E-ß-farnesene or EBF) is released in cornicle secretions that can also physically disrupt natural enemy attacks. Since these cornicle secretions may benefit neighboring conspecifics as well as the aphid under attack the likelihood of secretion might change depending on aphid density. We examined soybean aphid (SBA, Aphis glycines) alarm behavior; i.e. both the production of and response to cornicle secretions. First, we hypothesized that EBF, a common alarm pheromone in aphids, was also present in SBA cornicle secretions and can elicit a response when presented to SBAs. Second, since aphids other than the secretor benefit from cornicle secretions we hypothesized that the likelihood of secretion would increase concurrently with the density of neighboring conspecifics. Third, because cessation of feeding is costly we hypothesized that alarm behavior in response to cornicle secretions would decrease as the concentration of conspecifics increased. We found that 1) SBA produce cornicle secretions that contain EBF, 2) SBA show alarm behavior in response to cornicle secretions but that this does not vary with aphid density, and 3) SBA are less likely to produce cornicle secretions when alone than in a small group, but that secretion does not increase with additional conspecific density. We show that SBAs use their cornicle secretions to warn their neighbors of probable attack by natural enemies, but that alarm behavior does not change due to the density of nearby conspecifics.