Ontogeny of Alarm Pheromone Secretion in Pea Aphid, Acyrthosiphon pisum

When attacked by a predator, an aphid may secrete a droplet of fluid from its cornicles containing a volatile alarm pheromone component, (E)--farnesene. This study investigated both qualitative and quantitative aspects of alarm pheromone production in the pea aphid, Acyrthosiphon pisum. The best predictor of cornicle droplet emission was reproductive phase, rather than instar, as prereproductive aphids were more likely to secrete cornicle droplets than either reproductive or postreproductive individuals. Analogously, alarm pheromone amounts were highest in prereproductive aphids. (E)--Farnesene quantities (mean ± SE) increased significantly from first instar (1.5 ± 0.6 ng) to second instar (11.2 ± 3.7 ng) and did not significantly change during third (12.8 ± 3.0 ng) or fourth instars (11.0 ± 3.7 ng). Alarm pheromone amounts then decreased significantly in adults (4.8 ± 2.3 ng). We suggest that prereproductive aphids have been selected to produce higher levels of pheromone because of their more clustered colony structure and higher levels of predation, as compared with adult aphids.