Ontogeny of alarm pheromone secretion in pea aphid, Acyrthosiphon pisum

When attacked by a predator, an aphid may secrete a droplet of fluid from i ts cornicles containing a volatile alarm pheromone component, (E)-beta -far nesene. This study investigated both qualitative and quantitative aspects o f alarm pheromone production in the pea aphid. Acyrthosiphon pisum. The bes t predictor of cornicle droplet emission was reproductive phase, rather tha n instar, as prereproductive aphids were more likely to secrete cornicle dr oplets than either reproductive or postreproductive individuals. Analogousl y, alarm pheromone amounts were highest in prereproductive aphids. (E)-beta -Farnesene quantities (mean +/- SE) increased significantly from first ins tar (1.5 +/- 0.6 ng) to second instar (11.2 +/- 3.7 ng) and did not signifi cantly 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 p roduce higher levels of pheromone because of their more clustered colony st ructure and higher levels of predation, as compared with adult aphids.