The aphid alarm pheromone is known to trigger wing induction in pea aphids (Acyrthosiphon pisum). In reaction to alarm pheromone, aphids drop off the plant or walk away. While searching for a new feeding site they repeatedly encounter other members of the aphid colony and this increased contact rate is assumed to be important for wing induction ("pseudo-crowding" hypothesis). Because the encounter rate is a function of aphid colony size, wing induction in aphids in the presence of a predator should be a function of the number of aphids on the plant. We placed two, seven or 13 adult pea aphids on bean plants, and exposed the different-sized colonies to synthetic alarm pheromone to test the density-dependence of predator-induced wing induction. The mean percentage of winged morphs among the offspring produced on the plants ranged from 10 to 80 percent and increased both with aphid number and exposure to alarm pheromone. There was no synergy between aphid number and alarm pheromone exposure indicating that both factors are additive. The implications for aphid metapopulation dynamics are discussed.
The subsocial bug Elasmucha dorsalis lays egg masses on the underside of the leaves of female plants of Aruncus dioicus. Each bug straddles her eggs and shields her offspring with her body, until they moult to the 2nd instar. Females that: attended aggregations of 2nd and later instar nymphs feeding on fruit of inflorescences often settled close to the basal part of or just below the aggregation, and faced towards the base of the inflorescence. The nymphal aggregations often Seemed to be too large for females to guard effectively. The position and orientation of females attending 2nd or later instar nymphs probably enabled them to detect predators walking towards the nymphs.