Dropping of pea aphids from feeding site: a consequence of parasitism by the wasp, Monoctonus paulensis

We examined the dropping behaviour of the pea aphid, Acyrthosiphon pisum (Harris), feeding on broad-bean plants in the laboratory. Aphid responses to foraging and oviposition by females of Monoctonus paulensis (Ashmead) were instar-specific and included kicking with the hind legs, walking away and dropping from the feeding site. Fourth nymphal instars were most likely to drop, followed by second, third, and first instars, in that order. Compared with first instars, the odds that a fourth-instar aphid will drop were 6-times higher independent of the stimulus, and 16-times higher after parasitoid attack. Dropping from the feeding site increases an aphid's mortality risk. If parasitoid offspring are more likely to survive in small pea aphids, it may be adaptive for M. paulensis to choose smaller hosts, regardless of possible fitness gains due to increased body size from development in larger aphids.