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

Abstract

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.

Host-instar selection in the aphid parasitoid Monoctonus paulensis (Hymenoptera: Braconidae, Aphidiinae): a preference for small pea aphids

“…Third, we tested the hypothesis that parasitoid prefer ence reflects differences in handling costs and oviposition success between host types (Iwasa et al, 1984;Gerling et al, 1990;Chau & Mackauer, 1997;Losey & Denno, 1998). Anti-parasitoid behaviour including dropping from the feeding site and kicking with the hind legs are gener ally better developed in older instars and adult aphids than in first instars.…”

“…Anecdotal observations by Calvert & van den Bosch (1972b) and Calvert (1973) suggest that M. paulensis selectively parasitizes small first and second nymphal instars, as was also observed by Griffiths (1960) in Monoctonus crepidis (Haliday). Chau & Mackauer (1997) provided a possible explanation of this apparent preference for small rather than large hosts. They showed that parasitoid offspring were most likely to survive in first instars of the pea aphid, which had the lowest mor tality risk due to dropping from the feeding site after parasitism as compared with other instars.…”

“…Older instars and adult aphids are larger than first and second instars and, potentially, con tain more nutritional resources; however, these resources may not always be accessible by the parasitoid (Sequeira & Mackauer, 1992). Moreover, large aphids can better defend themselves against attacks and therefore may be more costly to the female than small aphids (Gerling et al, 1990;Chau & Mackauer, 1997;Losey & Denno, 1998). Direct costs to the female include the risk of injury due to host defenses and lost opportunity time if an attack fails.…”

“…They showed that parasitoid offspring were most likely to survive in first instars of the pea aphid, which had the lowest mor tality risk due to dropping from the feeding site after parasitism as compared with other instars. However, Chau & Mackauer (1997) did not examine host choice in M. paulensis and determine if observed preferences could be accounted for by attributes of the female, differences in host quality, or a combination ofboth.…”

Alarm Pheromones—Chemical Signaling in Response to Danger

“…Typical responses range from the cessation of feeding and removal of the stylet from host plant tissues to walking, jumping, or falling away from the source of emission (Edwards et al, 1973;Wientjens et al, 1973;Phelan et al, 1976;Nault, 1977a, b, 1978;Roitberg and Myers, 1978;Wohlers, 1980;Clegg and Barlow, 1982;Chau and Mackauer, 1997;Losey and Denno, 1998;Shah et al, 1999;Braendle and Weisser, 2001). In the sugar-cane woolly aphid, Ceratovacuna lanigera Zehntner (Homoptera, Pemphigidae), the alarm pheromone reportedly elicited aggressive behavior from conspecifics (Arakaki, 1989).…”