One giant leap for aphid-kind.

One giant leap for aphid-kind

In certain situations, pea aphids infested with deadly parasitic wasps jump to almost certain death. Scientists now suggest that the aphids apparently are pushed by instinct to protect other aphids from being parasitized by the wasps, thereby saving the aphid colony by their own demise. Recent studies of this complex suicidal behavior in aphid populations provide "the first convincing evidence in support of the host suicide hypothesis,' say researchers at Simon Fraser University in Burnaby, British Columbia.

Described in 1981 and rooted in evolution theory, the host suicide hypothesis suggests that individual animals containing parasites deadly to their relatives will endanger their own lives in a form of suicide, thereby ridding the population of any threat from the parasite. In studying host suicide, Murdoch K. McAllister and Bernard D. Roitberg used pea aphids bred from distinct British Columbia populations: one from the coastal town of Chilliwack, the others from the hot, dry areas near Kamloops and Okanagan Falls. Because the aphids reproduce asexually, members of a colony are genetically identical. This implies that self-sacrifice does not erase genes from the aphids' genetic pool, say the scientists in the Aug. 27 ANTURE.

Roitberg and others had reported earlier that when confronted by predatory beetles, pea aphids follow one of three possible escape plans: remaining still after backing up a short distance, running away on the leaf's surface or dropping from the plant. Because many predators of the aphids are ineffective hunters, in many cases dropping from the plant is not necessary to avoid the beetles, say Roitberg and McAllister.

In their controlled-climate laboratory, they compared the escape-response behavior of parasitized and unparasitized aphids living on bean leaves. They forced the aphids to choose escape routes by frightening them with either the approach of ladybird beetles or the presence of alarm pheromones "gently squeezed' from other aphids held nearby. Pheromones are chemical signals sent from one animal to another. Aphids do not systematically drop from plants without alarm pheromones or other "predator signals' being present.

Parasitized aphids from the drier areas are about twice as likely to drop off their leaves than are their unparasitized kin, concluded the scientists. No such preference for "suicide' by parasitized aphids has evolved in Chilliwack populations, however. According to the authors, falling off a plant in locations like Kamloops can be fatal, because aphids lying on the arid ground dry up quickly. The moist weather in Chilliwack, however, makes "desiccation on the ground' less certain, and leaping less likely to benefit the community.