The more corpses the better

Written by Thomas Hesselberg

Sunday, 05 April 2009

A colony of aphids feeding on a fennel stalk. Wikimedia Commons.

New research show that aphids, contrary to expectation, are more likely to stay put, when their neighbours in a colony are killed by parasitic wasps. However, results show that this is not because the aphids have a death wish, but rather because the parasitic wasps reduce the time spent searching for prey in areas with many dead aphids.

Many animals, especially in aquatic systems, have been shown to react to dead members of their own species by using their presence to assess the predation risk. The more dead individuals they encounter the higher the perceived predation risk. In aphids such a system would also work well since they live in high density colonies on plants and tree, where they feed on the sugar rich liquid. Aphid colonies are often farmed and protected by ants, but nonetheless suffer a very high predation rate from predators such as ladybird beetles, lace wings and parasitic wasps. A team of French scientists from the University of Rennes investigated how aphids respond to death colony members predated by parasitic wasps, which are especially suitable for such studies since the wasps leave the empty corpses intact on the plant.

The biologists reared aphid colonies in the laboratory and obtained corpses (mummies) by introducing parasitic wasps into some of those colonies. Aphids were then placed individually on plants containing one to several mummies or on control plants containing none. They were then allowed to produce offspring (aphilds can reproduce asexually by making clones) and the proportion of winged offspring was counted. Winged offspring are produced when aphids want to disperse or to escape from unfavourable conditions. The scientists, furthermore, tested the predation behaviour of individual parasitic wasps by exposing them to aphid colonies containing none or several mummies.

The results of the first experiment showed that, contrary to the expectations of the scientists, aphids produce a lower proportion of winged offspring with more mummies present. However, the second set of experiments showed that parasitic wasps spend less time searching for prey among colonies with a high number of mummies. In this light, the behaviour of the aphids to stay put when more corpses are around make good sense as the dead aphids offer indirect protection to the living.

Source
Virgil Fievet, Pauline Le Guigo, Julianne Casquet, Denis Poinsot, and Yannick Outreman (2009). Living with the dead: when the body count rises, prey stick around. Behavioral Ecology doi:10.1093/beheco/arp014.

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