ADAPTIVE SUICIDE IN PEA APHIDS - DECISIONS ARE COST SENSITIVE

Inclusive fitness theory predicts that host suicide evolve when a 'suicidal' host's genetic contribution, resulting from reduced parasitims in kin, exceeds the sacrifice of its own reproductive contribution. 'Suicidal' behaviour was examined in pea aphids, Acyrthosiphon pisum, parasitized by Braconid wasps, Aphidius ervi, when reproductive potential varied. Fourth instar aphids parasitized at the second instar (which produce no offspring before mummification) chose the most dangerous escape behaviour when approached by a ladybird beetle, Hippodamia convergens, predator while fourth instar (which can expect to produce seven to eight offspring) behaved no differently from unparasitized individuals. Therefore, 'suicidal' behaviour in aphids is cost sensitive. Coastal and interior aphid biotypes were cross-parasitized with wasps from non-local habitats. Coastal aphids, unlike interior aphids, are not faced with different mortality risks in their predator escape behaviour and do not behave differently when parasitized. The fact that parasite type had no effect on aphid behaviour, so long as the same biotype was infected, makes it unlikely that modified behaviour in infected interior aphids is merely a proximate effect of interior parasites. © 1990 The Association for the Study of Animal Behaviour.