The effects of mutualistic ants on aphid life history traits

The relationship between homopterans and ants is generally thought to be mu tualistic, as both partners seem to benefit from an association. In aphids, previous studies have shown that ant tending improves the survival and rep roduction of aphid colonies, mainly by protection of aphids from enemy atta ck. However, the effects of ant tending on the fitness of individual aphids have rarely been addressed. We investigated the effects of ant tending on life history traits of aphids feeding singly on a host plant, in the absenc e of natural enemies. A factorial design allowed us to control for variatio n in the level of tending effort among individual ant colonies. The presenc e of workers of the ant Lasius niger had a strong positive effect on the fi tness of individuals of the aphid Metopeurum fuscoviride. Ant-tended indivi duals lived longer, matured earlier, had a higher rate of reproduction, and a higher expected number of offspring than aphids not tended by ants. An a phid's longevity was significantly correlated with the daily mean number of workers tending it. The strong dependence of aphid fitness on the level of ant tending shows that ants can influence aphid life history traits even w hen aphids occur singly on plants.