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.