1. Aphids show a range of associations with ants from nonattendance to obli
gate myrmecophily. Aphis fabae cirsiiacanthoides is facultatively associate
d with ants, while Symydobius oblongus is an obligate myrmecophile. The sel
ection pressures that have shaped these associations are unknown.
2. The consequences for these aphids of their different degrees of associat
ions with ants were determined, in terms of costs and benefits to individua
ls and colonies in laboratory and field experiments. In the laboratory, ind
ividuals of A. f. cirsiiacanthoides performed worse and those of Symydobius
oblongus performed better when attended by the ant Lasius niger than when
unattended. For example, when ant-attended, A. f. cirsiiacanthoides develop
ed more slowly, were smaller, and invested less in gonads, whereas S. oblon
gus developed more quickly, were larger, and invested more in gonads. In ad
dition, the ant regulated the population size of S. oblongus to an average
of 50-70 individuals per birch sapling by removing aphids, but did not regu
late the population size of A. f. cirsiiacanthoides.
3. Under field conditions, ant-attended colonies of both A. f. cirsiiacanth
oides and S. oblongus achieved higher peak numbers and lasted longer, and a
nt-attended colonies of A. f. cirsiiacanthoides produced more alate dispers
ers than unattended colonies.
4. The implications of divergent selection pressures for the development of
myrmecophily in aphids are discussed.