Ant attendance in aphids: why different degrees of myrmecophily?

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.