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Résumé :
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1. Aphids show a range of associations with ants from nonattendance to obligate myrmecophily. Aphis fabae cirsiiacanthoides is facultatively associated with ants, while Symydobius oblongus is an obligate myrmecophile. The selection pressures that have shaped these associations are unknown. 2. The consequences for these aphids of their different degrees of associations with ants were determined, in terms of costs and benefits to individuals and colonies in laboratory and field experiments. In the laboratory, individuals 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 developed more slowly, were smaller, and invested less in gonads, whereas S. oblongus developed more quickly, were larger, and invested more in gonads. In addition, 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 regulate the population size of A. f. cirsiiacanthoides. 3. Under field conditions, ant-attended colonies of both A. f. cirsiiacanthoides and S. oblongus achieved higher peak numbers and lasted longer, and ant-attended colonies of A. f. cirsiiacanthoides produced more alate dispersers than unattended colonies. 4. The implications of divergent selection pressures for the development of myrmecophily in aphids are discussed.
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