Aphid-mediated coexistence of ladybirds (Coleoptera: Coccinellidae) and the wood ant Formica rufa:: seasonal effects, interspecific variability and the evolution of a coccinellid myrmecophile.

It is generally believed that most homopteran-eating insects avoid ant-tended colonies of Homoptera, due to the ant aggression they encounter there. However, because homopteran colonies which are ant-tended often persist For longer than untended colonies, some homopteran-eaters may utilise ant-tended Homoptera when untended colonies are scarce. Furthermore, a few homopteran-eaters are myrmecophilous, habitually coexisting with ants. To investigate these phenomena, a study was made of aphids and aphidophagous coccinellids (ladybirds) on Scots pine, Pinus sylvestris, growing in areas foraged and unforaged by the wood ant Formica rufa. The non-tended aphid Schizolachnus pineti exhibited a marked population decline in late summer but persisted in both areas at very low density. Facultatively tended Cinara aphids exhibited higher population densities when associated with F. rufa, and remaining colonies of these aphids were only found associated with ants in late summer. Coccinellids exhibited considerable interspecific variability in their level of association with F. rufa, and there was some evidence of an increase in certain species' frequencies of occurrence with the ant when Cinara aphids were all ant-tended, in late summer. Coexistence with ants appears to be associated with either an intolerance of low aphid densities. in Coccinella septempunctata and Harmonia quadripunctata, or with extreme dietary specialisation, in Myzia oblongoguttata. Similar factors to those which bring C. septempunctata into contact with ants were probably of importance in the initial stages of the evolution of myrmecophily of its congener, Coccinella magnifica.