Chemical defence,offence and alliance in ants–aphids–ladybirds relationships

Chemicals, which mediate the interactions between aphids, ladybirds and ants, are reviewed. Special emphasis is laid on autogenous and plant-derived chemical defence in aphids and ladybirds. Evidences for chemical cues used during foraging and oviposition in ladybirds are assessed. Possible mutualistic interaction between plants and the third trophic level is illustrated by the as yet few reports of indirect plant-defence volatiles induced by aphids or coccids attracting parasitoids or ladybirds. The use of chemical signals allowing aphid parasitoids or ladybirds to squeeze into ant–aphid mutualistic association is briefly described. Questions are raised and hypotheses suggested which could stimulate further research on aphid host-plant influence on ladybird foraging behaviour and fitness, and on the cues used by aphid-web partners for their mutual recognition.