The Argentine ant, Linepithema humile (Hymenoptera : Formicidae : Dolichoderinae) is sensitive to semiochemicals involved in the spacing behaviour inthe bird cherry-oat aphid Rhopalosiphum padi (Sternorrhyncha : Aphididae)

The behavioural response of an aphid-tending;ant, the Argentine ant Linepit hema humile, to semiochemicals related with spacing behaviour in the aphid Rhopalosiphum padi (L.) was evaluated. The compounds involved were 6-methyl -5-hepten-2-one (MHO), 6-methyl-5-hepten-2-ol (MHOH), and 2-tridecanone (2- T). Hexane solutions of these semiochemicals either singly or mixed, and ex tracts obtained by trapping volatile compounds from wheat seedlings either alone or infested with two different densities of aphids were used as stimu li in olfactometric bioassays. Ants showed attraction to volatile extracts from plants infested at both densities, and slightly preferred volatile com pounds from lightly over heavily infested plants in a choice test. On the o ther hand, while a dose-dependent repellence response was elicited by the n aturally-occurring mixture of MHO, MHOH and 2-T, single compounds did not e licit significant responses in olfactometric bioassays. The function of the se semiochemicals as indicators of quality of the aphid colonies as trophic resource to ants is discussed.