Behavioural correlates of genetic divergence due to host specialization inthe pea aphid, Acyrthosiphon pisum

Host specialization plays a central role in the diversification of herbivor ous insects and yet we know very little about the evolution of this trait. Populations of the pea aphid, Acyrthosiphon pisum (Harris), are specialized and locally adapted to either alfalfa or clover. Preference for either pla nt produces assortative mating, and gene flow between alfalfa and clover po pulations is consistently restricted (Via, 1991a,b, 1994). Behavioural stud ies of freely-moving aphids on the plant surface for 30 min suggest that ho st preference is chemically mediated, involves chemoreception, and is most likely due to secondary compounds located in the epidermal or mesophyll cel ls. Pea aphids do not distinguish between hosts and non-hosts at a distance but determine whether the plant is suitable or not after only a short prob e. Thirty-minute recordings of the activities of aphid stylets using an ele ctrical monitoring system (EPG), where aphids are attached to a gold wire w ith silver paint, provide a different picture, suggesting that EPG experime nts do not accurately reflect natural behaviour during the first 30 min of the aphid-plant interaction.