Factors affecting host plant selection in alfalfa aphids.

Alfalfa ( L.) hosts several species of aphid, (Harris), Koch and (Monell). The preference of the aphids of alfalfa plants for dense assemblies or individual plants, as well as for healthy or infested plants, was investigated in the field as in the laboratory. Years of field research have revealed the specific preferences of all three species of aphid. and are most commonly found in alfalfa crops, while is mostly found on alfalfa weeds. Also, a single species of aphid alone is usually present on a plant. In order to determine the reason for this clear preference and to establish whether at the very beginning, i.e. at the stage of choosing a host, aphid species distance themselves from each other, we tested the effect of the volatiles of healthy and infested plants on their attractiveness to aphids. is repelled by the volatiles of dense crops and plants previously infested with one of the other two species. and choose a dense assembly of plants, repelled by the volatiles of plants previously infested with . displays the weakest competitive traits, and the strongest. This research showed that competition between aphid species does not occur only when they find themselves on the same plant at the same time, fighting for resources, but also in the choice of plant, in order to avoid later competition.