Abstract
During coevolution of plants and herbivores, natural selection is assumed to increase preference-performance covariance, but some recent studies put that assumption in doubt. Our study of a gall aphid Neothoracaphis yanonis demonstrated that the stem mothers produce galls more abundantly on the leaves at the lowest position on a shoot, and on the basal intermediate part (B-part) of a leaf blade. The leaf position was determined by the synchronization between the appearance of stem mothers and the leaf opening phenology, but the leaf part was chosen by preference of the stem mothers. On this leaf position and leaf part, aphids produced larger galls containing more abundant alates. Thus, our observations support the preference-performance hypothesis. As gall density increased, gall size declined, the percentage of indehiscent galls increased, and the relative abundance of upper-type galls increased. The density effect resulted in promoting the dispersal of stem mothers and reducing fitness of aphids which located on less favorable sites.
