Genetic differentiation as a result of adaptation to the phenologies of individual host trees in the galling aphid Kaltenbachiella japonica

.1 The adaptation of the non‐migratory galling aphid Kaltenbachiella japonica (Matsumura) to the budburst phenologies of individual host trees ( Ulmus davidiana var. japonica Nakai) was investigated. There was a large variation in budburst time between individual host trees. We tested a hypothesis that aphid populations on respective host trees are genetically differentiated through adaptation to the budburst phenologies of host trees. 2 There was significant correlation between the budburst time of eight host trees in two successive years. 3 Tree‐associated populations differed significantly in egg‐hatching time, and the mean hatching time was significantly correlated with the mean budburst time of respective host trees. 4 Mating was made between male and female sexuales of the same clone (i.e. self‐mating) to obtain selfed eggs. Hatching patterns of selfed eggs showed that there was a substantial amount of genetic variance in hatching time between clones (galls). Nested ANOVA demonstrated that the between‐tree component contributed more to the phenotypic variance than the within‐tree or within‐gall components. 5 Reciprocal crosses between males and females of different clones furthermore demonstrated significant differences in hatching time between cross combinations, but no significant differences between the two reciprocals within combinations. This result suggests that there are no maternal effects for the timing of egg hatch and that the differences between selfed lines are attributable to genetic variance. 6 The fine‐scale adaptation hypothesis was supported by egg‐hatching experiments, which further suggest that budburst phenology varying between individual trees is acting as intense selective pressure on the egg‐hatching time of associated insects.