PHENOTYPIC SELECTION ON THE PROCESS OF GALL FORMATION OF A TETRANEURA APHID (PEMPHIGIDAE)

1. To clarify the evolutionary interactions between a galling aphid Tetraneura sp. and its host plant Ulmus davidiana var. japonica Nakai, we examined the pattern of phenotypic selection acting on the process of gall formation by multiple regression analyses and field experiments. 2. Two hypotheses have been advanced to explain variation in performance of gall insects within host plants. The 'spatial heterogeneity hypothesis' postulates that size differences among leaves or shoots chosen by gall formers are primarily responsible for differential performance. The 'synchronization hypothesis' emphasizes the effect of timing of insect attack relative to bud burst of the host plant as a source of the variation. 3. Multiple regression analysis showed that leaf size and shoot length did not contribute at all to fecundity in galls. In contrast, the timing of gall formation best explained variation in fecundity and thus supported the synchronization hypothesis. Field experiments demonstrated that the earlier fundatrices started galling, the more offspring they produced in the galls. 4. The hatching of larval fundatrices continued for more than 20 days on single trees, and they chose leaves of different positions depending on the hatching time. Gall sites where late-hatching larvae achieved higher fecundity were different from those where early-hatching larvae did. 5. The advantage of earlier hatching was negated by environmental stress. As a result, very early or late hatching resulted in a great decrease in expected fecundity. 6. Although exploitative competition was evident between galls sharing leaves, interference competition was not found among larvae inducing galls. We suggest that large within-tree variance in hatching time, which resulted probably from phenological variation among host trees, is partly responsible for the absence of territorial behaviour in this species.