Invertases are sucrose hydrolyzing enzymes often associated with plant tissues acting as physiological sinks, and plant galls are physiological sinks. We investigated several types of invertase and their potential benefits in galls of the aphid Hormaphis hamamelidis. Invertase activities in galls differed from those in leaves throughout the growing season. Vacuolar invertase activities (per g FW) were always greater in galls than leaves. In contrast, cell wall invertase activities per g FW started low in galls, but increased with time and were greater than those in leaves after 1 month. Gall growth was most closely related to vacuolar invertase activity, whereas leaf growth was correlated with both vacuolar and cell wall invertase activities. In separate correlational studies of aphid fecundity and invertase activities, cell wall invertase activity per gall accounted for 15-21% of the variation in offspring per gall. Gall dry weight explained more of the variation in offspring per gall (34.2%) than did gall volume (17.8%), a likely indication of the importance of sink strength over the life of the gall. Increased invertase activity probably underlies the enhanced sink strength commonly observed in galls. Hormaphis hamamelidis fundatrices appear to maximize reproductive output by eliciting multiple beneficial responses from their hosts. Initial gall growth correlated positively with high vacuolar invertase activity, while later in the season aphid fecundity correlated positively with cell wall invertase activity.