Gall-inducing insects require close phenological synchrony with their host plants. It has been hypothesized, but seldom shown, that this is because, to induce galls the insects require physiologically active plant tissue, which exists only during a limited stage of plant development, forming a phenological window for gall induction. We examined this hypothesis in the system of a shrub, Aucuba japonica Thunberg, and its specialist fruit gall midge Asphondylia aucubae Yukawa et Ohsaki. Female midges lay eggs into young host fruit and stimulate the integument, a reactive tissue in the fruit, to induce galls. Our results showed that the lifetime of the integument defines the end point of the phenological window. The integument was rapidly degenerated as fruit matured, and eventually disappeared. The degeneration of the integument began simultaneously with the adult emergence season of the midge, and was greatly accelerated in the latter half of the adult midge season, forming an apparent end point of the window. We experimentally caged host infructescences to keep them free from midge oviposition, and released one field collected ovipositing female into each cage. When the treatment was performed before the end point of the window, gall induction success of the midge was much higher. The integument, however, does not define the start point of the window because gall induction success was not lower when the midge was released very early in the adult emergence season. We suggest that the start point is defined by another factor, likely the hard endocarp of the fruit.