Gall midge larvae are associated with specific fungal symbionts in ambrosia galls. We found that the weevil Wagnerinus costatus (Hustache) (Coleoptera: Curculionidae: Ceutorhynchinae) frequently attacked ambrosia galls formed by the midge Asphondylia diervillae Felt (Diptera: Cecidomyiidae: Asphondyliini) on the buds of Weigela hortensis (Sieb. et Zucc.) (Caprifoliaceae) in three locations in central Japan. From late April to early May, female weevils laid single eggs in fully developed ambrosia galls, and upon hatching weevil larvae bored into these galls. The rate of parasitism by weevils was consistently high, from 90.2 to 94.5% in each gall midge population. Weevil larvae fed on the hyphae of fungal symbionts as well as on gall tissue. In most cases, parasitism by weevils did not influence the survivorship of A. diervillae or its parasitoids, because oviposition and feeding by the weevil occurred when gall midges had already developed into mature larvae or pupae within hard chambers surrounded by fungal hyphae. Although 11.4–56.3% of gall midge pupae died, 80.0–86.1% of this mortality was not due to parasitism by weevils but to parasitism by hymenopteran parasitoids [Pseudocatolaccus sayatamabae Ishii (Pteromalidae), Tetrastichus spp. (Eulophidae), and Bracon asphondyliae (Watanabe) (Braconidae)]. Weevil eggs were uniformly distributed among galls. A proportion of the weevils were attacked by the egg endoparasitoid Anaphes sp. (Mymaridae) and by unidentified larval ectoparasitoids. In rare cases of multioviposition by female weevils, interference competition among the larvae could occur. As a result of larval competition and parasitism, two or fewer larvae matured per gall; therefore, female weevils evidently avoided multiple oviposition, resulting in a uniform egg distribution.