• Parasitoids;
  • Eurytoma;
  • Eurosta;
  • Solidago;
  • size-dependent mortality;
  • plant galls

Abstract. 1. Eurytoma gigantea Walsh is a specialist parasitoid of the tephritid gallmaker Eurosta solidaginis (Fitch).

2. In the natural environment the incidence of parasitism by Eurytoma is greater in small galls than in large ones.

3. Laboratory experiments demonstrated that small galls are not more frequently discovered; however, oviposition attempts on small galls were more likely to be successful.

4. Eurytoma spends much time probing galls too big to penetrate; this leads to a decrease in foraging efficiency when many large galls are present.

5. The chance of successfully penetrating a gall depends on the thickness of the gall wall and the length of the parasitoid's ovipositor.

6. A simulation model was constructed which shows that a gallmak-er's chance of being parasitized depends on gall size, the number of parasitoids that discover the gall, and their ovipositor lengths.