Oviposition site selection and intraspecific competition influence larval survival and pupal weight of Strobilomyia neanthracina (Diptera : Anthomyiidae) in white spruce

Densities of coniferous seed cones fluctuate greatly from year to year, and thus cones may be a limiting resource for obligate endoparasites of seed c ones during years of low cone abundance. We examined this for the spruce co ne maggot, Strobilomyia neanthracina Michelsen, whose larvae feed entirely within the cones of white spruce, Picea glauca (Moench) Voss. During a year of very low cone densities, we tested the predictions that egg density, la rval survival, and pupal weight would be positively related to cone size, a nd that larval survival and pupal weight were negatively related to the num ber of fly larvae developing in a cone. Although previous studies demonstra ted that S. neanthracina avoids ovipositing on cones containing a conspecif ic, we observed that more than 90% of the cones examined contained three or more eggs, This suggests that cones were a limiting resource for females. The number of eggs laid per cone increased with cone size but the slope of this relationship varied significantly among clones of trees in this seed o rchard. Survival of fly larvae and weight of pupae were positively related to cone size and negatively related to the number of hatched eggs per cone in at least one of two samples of cones. Reductions in pupal weight with de creased cone size and increased egg densities suggests that exploitation co mpetition occurred among third (last) instar larvae. However, because most larval died during the second instar, before consuming much food, most larv al mortality was probably due to interference competition among larvae. Tre e clone significantly influenced larval survival in one cone sample and the relationship between survival and number of hatched eggs per cone in anoth er. Our results support a link between the preference by spruce cone flies for larger cones and the performance of their progeny. Reduced larval survi val when more than one individual developed in a cone indicates that select ion of unoccupied cones, when available, is adaptive.