A three-year field study was carried out to investigate proximate and ultimate influences on oviposition site selection by Strobilomyia laricis Michelsen and S. viaria (Huckett) (Diptera: Anthomyiidae), two sympatric flies whose maggots develop in young seed cones of eastern larch, Larix laricina (Du Roi) K. Koch. In 1990 and 1991, when the cone crop was light, the egg distribution at peak egg lay was uniform for both species, suggesting that females of both species recognized and avoided oviposition sites previously occupied by conspecifics. Strobilomyia viaria, which emerged two weeks after S. laricis, also avoided ovipositing on seed cones occupied by S. laricis. Thus, the egg distribution of S. laricis determined, to a large extent, the distribution of S. viaria. In 1992, a year when the cone crop was very light, and egg densities per cone were high, ovipositing females of both species apparently preferred cones with few versus many conspecifics. Host size also influenced oviposition site selection in S. laricis and S. viaria. In two of three years, both species selected longer cones for oviposition. A significant proportion of the variation in egg densities of both species was attributable to trees but not to cone density per tree. Egg distribution was influenced occasionally by cone phenology and cone location within tree crowns.
During the only year when egg densities per cone were high, preference for unoccupied oviposition sites resulted in increased larval survival, and in one of the other two years, preference for longer cones resulted in greater weight by the end of larval development, suggesting that oviposition preferences were adaptative. Cone phenology (at the time of oviposition) and position (crown aspect or level) had no significant influence on larval weight. Our results are consistent with the hypothesis that oviposition preference and offspring performance should be closely linked when females oviposit on the host in which their offspring must complete development.