PROXIMATE AND ULTIMATE FACTORS INFLUENCING OVIPOSITION SITE SELECTIONBY ENDOPARASITES ON CONIFER SEED CONES - 2 SYMPATRIC DIPTERAN SPECIESON LARCH

A three-pear field study was carried out to investigate proximate and ultimate influences on oviposition site selection by Strobilomyia lari cis Michelsen and S. viaria (Huckett) (Diptera: Anthomyiidae), two sym patric flies whose maggots develop in young: seed cones of eastern lar ch, Larix laricina (Du Roi) K. Koch. In 1990 and 1991, when the cone c rop was light, the egg distribution at peak egg lay was uniform for bo th species, suggesting that females of both species recognized and avo ided oviposition sites previously occupied by conspecifics. Strobilomy ia viaria, which emerged two weeks after S. laricis, also avoided ovip ositing on seed cones occupied by S. laricis. Thus, the egg distributi on 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 de nsities per cone were high, ovipositing females of both species appare ntly 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 ovipositio n. A significant proportion of the variation in egg densities of both species was attributable to trees but not to cone density per tree. Eg g distribution was influenced occasionally by cone phenology and cone location within tree crowns. During the only year when egg densities p er cone were high, preference for unoccupied oviposition sites resulte d in increased larval survival, and in one of the other two years, pre ference for longer cones resulted in greater weight by the end of larv al development, suggesting that oviposition preferences were adaptativ e. Cone phenology (at the time of oviposition) and position (crown asp ect or level) had no significant influence on larval weight. Our resul ts are consistent with the hypothesis that oviposition preference and offspring performance should be closely linked when females oviposit o n the host in which their offspring must complete development.