INTRASPECIFIC COMPETITION AND FACILITATION BY A SHOOT-GALLING SAWFLY

Euura lasiolepsis, which forms galls on arroyo willow Salix lasiolepsis, competed for oviposition sites where larval survival was highest. The probability of larval survival is positively related to shoot length. At low densities only the most rapidly growing shoots were attacked. Euura discriminates against ovipositing on sites with oviposition scars. As sawfly density increased and most high quality oviposition sites became scarred the range of shoots attacked increased to include slower growing shoots. Competition for oviposition sites occurred at all densities. Individual female sawflies oviposited significantly fewer times over their lifetime at high sawfly densities. At high densities, when oviposition scars limited access to rapidly growing shoots, females retained eggs rather than oviposit on very short shoots. Presence of galls on a shoot facilitated further oviposition on that shoot. One or two oviposition sites are available at a time on a willow willow shoot, and a sequence of oviposition sites becomes available through time. Gall formation slightly increased shoot growth, increasing the probability that subsequently available oviposition sites would be attacked, but the number of galls on a shoot did not significantly alter the probability of larval survival. -from Authors