Effect of Plant Resistance, Competition, and Enemies on a Leaf-Galling Sawfly (Hymenoptera: Tenthredinidae)

Abstract

The relative importance of three sources of larval mortality (host plant resistance, intraspecific competition, and natural enemies) was evaluated for a population of leafgalling sawflies, Pontania sp., near P. pacifica Marlatt, on arroyo willow, Salix lasiolepis Bentham, at a site in Flagstaff, Ariz., from 1981 to 1984. The ratio of the percentage of sawfly larvae killed by natural enemies to the percentage lost to host plant defenses (i. e., abortion, defined as an aborted formation of a gall resulting from death of the egg or small larva) was 1.53:1. Mortality from abortion was divided into a host plant resistance component (constitutive abortion, e.g., a plant resistance factor preventing establishment of the egg or small larva) and an intraspecific competition-mediated component (because abortion increased as within-leaf gall densities rose). Natural enemies caused twice as much mortality as plant resistance and 6.36 times greater mortality than intraspecific competition. These results support the hypothesis that third-trophic-level effects exert a stronger selective pressure on many insect populations than competition for limiting resources. There was evidence that intraspecific competition for limiting food resources occurred for these Pontania sp. sawflies; increased within-leaf gall densities had a density-dependent, linear, negative effect on gall size, and consequently, on larval biomass because sawfly size was positively correlated with gall size. However, only about 3% of the variation in gall size was explained by within-leaf gall density. Thus, gall (and larval) size were only weakly affected by intraspecific competition.