Competition Versus Host Plant Phenotype in Species Composition: Willow Sawflies

Abstract

The influence of host plant phenotype and interspecific competition on the abundance of four willow sawfly species was tested on potted cuttings of the arroyo willow, Salix lasiolepis, in northern Arizona. Interspecific competition by a stem—galling sawfly, Euura lasiolepis, was demonstrated on a leaf galler, Pontania sp., a leaf folder, Phyllocolpa sp., and a petiole galler, Euura sp. The competitive influence of the stem galler on these species is likely to be absent at low stem gall densities. Density of the stem galler is spatially patchy, so competition should be variable among willow clones in the field. The three other sawfly species form galls that are distal to stem galls and/or do not affect the shoot in a way detrimental to the stem galler, therefore this competition appears to be asymmetrical, an "amensalism." All sawfly species exhibited significant differences in gall densities among potted, replicate willow clones under uniform environmental conditions. This suggests underlying genetic variation among clones in susceptibility to sawfly oviposition. Each sawfly species seemed to respond to variation among clone phenotypes idiosyncratically. Under experimental conditions, host plant phenotype accounted for 1.5 to 3.2 times as much variation in the density of subordinate species as did interspecific competition; under field conditions host plant phenotype is likely to play a more important role than competition in the community patterns of herbivorous insects.