Genetic Variation Among Plants and Insect Community Structure: Willows and Sawflies

Abstract

The effects of both genotype and spatial position of host plants on density, proportion, and total community structure of herbivore species were experimentally tested for a guild of gall—forming sawflies on their host plant, the arroyo willow, Salix lasiolepis. Genetic differences among individually potted stem cuttings from 6 and 12 willow plants were found to have a significant effects on the densities of each of the four sawfly species and on their proportional representation on willow clones. Analyses also showed that the total community structure (proportions of species) varied among clones. Proportional overlap values showed that two the of willow clones differed markedly from the others regarding the predominant species on them. However, phenotypic correlations showed that the sawfly species were positively correlated among plants during 2 yr. Genotypic correlations were positive and mostly significant in 1985, but were mostly nonsignificant in 1986. Estimates of broad—sense heritability indicated that between 20 and 50% of variation in the density of two species, the stem galler (Euura lasiolepiis) and the leaf folder (Phyllocolpa sp.), was due to clone genotype, whereas only 9—28% of variation in the density of the leaf galler (Pontania sp.) and the petiole galler (Euura sp.) was due to clone genotype. Environmental variation due to spatial position of plants contributed little to variation in density of species in experiments in 1985, but was an important source of variation in 1986 for three species. As a consequence species proportion for all species varied significantly among rack positions of the willow pots. In 1986, clone x position interactions were not significant for species density or for species proportion, with the exception of the leaf folder, which showed marginal significance. Thus, there was little evidence of genotype x position interactions. These studies suggest that plant genetic variation may be an important variable influencing community structure of phytophagous insects on host plants with plant populations. Plant morphological traits that are important to sawfly oviposition in the field (e.g., mean shoot length) differed significantly among willow clones in both years and were correlated with the variation in density of three of the four sawfly species among all plants in both years. In 1985, correlations between shoot length and species density based on clone means were significant for all four species, but in 1986, none of the correlations was significant, although for three of the four species correlation coefficients were similar in magnitude to those from 1985. Species density of the leaf folder, stem galler, and petiole galler were also correlated with leaf length among plants and clones in 1985. These data tentatively suggest that there are positive genetic correlations between plant characteristics and susceptibility to sawfly galling