Effects of genetic and environmental variation on resistance of willow to sawflies

An experiment was performed to test the effects of clone genotype, spatial variation (sites), temporal variation (years), and their interaction on the densities of four sawfly species that form galls on willow. Clone genotype had a significant effect on the densities of three species over both levels of spatial and temporal variation. Genotype was not important at one site, but was significant for 3 species at another site. Clonal heritability of resistance to the stem-galling sawfly was small when averaged over both sites and years, but for sites and years separately it varied from 0.16 to 0.35. The effect of year was not significant overall or at the Northland Press site but was significant at the Museum of Northern Arizona site. The site effect was also not significant overall, but was significant for the stem galler when years were considered separately. Clone genotype did not interact with year and site variation overall, or for years considered separately. When sites were considered separately, there was one significant year by clone interaction for the leaf folder. Year by site interactions were significant for 3 of 4 sawflies, with the Museum of Northern Arizona site having greater variation in sawfly densities between years than the Northland Press site. Year by site interaction seemed related to differences in density of the leaf folder and stem galler on nearby field plants, but this was not true for the leaf galler and the petiole galler. Phenotypic and genetic correlations of sawflies densities were generally positive and were significant half of the time, indicating that these species responded similarly to variation among clones.