Defoliation patterns and genetics of insect resistance in cottonwoods

In 1995, an outbreak of a leaf beetle, Phratora californica Brown (Coleoptera: Chrysomelidae), began in a three-generation Populus trichocarpa Torr. & Gray × Populus deltoides Bartr. pedigree planting near the lower Columbia River in Oregon. This outbreak provided us with an opportunity to assess leaf beetle feeding patterns and the genetics of cottonwood resistance to defoliation. We developed a method for estimating damage levels by training personnel to visually estimate percent damage in leaf samples. Digital image analysis was used to measure damage to the leaves used in the training. Based on a sample of 300 trees from 100 genotypes, herbivory was found to be greatest in the upper canopy and in the fall. Broad-sense heritability was estimated to be 0.88 and 0.80 for July and October, respectively, demonstrating that resistance to P. californica is under relatively strong genetic control. Resistance in the F2 likely came from the P. trichocarpa parent, because this parent was less susceptible, on average, than the P. deltoides parent. However, the difference between parents was not great, and any further genetic analysis of resistance to Phratora californica should employ crosses between individuals with more strongly contrasting phenotypes.