Submitted to: Canadian Journal of Forest Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 17, 1999
Publication Date: N/A
Interpretive Summary: Cottonwood trees are currently grown in highly managed plantations as a source of paper pulp, fuel and structural wood. As the demand for these products increases, so does production. Systems for managing pests are needed for this newly emerging crop, especially systems with low pesticide use. Cottonwood varieties have already been developed using plant breeding techniques and understanding the genetics of insect resistance would help scientists breed for pest-resistant varieties. A small bluegreen leaf beetle, called Phratora californica, is a new cottonwood pest that can seriously damage leaves. This beetle first became a problem along the lower Columbia River in Oregon. Large numbers of beetles were defoliating a breeding experiment involving a cross between two cottonwood species, black cottonwood and eastern cottonwood. In order to select trees for breeding, we first need a method for quantifying damage. Three methods for sampling and measuring damage were tried, and a visual estimate of percent damage t leaf samples from throughout the tree was found to be the most efficient. We sampled for damage early in the season when insect larvae were present, and again in the fall when only adults were feeding. We used the breeding experiment which included the two parent trees, two first generation offspring, and 100 in-bred, second generation offspring, to measure damage and estimate the heritability of insect resistance in this family. Heritability was 0.8-0.88. Levels this high mean that genetics play a major role in determining resistance to beetle damage. In order to breed for resistance, strong genetic control is necessary. The genes for resistance in this family likely came from the black cottonwood. Crosses using this species may provide more resistance than crosses using eastern cottonwood.
The genetic basis for resistance to insect herbivores in Populus is relatively unstudied. In 1995 an outbreak of a leaf beetle, Phratora californica (Coleoptera: Chrysomelidae), began in a three-generation Populus trichocarpa x P. deltoides pedigree planting near the lower Columbia River in Oregon. Assessing the level of foliar damage sustained by ytrees in this cross, we were able to estimate the heritability of resistance. Two methods for estimating damage were compared in 1995. A visual, categorical, estimate was only moderately correlated with a more quantified estimate, percent leaf area damaged, that was based on digital image analysis. In the fall of 1995, we found leaf age (or position) significantly affected the degree of damage caused by Ph. californica. Young leaves, with a leaf plastochron index (LPI) of 2, sustained more feeding damage than older leaves at LPI 5 and LPI 10. In contrast, older leaves sampled in the fall of 1996 were more damaged than leaves at LPI 2. In 1996 herbivory was greater in the upper canopy than the lower, in July and October. The level of herbivory was greater in October than July. Clonal variation in susceptibility to beetle feeding varied significantly in 1996. 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 came from the P. trichocarpa grandparent because clones from this tree were less susceptible, on average, than from the P. deltoides tree. The difference between grandparents was not great and any further genetic analysis of resistance to Ph. californica should emply the progency of a cross between individuals with more strongly contrasting phenotypes.