Cottonwood leaf beetle (Coleoptera: Chrysomelidae) performance in relation to variable phytochemistry in juvenile aspen (Populus tremuloides Michx.)

Larval performance of the cottonwood leaf beetle, Chillsomela scripta F., was evaluated in relation to genetic variation in phytochemical characteristics among first year micropropagated ramets of five aspen clones (Populus tremuloides Michx.). Foliage from the juvenile ramets used in this experiment exhibited moderate variation in nitrogen, phenolic glycosides, and condensed tannin concentrations among clones, and overall, had very high levels of phenolic glycosides (15-22% dry weight) and low levels of condensed tannins (4-6% dry weight). Results from performance assays indicate that genetic differences among aspen clones resulted in only marginal differences in larval performance of this specialist leaf beetle. Although tannin levels were quite low in the juvenile trees, larval growth rate was reduced by 30% with increasing condensed tannin concentrations (R-2 = 0.209). Recent evidence suggests that aspen undergoes ontogenetic shifts in foliar concentrations of secondary metabolites resulting in decreased phenolic glycoside and increased condensed tannin concentrations as trees age. The high phenolic glycoside and low condensed tannin phytochemical profile of juvenile aspen appears to make it an ideal host for cottonwood leaf beetles.