Red alder defense mechanisms against western tent caterpillar defoliation

Red alder (Alnus rubra Bong.) is a tree of high economic and ecological importance, yet is subject to severe defoliation during episodic outbreaks of tent caterpillars (Malacosoma spp.). We evaluated variation in western tent caterpillar (Malacosoma californicum Packard, 1864) (WTC) resistance among and within red alder populations and clones and investigated potential defense mechanisms. Bioassay feeding trials were conducted with WTC on 20 red alder clones from 10 provenances (two clones per provenance). Phenology and quality of red alder leaves were analyzed to determine whether bud burst, leaf chemical content, water content, or physical traits are determinants of WTC preference. Alder clones differed in percent leaf area eaten by WTC and in leaf defense traits. The concentrations of total phenolics, condensed tannin, and the diarylheptanoid oregonin were negatively correlated with the percent leaf area eaten by caterpillars. In addition, a potential threshold was observed above which the concentration of each of the chemicals appeared to reduce WTC feeding. In particular, foliar concentrations of oregonin >20% of leaf dry mass were consistently associated with reduced feeding. The effects of oregonin concentration in red alder leaves on tent caterpillar feeding is a novel finding.