Complementary roles of two classes of defense chemicals in white spruce (Picea glauca) against eastern spruce budworm (Choristoneura fumiferana)

As a boreal tree species, the white spruce is under a constant threat by spruce budworm outbreaks. Studies in white spruce reported two classes of defense chemicals, monoterpenes and phenolics; currently it is unknow whether both classes are complementary to one another or one plays a more critical role against spruce budworm than the other. We characterized the variations in foliar monoterpenes and phenolics of white spruce families from a progeny trial site in western Canada and cluster trees in two phenotypes based on their monoterpene profiles. Using the monoterpene and phenolic profiles of representative families in each phenotype, we amended media either with phenolics alone or with monoterpene and tested growth and development of spruce budworm on both types of diets. Defense phenotypes and families differentially affected the budworm responses depending on the proportion of monoterpenes and phenolics in a particular phenotype and family. Specifically, the most toxic families that caused 100% budworm larval mortality had the highest proportional monoterpenes, ranging from 40% to 60%; in these families the proportion of phenolics ranged from 40% to 60%. In contrast, families with the relatively higher proportion of phenolics (³62%) and lower proportion of monoterpenes (£40%) were less toxic or not toxic to budworms. These results altogether suggest that the proportions of both monoterpenes and phenolics appear to be important in overall defenses in white spruce trees but the proportion of monoterpenes in the profile appears to determine the overall toxicity of white spruce foliage the spruce budworm.