Quantitative defense theory and patterns of feeding by oak insects.

Patterns of herbivory over a two year period on Quercus emoryi (Fagaceae) were correlated with seasonal and yearly changes in tannin and protein content. Quantitative defense theory predicts that tannin and protein content in apparent plants should be negatively and positively correlated, respectively, with degree of herbivory. Most herbivory occurred early in the growing season, but the pattern varied between the two years. Tannin and protein content sometimes varied negatively and sometimes positively with degree of herbivory; they did not consistently covary with herbivory. Protein content was positively correlated with herbivory in 1981-1982 but not in 1982-1983. Condensed tannin content was negatively correlated with herbivory in 1981-1982 but not in 1982-1983. Hydrolysable tannin content was positively correlated with herbivory activity. Multiple regression analyses indicated these phytochemical variables explained either no significant variation in herbivory (1982-1983) or did so in a fashion opposite (1981-1982) to the predictions of the theory of quantitative defense.Feeding by oak insects was not solely a function of seasonal changes in quantitative defenses and nutrients. Obviously, population dynamics of the insects are sensitive to factors other than phytochemistry of the trees and I discuss other factors that can influence patterns of herbivory.