Variation in leaf trichomes and nutrients of Wigandia urens (Hydrophyllaceae) and its implications for herbivory

Leaf trichome variation was studied in a population of Wigandia urens (Hydrophyllaceae) in relation to water availability, diversity of herbivorous insects, and grazing rates. Plants of W. urens have glandular and urticant (stinging hairs) trichomes, and it is possible to distinguish two types of leaves: "smooth" leaves with only glandular trichomes and "bristly" leaves with both types of trichomes. Density of urticant trichomes was negatively correlated with weekly mean precipitation. Fourteen species of insects in five different orders feed on leaves of W. urens throughout the year. Grazing rates varied according to type and age of leaves. Grazing rates were lower in smooth than bristly young leaves but the rates were similar for mature leaves. Secondary metabolites, screened using qualitative methods, showed similar classes of compounds in both types of leaves. Bristly leaves had significantly higher concentrations of nitrogen, phosphorous and water and these nutritional differences were well correlated with the differences in grazing rates between bristly and smooth leaves.