Patterns of Resource Use by a Drosophilid (Diptera) Leaf Miner on a Native Crucifer

Abstract

Distribution and damage of Scaptomyza nigrita Wheeler on its host (bittercress, Cardamine cordifolia A. Gray), a native perennial crucifer, were examined over two growing seasons in relation to leaf position. Concentrations of defensive compounds (glucosinolates) and of nutritive compounds (total nitrogen, free amino acids, soluble carbohydrates) were also examined. The fly-host plant relationship was studied in sun and shade habitats at two sites. Oviposition and leaf-mining damage were concentrated on the lower central leaves of a stem in both habitats. These mature leaves have lower glucosinolate concentrations than new leaves. Adult densities and larval feeding damage were consistently and significantly greater on plants in the sun than on those in the shade. Higher S. nigrita densities in the sun habitat and slightly higher soluble carbohydrate concentrations in sun leaves at the beginning of the growing season, rather than variation in defensive glucosinolate levels, are the most likely mechanisms determining higher levels of leaf mining on host plants in the sun habitat.