Folivory on Baccharis halimifolia by larvae of the chrysomelid beetle Trirhabda bacharidis depresses densities of the most common insect herbivores on B. halimifolia : the leaf mining flies Amauromyza maculosa and Liriomyza trifolii, and the gall making fly Neolasioptera lathami. Larval T. bacharidis were excluded from B. halimifolia plants in a field experiment using Tanglefoot® pest barrier to stop larvae, which emerge from eggs laid in the plant bark, from feeding on the leaves. Densities of A. maculosa, L. trifolii, and N. lathami were significantly higher on plants from which larval T. bacharidis had been excluded than on controls. Laboratory experiments showed that larval T. bacharidis did not prefer to eat mined leaves and the miners within. Thus, observed density reductions are the result of exploitative competition with T. bacharidis larvae. Competition between T. bacharidis larvae and other folivores is probably frequent in this system because T. bacharidis larvae occurred on 69% of 172 B. halimifolia plants in the study areas and defoliated 68% of the available vegetation.