The potential for interspecific competition between two abundant defoliators on oak: leaf damage and habitat quality

. 1. The pedunculate oak, Quercus robur L., suffers high annual levels of spring defoliation in Wytham Woods, Oxon. The two major defoliators, Tortrix viridana L. and Operophtera brumata L., sometimes reach high enough densities to completely defoliate trees. 2. Experiments with larvae of both species enclosed on oak trees in Wytham Woods suggest considerable potential for intra‐ and interspecific competition between T. viridana and O. brumata. 3. T. viridana and O. brumata consume equivalent amounts of leaf tissue per unit caterpillar density. Despite this, O. brumata is the superior competitor, measured by mortality during the larval feeding period. 4. Evidence is presented that T. viridana larvae depend upon the protected microclimate of leaf rolls to maintain water balance, and that leaf damage may disrupt the construction of, and microclimate within, leaf rolls. O. brumata may therefore be competitively superior because it indirectly interferes with the water balance of T. viridana. 5. The importance of interspecific competition in the dynamics of T. viridana and O. brumata populations is discussed. Stochastic mortality factors acting on first instar larvae of both species probably make intense competition a rare event.