Patterns of relative densities of a dominant leafmining species,Camerariasp. (agrifoliellaBraun group), on eight trees ofQuercus emoryiTorrey were documented over five growing seasons. Relative abundances tended to remain constant among trees throughout the five growing seasons, and densities were consistently greater in the shaded regions of trees than in the sunny regions. To test the hypothesis that variation in leaf size affects colonization, dispersion, and survival ofCamerariasp., equal numbers of leafminers were introduced into field enclosures and allowed to colonize branches varying in mean leaf size. Densities of leafminers were lower and dispersion was more aggregated on small-leaved branches than on large-leaved ones. Mortality of larval leafminers was either inversely density-dependent or density-independent at the spatial scale of the branch. However, log linear analysis showed that survival of leafminers was a negative function of clumping of leafminers among and within leaves. Small-leaved branches and trees appear to be less susceptible to leafminer attack because adult females deposit fewer eggs overall, and eggs are more frequently laid singly on leaves of inadequate size for development or are more frequently clumped on leaves with competing conspecifics. It is suggested that variation in leaf size, coupled with variation in other morphological and phenological traits, acts as a ceiling on densities ofCamerariasp. on Emory oak at the study site by altering dispersion such that interference and exploitative competition increase.