A margarodid scale,Matsucoccus resinosaeBean&Godwin (=M. matsumuraeKuwana), and an adelgid,Pineus boerneriAnnand, are serious introduced pests of red pine,Pinus resinosaAiton, in parts of New England. Studies in a field plot revealed that both insects had significantly greater survival and fecundity onPinus resinosathan onPinus thunbergianaFranco, their native host in Asia. The performance ofP. boernerifeeding on both the branches and needles ofP. resinosawas significantly reduced by the presence ofM. resinosae, which fed only on the branches. The presence ofP. boernerihad no significant reciprocal effect on the fitness ofM. resinosae. Interspecific competition was a significant limiting factor forP. boerneri, and the needles did not provide a refuge from competition. In each of nine pine stands sampled in Connecticut, densities of adelgids and scales during a 7-yr period rose, but then fell sharply as trees became visibly injured. Abundance ofP. boernerion bark and on needles declined sharply in six pine stands following invasion byM. resinosae. In three of these stands,P. boerneriwas excluded by its competitor within 3 yr. Although competition occurred among these insects, it did not limit the growth of pest populations to a level at which the host could benefit from this antagonisitic interaction.