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The role of interspecific competition as a regulating force in natural populations has been controversial, especially for phytophagous insect communities. A series of manipulative experiments using enclosure cages were conducted to evaluate the role of interspecific competition between a weevil and a fly, two seed feeding agents released against spotted knapweed in North America. The fly, an inferior biological control agent, was the superior competitor. Consequences of the antagonistic interaction included reduced seed destruction compared to if just the weevil was released on its own. The role of plant phenology on insect herbivore density was also assessed. The implications of phenologyinduced variation in insect density were evaluated with respect to competition between the fly and the weevil and were found to be important. Hypotheses of four plantmediated mechanisms of interspecific competition were also tested. Results support resource preemption as a competitive mechanism. / vii, 97 leaves ; 29 cm