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The biology and infestation patterns of three seed-feeding insects were investigated in Platte thistle populations (Cirsium canescens Nutt.). The insects have been identified as Paracantha culta (Wiedemann), Orellia occidentalis (Snow), and Homoeosoma stypticellum (Grote). Factors determining infestation patterns were studied by collecting 75 post-bloom heads from three Sandhills sites on June 16, 1977. Mean number of insects per head was 0.55 for P. culta, 4.3 for O. occidentalis, and 3.3 for H. stypticellum. Insects were present in 97 percent of the heads. Competition between species was reduced by differences in time of active growth of the insects within a head and by their segregating spatially into different heads.