Natural enemies have not been shown previously to play a significant role in regulating thrips in field crops. Most thrips are r-selected with population attributes that are believed to result in thrips outstripping the capacity of natural enemies to regulate thrips populations. In replicated field experiments conducted in 1995, insecticide applications to peppers were observed to increase rather than decrease populations of Frankliniella spp. thrips in flowers. We conducted experiments in 1996 and 1997 to determine if this phenomenon resulted from exclusion of predators. Species of thrips abundant in the pepper flowers were Frankliniella occidentalis (Pergande), F. tritici (Fitch), and F. bispinosa (Morgan). The only common predator in the flowers was Orius insidiosus (Say). Different treatments of biological and synthetic insecticides were used which altered the population densities of prey and predator. Covariate analyses over time were used each year to separate insecticide and predator effects on thrips populations. The predator significantly suppressed populations of all three thrips species. F. occidentalis and F. tritici reproduced in pepper flowers, but most of the larval thrips in untreated plots were F. occidentalis. Near extinction of F. occidentalis adults and larvae occurred within days after predator:prey ratios reached 1:40. Exclusion of the predator with synthetic insecticides resulted in continued abundance of thrips. We conclude that O. insidious was an effective predator that suppressed populations of Frankliniella spp. in field peppers during a period when thrips were rapidly colonizing and developing in the flowers. Species of Orius are abundant predators nearly worldwide, and the role of density dependence in regulation of thrips populations should be reevaluated.
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