Observations of population increases and injury by spider mites (Acari: Tetranychidae) on ornamental plants treated with imidacloprid

A series of experiments were conducted to investigate previous observations that soil applications of imidacloprid resulted in increased spider mite populations and spider mite injury on ornamental plants. Marigolds, Tagetes erecta L., treated with either a soil drench or soil granular formulation of imidacloprid were observed to sustain significantly greater damage from twospotted spider mite, Tetranychus urticae Koch, than untreated plants. Populations of the honeylocust spider mite, Platytetranychus multidigituli (Ewing), were as much as 3-4 times higher on honeylocust, Gleditsia triacanthos L., which had received an imidacloprid soil drench, compared with control plants. Effects on spider mite populations or injury were not observed in greenhouse trials involving marigolds, suggesting that this effect was not caused by either hormoligosis or phytotoxicity. However, mortality of Orius tristicolor (White), a hemipteran predator of spider mites, was observed to increase significantly when confined with leaves collected from plants that had received soil treatments of imidacloprid. The adverse effects to predators may be an important reason for the increase in spider mite injury and abundance resulting from soil treatments of imidacloprid in Colorado.