Effect of two-spotted spider mites and miticides on alfalfa hay produced for a late May cutting

Abstract

Three miticide treatments (milbemectin, clarified neem oil, and chlorpyrifos plus dimethoate) were compared with an untreated check to obtain information on the effects of twospotted spider mite (Tetranychus urticae) feeding on alfalfa yields, quality and economics, as well as crop responses to miticides. Mite infested alfalfa was treated just prior to first irrigation after cutting in spring 2003, and numbers of spider mites and western flower thrips (Frankliniella occidentalis) were obtained at weekly intervals thereafter for the next seven weeks. Data for plant parameters (stem widths, numbers of leaves, internode lengths) were obtained at harvest, as were hay yields and quality. The milbemectin treatment resulted in quickest reduction of spider mites (88.5% at 5 days after application) and also resulted in significantly higher yields than the untreated check (0.18 tons of hay/acre), attributed to the longer internodes and resultant tallest plants and significantly thicker stems than the untreated check. Clarified neem oil and chlorpyrifos plus dimethoate treatments did not control spider mites as quickly as milbemectin and yields were increased by only 0.04-0.05 tons hay per acre in this experiment compared with the untreated check. These two treatments also differed in their effects on stem widths and internode lengths, indicating that some results noted were a result of interactions of miticides with alfalfa as well as spider mite control.