Systemic use of spinosad to control the two-spotted spider mite (Acari: Tetranychidae) on tomatoes grown in rockwool.

Spinosad is a reduced-risk insecticide derived as a fermentation product from the soil actinomycete Saccharopolyspora spinosa. It is toxic by ingestion and contact and has a unique mode of action on the insect nervous system. Spinosad exhibits a high degree of selective toxicity towards the insect orders Lepidoptera, Diptera and Thysanoptera, but is less toxic to many beneficial arthropods. To determine if spinosad could be valuable as an alternative acaricide for the control of Tetranychus urticae, laboratory toxicity experiments with leaf-disk bio-assays were performed on a laboratory susceptible and several resistant strains. LC50 values were rather high in comparison with newly developed commercial acaricides. Surprisingly, when spinosad was applied to the roots of tomato plants in rock wool, excellent control of spider mites was obtained. Apparently, spinosad has systemic properties and quantities as low as 1 mg/plant could protect tomato plants from mite infestation. Different substrates with varying percentage of clay and organic matter were tested in comparison with rockwool and showed that sufficient control was restricted to the rockwool substrate. Consequently, a dose-response experiment with tomato plants grown in rockwool was set up. The persistence of spinosad toxicity when applied via the roots was determined, and pointed to a long lasting control (up to 30 DAT). Spinosad amounts in leaves after systemic application were determined with an immunological technique to quantify spinosad uptake. Correlations between mite control, spinosad uptake and leaf concentrations can be helpful to determine the necessary dose in field situations.