Residual activity of spinosad applied as a soil drench to tomato seedlings can be a driver of Tuta absoluta insecticide resistance evolution

Abstract

Abstract Tuta absoluta (Lepidoptera: Gelechiidae) is difficult to control by means of foliar insecticides, partly due to the endophytic feeding behavior of larvae. The biopesticide, spinosad, is applied as foliar spray for control of T. absoluta and has systemic properties when applied as a soil drench to the growing medium of tomato plants. This study was done to determine the (i) instar-dependant tolerance of larvae to spinosad, (ii) efficacy of spinosad drench application for the control of larvae, (iii) residual period of systemic activity of spinosad in leaves and fruit after drenching, and (iv) effect of spinosad drenching on tomato plant growth parameters. The lethal concentrations (LC50 and LC80) of spinosad administered as soil drench were estimated by exposing second-instar larvae to leaves of plants drenched with different concentrations of spinosad. Systemic persistence of spinosad in leaves of these plants was determined by means of of liquid chromatography-tandem mass spectrometry. Larval mortality data were recorded over time after drenching. Although the spinosad concentration in leaves decreased over time, high larval mortalities were recorded for the duration of the experiment, which lasted 25 days (equivalent to one T. absoluta generation). This study provides evidence of the systemic properties of spinosad and its efficacy for control of T. absoluta larvae over a prolonged period. However, drenching this insecticide violates the recommendation of the Insecticide Resistance Management Committee to avoid treating consecutive insect generations with the same mode of action and can therefore be a driver of insecticide resistance evolution.