Residual activity of spinosad applied as a soil drench to tomato seedlings for control of Tuta absoluta

Abstract BACKGROUND Tuta absoluta (Lepidoptera: Gelechiidae) is difficult to control by means of foliar insecticides, partly because of the endophytic feeding behavior of its larvae. The biopesticide spinosad is applied as a foliar spray for control of T. absoluta and has systemic properties when applied as a soil drench to the growing medium of tomato plants. The aims of this study were to determine the: (i) instar‐dependent 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. RESULTS The estimated LC50 value (Lethal Concentration at which 50% of the larvae died) differed between instars. The LC50 for second‐instar larvae (0.41 ppm) to spinosad was significantly lower than that for third‐ (0.64 ppm) and fourth‐instar (0.63 ppm) larvae. The LC80 value (Concentration at which 80% of the larvae died) for fourth‐instar larvae (2.48 ppm) was 2.6‐ and 1.7‐fold higher than that for the second‐ and third‐instar larvae, respectively. The spinosad concentration recorded in leaves at 25 days after treatment (DAT; 0.26 μg g−1) was significantly lower than that in leaves sampled at 3, 10 and 15 DAT. High larval mortalities were, however, recorded for the duration of the experiment, which lasted 25 days (equivalent to one T. absoluta generation). CONCLUSION Systemic spinosad effectively controlled T. absoluta larvae over a prolonged period. However, drenching this insecticide violates the recommendation of the Insecticide Resistance Action Committee to avoid treating consecutive insect generations with the same mode of action and can therefore result in the evolution of insecticide resistance. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.