Stability of Resistance to Deltamethrin in Tomato Leaf Miner, Tuta absoluta (Lepidoptera: Gelechiidae) and Control Failure Likelihood

Resistance to insecticides is one of the several factors responsible for control failures of Tuta absoluta (Meyrick), but this phenomenon has been very little studied to date. The aims of the present study were to: 1) assess the susceptibility of seven greenhouse populations of T. absoluta to deltamethrin, in comparison with the most susceptible population 2) assess the likelihood of deltamethrin control failures in the surveyed populations. 3) determine the stability of resistance to deltamethrin in greenhouse populations of Tuta absoluta. Bioassays were done using 2nd instar lande and dipping method to determine the resistance level. Calculated resistance ratios varied from 1.25- to 13.91 fold. Additionally, the control failure likelihood, the narrow meaning of insecticide resistance, was surveyed using the insecticide recommended label rate as the discriminating concentration, and the 80% efficacy in greenhouse populations of the tomato leaf miner. The 80% mortality is the lowest expected efficacy level without control failure due to resistance to insecticides. Mortality due to treatment with deltamethrin at the label rate was considered significantly lower than 80%, when the recommended label rate was lower than the lower threshold of the 95% fiducial limits of LC80. Deltamethrin exhibited the expected control efficacy (between 85% and 100%) against the tomato leaf miner populations at the recommended label rates with the exception of two populations that were resistant to this insecticide. The significant decrease in resistance ratio over seven generations without any selection pressure indicated that resistance to deltamethrin remained obviously unstable.