Residual contact vial method for the rapid on-site detection of insecticide resistance in Thrips palmi

A residual contact vial plus water (RCVpW) bioassay method, in which water was supplemented to minimize control mortality, was established to monitor insecticide resistance in field populations of the melon thrips, Thrips palmi. In the RCVpW, median lethal doses (LD₅₀) of six insecticides commonly used in T. palmi control, were determined at 8 h post-treatment, using a susceptible RDA strain according to the RCVpW protocol. Diagnostic doses for on-site resistance monitoring of the six insecticides, which were determined as doses two-fold higher than required to achieve LD₉₀ in the RDA strain, were in the range of 0.299 to 164.3 μg^?1 cm². Insecticide resistance levels in five field populations of T. palmi were evaluated to test the applicability of RCVpW in monitoring the pest. Although the RDA strain exhibited 100% mortality to diagnostic doses, field populations showed a reduced mortality in response to all test insecticides, indicating different degrees of resistance. In particular, all test field populations exhibited a significantly low mortality in response to spinosad, suggesting a wide distribution of spinosad resistance. Synergistic bioassay revealed that cytochrome P450-mediated metabolic factor is involved in spinosad resistance in the Korean population. Interestingly, an apparently reduced mortality to emamectin benzoate and chlofenapyr was observed in some field populations, perhaps suggesting uneven distribution of resistance to these insecticides in field populations. Our study showed that the RCVpW protocol can be employed both as an on-site resistance monitoring method for major thrip species, and in the selection of appropriate insecticides for their control.