Susceptibility of Oriental fruit moth (Lepidoptera: Tortricidae) to two pyrethroids and a proposed diagnostic dose for field detection of resistance

Laboratory colonies of Oriental fruit moth, Grapholita molesta (Busck), were reared on ‘Gala’ apples and lima bean diet. Neonates from these colonies were placed on wheat germ diet containing a range of concentrations of esfenvalerate or lambda-cyhalothrin. Mortality was assessed after 96 h. For a long-term laboratory colony, LC₅₀’s of esfenvalerate and lambda-cyhalothrin were 0.35 and 0.12 ppm, respectively, for progeny of insects reared on apples. For a colony established from Calhoun County, IL in 2007, LC₅₀’s of esfenvalerate and lambda-cyhalothrin were 0.37 and 0.10 ppm, respectively, for progeny of insects reared on apples. LC₅₀’s of these insecticides did not differ significantly from these values for either colony when progeny of insects reared on lima bean diet were tested. We observed no consistent evidence of pyrethroid resistance in the Calhoun colony after laboratory culture for 21-23 generations. We estimated the dose-response relationship for esfenvalerate applied topically in 1 µl of acetone to male moths and estimated the LD₉₉ to be 0.022 µg per moth. Application of 0.022 µg esfenvalerate per moth to ca. 600 male moths from two putatively susceptible populations resulted in mean survivorship approximately equal to the expected level of 1.0%. Application of the same dose to ca. 375 moths captured in two Calhoun County orchards with histories of pyrethroid use resulted in mean survivorship of 9.4% and 82%. We propose that 0.022 µg of esfenvalerate in 1 µl of acetone can be used as a diagnostic dose for monitoring pyrethroid resistance.