Increased Susceptibility of Pyrethroid-Resistant Tobacco Budworm (Lepidoptera: Noctuidae) to Chlorfenapyr

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Abstract
Chlorfenapyr is a new pyrrole insecticide with a new mode of action. Its toxicity to adult Heliothis virescens (F.) is negatively correlated with cypermethrin toxicity by the equation y = 1.065x + 3.513 (r = 0.97), where x = log LC50 cypermethrin, y = log LC50 chlorfenapyr. Chlorfenapyr was most toxic to adults of a pyrethroid-resistant strain (LC50 = 70.1 µg per vial in adult vial tests at 48 h). The negative correlation was not observed in neonates fed with chlorfenapyr impregnated diet. Cross-resistance to chlorfenapyr in Pyrethroid-R larvae was 2.3-fold with the diet method, compared with 28-fold resistance to cypermethrin. Cross-resistance was only significant in diet tests. Chlorfenapyr was as effective as insecticides registered for cotton against H. virescens. Inheritance data showed that only 1 allele is necessary from the Pyrethroid-R strain to produce complete susceptibility to chlorfenapyr in adults. The susceptibility to chlorfenapyr is a dominant trait, and the resistance to chlorfenapyr is a recessive trait. In a backcross experiment, one-half the progeny were highly susceptible to chlorfenapyr and one-half were highly resistant, indicating segregation of a single factor. The mechanism for this effect could be caused by pleiotropy (i.e., a gene for pyrethroid resistance controls increased detoxication to cypermethrin while also increasing the susceptibility of chlorfenapyr). The pyrethroid-resistant gene is likely to be identical to, or closely linked to, the gene that gives increased susceptibility to chlorfenapyr.

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