Molecular Evidence for a Decline in Pyrethroid Resistance from the kdr Locus in Heliothis virescens

The tobacco budworm, Heliothis virescens, is an historically important pest of cotton that evolved high levels of pyrethroid resistance in the mid-1990s. Following their commercialization in 1996, transgenic cotton expressing Bt toxins were rapidly adopted throughout the Southern US to control H. virescens populations. This resulted in an overall decline in pyrethoid use for cotton production, and promoted a reversal of pyrethroid-resistance phenotypes in some, but not all, populations of H. virescens. Our research examined the molecular evidence for the reversal of H. virescens pyrethroid resistance in two important cotton-growing areas of the Southern US. We describe a PCR-based assay, similar to that of Park and Taylor (1993), that enabled us to rapidly and inexpensively examine temporal changes in the frequency of the L1029H mutation associated with pyrethroid resistance. We applied this assay to archived samples from Louisiana and Texas collected between the years 1997-2015. For both populations, the frequency of the resistance allele declined over time, suggesting that pyrethroid-susceptible phenotypes are becoming more prevalent in the Southern US. We then discuss the implications of these results for insecticide resistance management.