Variation in P450-mediated fenvalerate resistance levels is not correlated with CYP337B3 genotype in Chinese populations of Helicoverpa armigera

Abstract

Metabolic resistance to synthetic pyrethroids in Helicoverpa armigera has recently been associated with the chimeric cytochrome P450 enyzme CYP337B3. One variant of the latter, CYP337B3v1, accounts for 40–50 fold resistance to fenvalerate in an Australian population while a second variant, CYP337B3v2, has been associated with ~7 fold resistance to cypermethrin in a Pakistani population. Here we show that ~250–1200 fold resistance to fenvalerate in populations of the species from northern and northwestern China is largely due to P450-based metabolism, and that CYP337B3v2 is also at high frequency in these populations but absent in a susceptible control strain. However we find little correlation between the level of resistance and CYP337B3v2 frequency, either across the resistant populations studied, or over time within them. While there is variation between populations in the levels of CYP337B3v2 expression, this is not correlated with the level of resistance either. These data suggest that much of the variation in the level of fenvalerate resistance in China is explained by P450s other than CYP337B3. We also find that both the level of resistance and CYP337B3v2 frequency decline in field populations transferred to the laboratory and remained there without fenvalerate exposure, suggesting a fitness cost to both characters in the absence of the pesticide pressure. However the declines in the two characters are not well correlated across populations, again consistent with a large contribution to the variation in resistance levels from factors other than CYP337B3.