Rapid evolution of pesticide resistance via adaptation and interspecific introgression in a major North American crop pest

Insect crop pests threaten global food security. This threat is amplified through the spread of non-native species and the evolution of pesticide resistance, which can be introduced to a population though de novo mutation or gene flow. We investigate these processes in an economically important noctuid crop pest, Helicoverpa zea, which has rapidly evolved resistance to several pesticides. Its sister species Helicoverpa armigera, first detected as an invasive species in Brazil in 2013, introduced the pyrethroid resistance gene CYP337B3 to South American H. zea via introgression. To understand whether this contributes to pesticide resistance in North America, we sequenced 237 H. zea genomes across 10 sample sites in the US. First, we report H. armigera introgression into the North American H. zea population. Two individuals sampled in Texas in 2019 carry H. armigera haplotypes in a 4Mbp region containing CYP337B3. Second, we show that the remarkable dispersal ability of H. zea results in a panmictic North American population. Third, we detect signatures of selection in non-admixed H. zea, identifying a selective sweep at a second pesticide resistance locus with a similar name: CYP333B3. We estimate that its derived allele conferred a [~]4.9% fitness advantage and show that this estimate explains independently observed rare nonsynonymous CYP333B3 mutations approaching fixation over a [~]20-year period. We also detect putative signatures of selection at a kinesin gene associated with Bt resistance. Our results show that pesticide resistance in H. zea evolved rapidly and recently via two independent mechanisms: interspecific introgression and rapid intraspecific adaptation.