Soybean aphid management has relied heavily on foliar insecticides to protect yield since 2000. In 2016, performance issues have been documented in commercial fields and research plots in northern Iowa counties. With support from the Iowa Soybean Association, we are exploring insecticide resistance for soybean aphid in Iowa and describing the mechanism of resistance. Laboratory bioassays identified field-collected population’s evolved resistance to pyrethroids. We performed leaf-dip bioassays to determine the susceptibility of three field-collected and three laboratory populations with distinct phenotypes (i.e., Biotypes 1, 3 and 4) to lambda-cyhalothrin. Resistance ratios calculated using the LC50 ranged from 3.77 to 45.82 in field-collected populations. Resistance to lambda-cyhalothrin is partly associated with a mutation of a single nucleotide that causes an amino acid change within the para-type sodium channel. This point mutation confers knock down (kdr)-type resistance to pyrethroids across a range of different insect species. We sequenced the genomic region containing domain IIS4-IIS6 of the para gene to determine if a mutation of the para-type sodium channel is present in pyrethroid resistant soybean aphids. Comparison of the sequences from field-collected and laboratory colony populations revealed that the kdr mutation (L1024F) was present in the resistant individuals (field-collected) but not the susceptible individuals (laboratory populations). These results suggest the kdr point mutation is associated with pyrethroid resistance in the soybean aphid. These results suggest that a genetic marker for the detection of individual soybean aphids that are resistant to pyrethroids could be crafted from this point mutation.
What does it mean for farmers: Pyrethroid insecticide resistance has been confirmed for soybean aphid in Iowa; however, the magnitude of resistance and occurrence in the landscape is not well understood. The mechanism for this resistance is being described and may lead to an easy to use marker to help farmers make decisions about switching to other modes of action.