Prevalence of pyrethroid resistant soybean aphid, Aphis glycines (Hemiptera: Aphididae), in Iowa

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), causes economically significant damage to soybean, Glycine max, in the north central region of the United States. Knockdown resistance (kdr) to field-applied rates of pyrethroids in A. glycines is associated with target site mutations in Domain II Segment IV-VI (DIISIV-VI) of the voltage-gated sodium channel subunit h1 (vgsc-h1). Between 2021 and 2023, aphids were collected from 10 commercial fields across northern Iowa at different timepoints and sub-field regions relative to the application of a pyrethroid. We also collected aphids from 4 suction traps in northern Iowa, designed to capture aphids migrating between winter and summer host plants. None of the field populations reached the economic threshold of 250 aphids per plant or decreased significantly following application of a pyrethroid. Nineteen vgsc-h1 genotypes with 1 or more target site mutation each were detected among 99.9% of the 1955 field collected aphids. The proportion of these 19 genotypes changed significantly between timepoints relative to the application of a pyrethroid, with the frequency of aphids with greater than 1 mutation increasing after a pyrethroid application. All aphids collected in suction traps also had genotypes encoding target site mutations. These results suggest that 4 mutations associated with pyrethroids resistance are highly prevalent among A. glycines infesting soybean in northern Iowa, may confer different levels of survival to in-field selection and have spread readily across the landscape in response to strong in field selection. Continued genetic monitoring information will be important to inform growers of the efficacy of their A. glycines management decisions.