Practical resistance to spinosad in an onion maggot (Diptera: Anthomyiidae) population in New York.

Onion maggot, Delia antiqua (Meigen), is a significant constraint on onion production across the temperate regions of North America. Early season infestations can reduce plant stands up to 75% if untreated. Effective management of D. antiqua has historically relied on insecticide seed treatments, with spinosad being among the most commonly used options. However, declining control efficacy observed on a New York onion farm over several years raised concerns about the potential development of insecticide resistance. The purpose of this research was to (i) evaluate the efficacy of spinosad seed treatment in managing D. antiqua in the affected field from 2021 to 2024 and (ii) to assess the population's susceptibility to spinosad using a laboratory bioassay. Results from field trials revealed that the percentage of control of D. antiqua using spinosad seed treatment relative to a noninsecticide treatment declined from 51% in 2021 to -83% in 2024. Laboratory bioassay results showed an increase in LC50 values from insects collected from nontreated and spinosad-treated plants in 2023. These results document the first case of reduction in spinosad seed treatment efficacy in D. antiqua that has practical consequences for pest control (practical resistance). Given the critical role of spinosad seed treatment in onion maggot management, these results underscore the need for evaluating resistance prevalence in other populations across North America.