Assessment of the influences of neonicotinoid seed treatments of Bt maize upon resistance management and environmental residues
The western corn rootworm (WCR) Diabrotica virgifera virgifera Leconte is a major pest of maize in the United States. Currently, it is principally managed using Bt maize hybrids and neonicotinoid seed treatments (NSTs), which were concurrently introduced in the early 2000s. This simultaneous release, and subsequent rapid adoption, created a situation in which Bt maize hybrids were never assessed in the absence of NSTs, and vice versa. Consequently, neonicotinoids’ influence on refuge function, primarily whether these insecticides aid or hinder the production of a sufficient population of susceptible beetles to delay resistance, has not been assessed. Moreover, a mounting suite of detrimental environmental effects of NSTs have been documented, lending some urgency to questions about their necessity.
To determine the influence of NSTs on refuge function, untreated and treated Bt maize fields were planted with 5% untreated refuge marked with 15N. Throughout the field seasons of 2019 and 2020, adults were collected from these fields and analyzed to determine their natal host plant. Results documented that the numbers of refuge beetles produced by the 5% seed blend are likely insufficient to result in rates of mating to delay resistance development.
To determine if the effect of using NSTs in combination with Bt maize hybrids is additive, synergistic, or neutral at managing secondary soil pests in the Midwest, four 16 block fields were planted in 2018, 2019 and 2020 comparing four treatments (1. Untreated, Bt seed; 2. NST, Bt seed; 3. Untreated, non-Bt seed; 4. NST, non-Bt seed). Compact method sampling, root rating, and yield were used to document the presence of secondary soil pests. All three sampling years documented low abundance of white grubs and wireworms. There was minimal influence of NSTs on maize yield (2018: p = 0.07; 2019: p = 0.62; 2020: p = 0.056) and root damage (0-3 scale) (2018: M = 0.0092; 2019: M = 0.0091; 2020: M = 0.0361). These same fields were used to document the presence of NSTs in soil, as well as residues in nearby waterways. Results documented greatest soil and water clothianidin levels earlier in the season and declined as the season progressed.