UVM field trials find mixed benefits from neonicotinoid seed treatments; residues and runoff vary by soil and management

Heather Darby, agronomist and soil specialist at University of Vermont Extension, briefed the committee on two years of field trials intended to inform when variances or exemptions for neonicotinoid seed treatments might be warranted.

"We have not seen any significant differences in plant populations between treated and untreated corn seed," Darby told the committee, describing planning-date trials that planted treated and untreated seed across six planting windows from May 10 into mid-June to capture seed-corn-maggot peak flights.

Nut graf: The trials showed seed-corn-maggot pressure varies by planting date and local conditions; in most cases treated and untreated plots had similar plant populations and yields, though one late planting date showed a meaningful but statistically inconclusive yield drop in untreated plots. Residues of neonicotinoids were detected in some soils and in tile and surface runoff following heavy rain; planter-dust loss in 2024 was low in wet, low-wind conditions but differed by seed brand and fluency agent.

Key findings and details

- Planning-date trials (2023———and preliminary 2024 results): Darby said the crew observed two common peak flights of seed-corn maggot, typically early May and again in early June. Plant populations and yields showed no consistent significant difference between treated and untreated seed across the study; one late planting (around June 9) coincided with a peak flight and showed a drop from about 36———37 tons silage per acre to roughly 32 tons in untreated plots.

- Pest context: Darby described seed-corn maggot, white grubs, and wireworms as the primary early-season pests in Vermont; these pests often increase following sod or manure incorporation. She said fields converted from long-term sod to corn or fields receiving tilled manure and tillage showed a higher probability of seed damage.

- Residues and persistence: At Borderview Research Farm (silty loam), samples taken before planting in 2023 showed detectable neonicotinoids at 2.5———6 inches in many plots; levels rose after planting and declined to below detectable limits by the following April (after a wet year). In contrast, a heavy-clay site in St. Albans Bay with several years of continuous treated corn showed detectable clothianidin a year later, indicating longer persistence in some soils.

- Water monitoring: At a commercial farm study with edge-of-field instruments, Darby said neonics appeared in surface runoff and tile samples after major rain events. She reported detectable neonics in surface runoff 47% of sampling events and in tile samples 43% of events, with the largest concentrations following a multi-inch rain on July 3.

- Planter dust and fluency agents: The team revisited Corn Dust Consortium methods at five planting sites in 2024. Because spring 2024 conditions were cold and wet with low wind, dust movement off fields was low. In controlled vacuum-collector tests, however, Darby reported that fluency agent choice and seed-brand coating method strongly affected dust loss: a polyethylene-wax product (described as a Bayer product in testimony) reduced loss more than using no fluency agent, while talc and graphite caused greater chipping and loss. Two seed brands treated at the same rate of LumaSure 1250 showed markedly different dust-loss behavior; one brand's coating chipped more readily under graphite.

- Alternatives and management: Trials testing untreated seed, fungicide-only seed, organic insecticide (spinosad), diamide insecticides and neonicid showed that insecticidal seed treatments increased the proportion of undamaged seed from very low values under heavy organic-matter conditions (e.g., manure over the row) to roughly 75% undamaged when any insecticide was used. Darby emphasized fungicides remain important in Vermont and that insecticide alternatives tend to be more costly and may have different non-target profiles.

Darby's team is expanding on-farm sites for a three-year study in collaboration with Cornell University to build a larger data set and develop a risk-management tool to guide exemptions. She said the goal is to identify conditions (planting date, soil type, prior management such as manure or sod) under which a written exemption or variance might be warranted by 2029, when treated-seed prohibitions could take full effect depending on New York's law.

Ending: The committee did not take formal regulatory action. Darby said the research will continue; she and committee members agreed to reconvene when additional 2024 data arrive.