Neonicotinoid insecticides are commonly used as seed-treatments on major agricultural row crops (e.g., corn). However, neonicotinoids are also highly persistent with long half-lives in soils (e.g., clothianidin: 1386 d). Due to their high solubility, neonicotinoids are readily transported in surface runoff into freshwater ecosystems such as rivers and wetlands. Previous field studies have documented neonicotinoid persistence in global surface waters as well as lethal and sub-lethal responses by aquatic invertebrates in laboratory settings; however, less is known about neonicotinoid occurrence in managed public wetlands of Missouri. We investigated the response of benthic invertebrate communities to direct application of neonicotinoids in managed wetland ecosystems. In 2016, 22 wetlands from nine conservation areas were sampled for water, sediment, and aquatic invertebrates during spring (pre-wetland drawdown) and fall (post-wetland flood-up). During the summer, portions of wetland areas were planted with either untreated corn (control) or corn treated with a neonicotinoid (i.e., thiamethoxam). We present chironomids as field-collected model organisms to assess potential effects of neonicotinoid seed-treatment application in wetlands on aquatic invertebrates. Using a series of linear mixed effects models, we evaluate water quality parameters and insecticide concentrations on invertebrate metrics such as morphology (e.g., size of individual), biomass, and production (e.g., abundance).
Water and sediment samples were analyzed for the six most common neonicotinoids (and fungicides) with 68% of wetland water in spring having low, but detectable clothianidin residues (mean: 0.006 µg/L; max: 0.012), whereas 10% of wetlands contained imidacloprid (max: 0.08 µg/L). In addition, 30% of wetland sediment samples contained a neonicotinoid (mean: 0.22 µg/kg; max: 2.5) with 65% of sediment containing detectable fungicide residues (mean: 0.15 µg/kg; max: 0.794). With sediment concentrations in spring being orders of magnitude higher than water, these insecticides may be persisting through the growing season and re-solubilized during fall flood-up. This has important implications for benthic organisms as well as wetland-dependant species (e.g., waterfowl) as some concentrations may surpass chronic exposure regulatory guidelines for the protection of aquatic life. High concentrations may further affect sensitive biota (e.g., Chironomidae) through direct toxicity or the reduction of ecosystem services (e.g., secondary production). Conversely, low concentrations of fungicides (ie., strobilurins) are also shown to affect typically insensitive nektonic species such as cladocerans. Here, we present our preliminary findings and discuss how this research improves our understanding of the potential impacts of neonicotinoid seed-treatment use on non-target aquatic invertebrate communities in managed wetland ecosystems.