Determine concentrations and persistence of imidacloprid, clothianidin and thiamethoxam in terrestrial agricultural settings on Missouri public areas
July 2015 - June 2019
- Missouri Department of Conservation
Imidacloprid, clothianidin and thiamethoxam are three active ingredients in a relatively new class of chemical insecticides known as neonicotinoids. Neonicotinoids are systemic insecticides widely used as pre-planting treatments on crop seeds to control insect agricultural pests. Neonicotinoids are a derivative of nicotine with a mode of action that binds more tightly with the nicotinic acetylcholine receptor (nAChR) in the central nervous system of insects, compared to vertebrates (Morrissey et al. 2015). Consequently, neonicotinoids are selectively more toxic to invertebrates than vertebrates (Tomizawa and Casida 2005). Neonicotinoids were developed and registered in the early 1990s and have been widely adopted for agricultural use throughout North America and Europe, in large part because of their low toxicity to vertebrates, high toxicity to insects, flexible use, and systemic nature (Morrissey et al 2015, Goulson 2013, Kollmeyer et al. 1999, Tomozawa and Casida 2005). Imidacloprid, clothianidin, and thiamethoxam are three of the most commonly used active ingredients in neonicotinoid-formulated insecticides. Neonicotinoid insecticides can be applied using a number of different methods including application as a drench, with irrigation water, as a foliar spray, as seed coatings or injected in soil or trees. They are most commonly used as pre-planting seed treatments to provide early season protection for corn and soybeans as well as a number of other agricultural crops, fruits, and vegetables; imidacloprid is also widely used in lawn and garden products as well as topical flea applications (Hladik et al. 2014). Neonicotinoids are systemic chemicals, meaning the active ingredient is translocated throughout a growing plant, transforming all parts of the plant toxic to insects that feed on the plant (Syngenta 2011). A benefit of neonicotinoids is that application rates are low and precise; when applied as a seed dressing, rates typically range from 0.25 to 1.25 mg active ingredient (ai)/seed. However, because these chemicals are systemic, all parts of a treated plant, including nectar and pollen, receive some dose of insecticide which increases the potential of adverse effects on non-target insects. The primary targets of neonicotinoid seed treatments are agricultural pests that feed on crop plants through chewing or sucking methods; however, all three chemicals have been implicated in honey bee deaths whereby the bees have contacted contaminated pollen, ingested contaminated guttation or been exposed to contaminated dust generated during planting activities (Krupke et al. 2012, Mason et al. 2013). Imidacloprid, when applied experimentally in field realistic doses, has been linked to reduced colony growth and queen performance in bumblebees (Feltham et al. 2014, Whitehorn et al. 2012). Similarly, Sandrock et al. (2013) found sub-lethal exposure of a solitary bee species to clothianidin and thiamethoxam in controlled experimental populations reduced reproductive output and resulted in male-biased offspring. Bees other than honey-bees may face additional hazards in an agriculture setting as they tend to have smaller foraging areas which results in fewer food choices (Pisa et al. 2014). Ground-nesting species of bees also face potential exposure to pesticides in soil and, as they tend to be smaller-sized, may experience effects at different concentrations than larger-bodied bees (Pisa et al. 2014). These and other concerns regarding potential detrimental effects on bee populations, specifically, and pollinator populations, in general, resulted in the Environmental Protection Agency (EPA) issuing new label requirements for insecticides containing neonicotinoids and re-opening the registration review dockets on all neonicotinoid pesticides with the reviews projected to be completed by 2018-2019 (EPA 2015). Another emerging issue related to neonicotinoids is persistence and accumulation in the soil and consequences to soil organisms such as earthworms that may be in direct contact with or ingest the chemicals (Dittbrenner et al 2010, Wang et al. 2012). Neonicotinoids are persistent in the environment, with reported half-lives ranging from 200 to >1000 days and since >90% of active ingredients found in neonicotinoids applied as seed treatments enter the soil, repeated application has the potential to accumulate high concentrations in soils (Goulson 2013, Jones et al. 2014). Jones et al. (2014) conducted a preliminary investigation to determine if imidacloprid, clothianidin, and thiamethoxam could be detected in cropfields and crop margins in eastern England that had not been planted for up to three years prior to their investigation. Total detected neonicotinoid concentrations were greater in the center of the tested fields, ranging from 0.51-16.4 µg/kg than the edges of the fields, which ranged from 0.04-7.4 µg/kg. They detected clothianindin in a field that had not been treated with clothianindin or thiamethoxam (clothianindin is a metabolite of thiamethoxam) for 2.5 years, and clothianindin in one field and thiamethoxam in seven fields which had not had these chemicals applied for three years. Imidacloprid was detected in 15 of 18 fields sampled even though this chemical had only been used in two of the fields three years prior to their study. De Perre et al. (2015) also found clothianindin to be persistent in the soil and that accumulation occurred with repeated plantings; however, the concentrations they detected in fields planted with application rates of 0.25 mg ai/seed and 0.50 mg ai/seed did not pose an acute risk to the species used in their study. It is uncertain how long neonicotinoids persist in the environment once direct application ceases as degradation is influenced by multiple factors such as differences in application rates, pH, temperature, soil type, and organic content (Morrissey et al 2015). It is also uncertain if plants that subsequently are planted to or invade a site with neonicotinoid soil accumulation will take up the chemicals. De Perre et al. (2015) speculated that reduced concentrations of clothianindin in runoff water samples after approximately 30 days were a result of the chemical binding with soil particles, thus making it less available for leaching. This would seem to reduce the likelihood of uptake by other plants. The influence of neonicotinoids on soil fauna is equally uncertain, particularly in terms of lethal and sub-lethal effects and potential trophic cascading effects. Earthworms play key roles in soils and, as part of the ecosystem services they perform, contribute toward breakdown of organic matter, soil fertility, soil aeration and drainage (Edwards and Bohlen 1992, Duiker and Stehouwer 2008). Earthworms are viewed as excellent candidates to assay soil health due to their ecological importance, high biomass in soil, and sensitivity to relatively low concentrations of contaminants (Dittbrenner et al. 2010). Although Eisenia fetida is the recommended test species by the International Standardization Organization (ISO), it is an epigeic, or litter-dwelling, earthworm rarely associated with agriculture and is considered to be more tolerant of contaminants than other earthworm species (Dittbrenner et al. 2010). Nevertheless, Wang et al. (2012) found five neonicotinoids, both in contact and artificial soil tests, to be highly toxic to E. fetida. Dittbrenner et al. (2011) assessed the effects of imidacloprid on Lumbricus terrestris, an anecic or deep-dwelling, and Aporrectodea caliginosa, an endogeic, or shallow-dwelling, species of earthworms more closely associated with agricultural fields. They found adverse effects of imidacloprid on the burrowing activities of both species in environmentally relevant concentrations for short- and long-term laboratory experiments. Similarly, Dittbrenner et al (2010) reported sub-lethal effects on L. terrestris and A. caliginosa when using changes in body mass and cast production to evaluate the effects of imidacloprid at environmentally relevant concentrations in a laboratory setting. Sub-lethal effects on soil organisms exposed to neonicotinoids raise questions regarding trophic interactions, as predators consume prey that may have bio-accumulated the chemicals. Douglas et al. (2014) detected a trophic transfer of neonicotinoids in which slugs, a molluscan agricultural pest, were tolerant to thiamethoxam but the predaceous beetles that fed on the slugs were sensitive to the chemical. They noted depressed activity by the predaceous beetles after feeding on contaminated slugs, increased damage to soybeans by slugs as the population was released from predation pressure, and overall reduction in soybean yield (Douglas et al. 2014). Hallmann et al. (2014) recently reported a negative correlation between local population trends of insectivorous birds and neonicotinoid concentrations in surface waters, and attributed the relationship to cascading trophic effects of neonicotinoids to aquatic food chains. Currently, the vast majority of corn seed planted in the Midwest is treated with either clothianidin or thiamethoxam while soybean seeds are more often treated with thiamethoxam (Krupke et al. 2012). The Department of Conservation plants approximately 67,000 acres of agricultural crop annually on Department areas. In 2013, almost 15,000 acres were planted to corn and 23,000 acres to soybeans. These agricultural operations on Department lands are used for optimal production of wildlife food and cover with use of accepted best farm management practices, and are managed to provide the best economic return consistent with resource management objectives. An important and vital aspect of the Department’s mission is to protect and manage the forest, fish and wildlife resources of the state (MDC 2014). Given the high use of neonicotinoid insecticides as a pre-planting treatment of corn and soybean seeds in the Midwest and their inherent characteristics, i.e., acute toxicity to insects, relatively long half- lives in soil, and high water solubility combined with the potential for them to pass through trophic levels via food webs, their use may be in conflict with the Department’s efforts to sustain healthy forests, fish, and wildlife if concentrations are present at lethal or sub-lethal levels to non-target taxa. It is, therefore, imperative to determine the concentrations and persistence of these chemicals on Department public areas in regards to upland agricultural plantings, associated cropfield borders, and within the soil and, depending on the outcomes of this investigation, recommend best management practices to reduce potential deleterious effects on non-target taxa.