Translocation of Neonicotinoid in Maize and Nontarget Impacts to Water and Soil
The neonicotinoids are a relatively new group of insecticides, first commercially available in the early 1990s, and have become the most widely used insecticide class in the world. Neonicotinoids can be utilized in a variety of ways however the vast majority are applied as a seed coating prior to planting. These neonicotinoid seed treatments (NST) are primarily used within field crops, where their main use is as a prophylactic, insurance based approach to pest management. By 2011 US adoption rates exceeded >80% in maize and 34-44% in soy. This widespread and rapid use has not been justified with a corresponding rise in pest pressure. Numerous peer-reviewed publications had reported inconsistent yield benefits for NST use at the start of this dissertation. Furthermore, an increasing number of reports have reported contamination of nontarget areas with neonicotinoids. In order to investigate one potential mechanism to explain both inconsistent yield benefits and estimate the contribution of NST to environmental contamination, I conducted a two-year field study in which I compared concentrations of clothianidin seed treatments in maize to that of maize without neonicotinoid seed treatments. I found the protection of NST extends to at least 34 d post planting and that in-plant concentrations followed an exponential decay pattern with initially high values followed by a rapid decrease within the first ~20 d post planting. A cumulative maximum of 1.34% of the initial seed treatment was successfully recovered from whole plant tissues in both study years with only 0.26% of the initial seed treatment being recovered from root tissue. My findings suggest NST may provide some protection from early season maize pests but, even at the highest per kernel concentrations of clothianidin tested, peak in-plant concentrations are poorly correlated with the key pest that they are labeled to control: the corn rootworm the key maize pest within the US. Additionally, the poor translocation efficiency of NST, in combination with the high leaching risk of these compounds, provides a route for environmental contamination. To further address the crop protection efficacy of NST, and determine if their mandatory use is justified (growers usually do not have access to untreated seed), I also conducted a meta-analysis on plant health and pest damage metrics from 15 yr of insecticide efficacy trials conducted on Indiana maize. Corn rootworm remains the key pest of maize in the United States however it is managed largely by Bt corn hybrids, along with soil insecticides and NSTs. Frequently, more than one of these pest-management approaches is employed at the same time. This meta-analytical approach allowed me to summarize the mean effect over 15 yrs worth of data and pest management techniques. The probability of recovering the insecticide cost associated with each treatment was also calculated when possible. With the exception of early-season plant health (stand counts), in which the NSTs performed better than all other insecticides, the vast majority of insecticides performed similarly in all plant health metrics, including yield. Furthermore, all tested insecticides (including NSTs) reported a high probability (>80%) of recovering treatment costs. Given the similarity in performance and probability of recovering treatment costs, I suggest NSTs be optional for producers, so that they can be incorporated into an insecticide rotation when managing for corn rootworm, the primary Indiana corn pest. This approach could simultaneously reduce costs to growers, lower the likelihood of non-target effects, and reduce the risk of pests evolving resistance to the neonicotinoid insecticides. The high soil half-life and low Kow of many neonicotinoids result in a high leaching risk. This, in combination with their annual repeated use is likely the reason an increasing number of detections of environmental neonicotinoids are being reported. In order to quantify the magnitude and timing of CLO concentrations in leachate from tile drain fields, an experiment was carried out using field lysimeters at the Purdue Water Quality Field Station. The maximum clothianidin concentration in leachate was found to be an approximate order of magnitude higher than previously reported (3.48 ng/ml). This is not surprising, as these data result from a direct and undiluted measure of neonicotinoid leachate reinforcing the role of NST to environmental contamination. Furthermore, my work demonstrates that NST concentrations within leachate are greatest at precipitation events that follow planting and conform to a first-order decay pattern of initially high concentrations, with a rapid and drastic concentration decrease as the growing season progresses. I also investigated the possibility of nontarget impacts resultant from neonicotinoid contamination in leachate. The systemic nature of NST allows them to readily be translocated by nontarget vegetation and at the start of this dissertation, non-target translocation had not been reported in aquatic vegetation. This question was explored with manipulative laboratory experiments to assess the uptake potential of aqueous clothianidin, a proxy for agricultural runoff and leachate, in the aquatic macrophyte, gibbous duckweed (Lemna gibba). Clothianidin was found to reach equilibrium within plant tissues by 12 hrs exposure at a concentration ~65% of the concentration within the water. Finally, bioassays utilizing clothianidin-contaminated duckweed were conducted on a duckweed-associated insect (Rhopalosiphum nymphaeae, waterlily aphid (Linnaeus)) to investigate potential impacts on higher trophic levels
Krupke, Purdue University.
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