Purdue Study: Corn Seed Treatment Insecticides Pose Risks to Honeybees, Yield Benefits Elusive

Purdue study finds no evidence that neonicotinoid insecticidal seed treatments boost corn yields while putting honeybees at risk of neonicotinoid exposure.

There’s no evidence that neonicotinioid insecticidal seed treatments increase yield, according to a Purdue University study. The study also showed that over 94% of honeybee foragers throughout the state of Indiana are at risk of exposure to varying levels of neonicotinoid insecticides, including lethal levels, during the planting of corn. (Watch a video abstract here.)

Neonicotinoids, including clothianidin and thiamethoxam, are a class of insecticide commonly applied as a coating to corn and soybean seeds to protect them from early-season pests. Since the coatings are sticky, a talc or graphite powder is added to vacuum systems in planters to keep the seeds from clumping. Powder exhausted from the planter contains neonicotinoids.

The U.S. is losing about one third of its honeybee hives each year, a significant problem since the bees pollinate many crops used to feed people and livestock. Neonicotinoids, which are highly toxic to honeybees, are being scrutinized as a possible contributor to the losses. Christian Krupke, a professor of entomology, showed in 2017 that exhausted insecticides collected on flowers that border agricultural fields and were present in hives near those fields.

Bees in those hives showed physical signs of insecticide poisoning, and dead bees tested positive for the neonicotinoids used as seed treatments of corn and soybeans. Now, Krupke, along with collaborators Jeff Holland at Purdue, Elizabeth Long at Ohio State University, and Brian Eitzer with the Connecticut Agricultural Experiment Station, have measured the drift of those neonicotinoids from fields and found that the insecticides can settle on flowers up to 100 meters from the edge of the planted fields, the farthest distance examined in the study. Their findings are published in the Journal of Applied Ecology.

The study is not without criticism. Officials for Syngenta, which makes and sells neonicotinoid seed treatments, say they remain an important tool for farmers. They say research about them should be consistent with how they are used in the field. In this case, Syngenta officials say researchers used unrealistic modelling to estimate dust drift from neonic-treated corn seed, exaggerating the potential exposure to foraging bees. And they only used a handful of small trial plots — less than three acres — to evaluate crop yields, the officials said. 

Neonicotinoids currently are going through EPA Registration Review, and the agency’s preliminary assessment has found neonicotinoid seed treatments pose minimal risk to pollinators, say Syngenta officials. Neonicotinoid seed treatments provide farmers with an economical level of risk management and means of protecting seeds and seedlings against early-season insect pests, say Syngenta officials. This is especially important as farmers reduce the number of seeds planted per acre to manage their expenses, they say. Syngenta officials point to a comprehensive 2014 study by AgInfomatics demonstrated that neonicotinoid seed treatments in the U.S. increased crop yields by an average of 3.6% to 71.3% across multiple crops, compared to untreated acres. For U.S. corn, the average yield was 17.8%.

Study Specifics

Mapping Indiana’s corn acreage, as well as the areas that may receive drift, the authors of the Purdue study say that 42% of the state is exposed to neonicotinoids during crop planting. Looking at public data on the location of apiaries and projecting the range that honeybees forage, they found that 94% of bees could fly through areas that contain lethal doses of the insecticides during the period when corn is planted.

“Our previous study showed that these neonicotinoids are likely to leave the field, but we wanted to demystify that distance and show how far the material moves, at what concentrations, and what the actual risk is,” Krupke said. “There was a misconception that any bees not living near corn were likely to be fine. But that’s not true, and it’s clear that these insecticides are reaching into the places bees forage, and that is putting them at risk.”

Krupke’s team set up dust-collection stations at 12 Indiana fields where corn was being planted. They collected samples for two years at distances up to 100 meters. Analysis of the collected dust showed lethal doses of neonicotinoids were reaching the farthest traps. Added to the clouds over the fields during planting, Krupke said bees are exposed to significant risk.

“As planter exhaust is blown up and away from the equipment, it gets into the airstream and is at the mercy of whatever is going on with the wind,” Krupke said. “It’s not all that different from the pesticide drift that we’ve talked about for years, but these products were supposed to solve that problem. Now we know that they also drift.”

In the same study, the researchers found no evidence that neonicotinoids increased yield in corn. The authors tested untreated corn seed and seeds coated with neonicotinoids and fungicides at both high and low doses at three locations around Indiana. There were differences in pest damage at one site, but those did not translate into yield loss. The authors conclude that the lack of benefit for corn yields in their study, as well as inconsistent findings in U.S. corn, soybean, and oilseed rape in Europe, “suggest that the current use levels of insecticidal seed treatments in North American row crops are likely to far exceed the demonstrable need, and our results likely reflect a scarcity of target pests.”

The industry continues to work on alternative seed lubricants to reduce dust movement at planting. To date, progress has been limited. According to a Penn State study analyzing USDA pesticide-use data, the rates of neonicotinoid use in corn have doubled since 2012. The authors say the risk to bees and other nontarget organisms could be more significant than their paper suggests. They only examined cornfields in this study, and soybeans are also typically treated with neonicotinoids.

The study transects were limited to 100 meters from field edges, but it’s possible that lethal doses reach farther. They did not account for the fact that bees create static charges on their bodies during flight, which means they may be attracting insecticide-tainted dust during flight and not just when landing on flowers.

Neonicotinoids are on almost all corn and most soybean seeds sold in the U.S., though Krupke said that this study and other reports of inconsistent yield benefits, show that widespread use is unnecessary and farmers could benefit from access to seeds not treated with insecticide. He will focus research on determining the circumstances in which neonicotinoids are useful for improving yield, and he will encourage farmer access to neonicotinoid-free seed, which is almost nonexistent in the current market.

“The good news is that because farmers often don’t need these additions to seeds or benefit from them, we can easily and rapidly reduce the risk simply by having untreated seeds available,” Krupke said. “That would also allow farmers to make some side-by-side comparisons in their own fields.”

The U.S. Department of Agriculture and the Indiana Corn Marketing Council supported the research.

Below is an abstract of the study.

Planting of neonicotinoid-treated maize poses risks for nontarget organisms over a wide area without consistent crop yield benefit C. H. Krupke1, J. D. Holland1, E. Y. Long2, and B. D. Eitzer3 1. Department of Entomology, Purdue University. 2. Department of Entomology, The Ohio State University. 3. Connecticut Agricultural Experiment Station.

1. Neonicotinoid insecticides are used as seed treatments on most grain and oilseed crops in the U.S., yet the extent and likelihood of spread of insecticide residues during planting has not previously been quantified.

2. Honeybees are the best model for estimating exposures in mobile insects. We measured neonicotinoid dust drift during maize sowing and used sites of maize fields, apiary locations, and honeybee foraging radii to estimate likelihood of forager exposure. We performed a concurrent multiyear field assessment of the pest-management benefits of neonicotinoid-treated maize.

3. Our results indicate that over 94% of honeybee foragers throughout the state of Indiana are at risk of exposure to varying levels of neonicotinoid insecticides, including lethal levels, during sowing of maize. We documented no benefit of the insecticidal seed treatments in terms of crop yield during the study.

4. Synthesis and applications: We demonstrate that the likelihood of exposure for nontarget lands and the primary agricultural pollinator species, Apis mellifera, is high during sowing of neonicotinoid-treated maize seed in a representative agricultural state in the U.S. This pest-management approach has potential to contaminate nontarget lands and waterways; we estimate that over 42% of the state of Indiana will encounter deposition of neonicotinoid residues during the period of maize sowing. However, we also demonstrate that the risks to pollinators and other sensitive organisms can likely rapidly and dramatically reduced without yield penalties, by aligning use rates of neonicotinoid insecticides with pest incidence.

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