A rice water weevil, Lissorhoptrus oryzophilus. Photo by Ken Walker, Museum Victoria, Melbourne Australia, bugwood.org.

In October 2014, the United States Environmental Protection Agency (EPA) issued a report that stated there were no economic benefits for soybean growers who use seed that is pre-treated with insecticides known as neonicotinioids. Shortly afterwards, Robert Johannson, the Acting Chief Economist of the U.S. Department of Agriculture (USDA), wrote an open letter to the EPA that disagreed — to put it mildly — with their assessment.

Richard Levine

“As a whole, USDA disagrees with that assessment,” he wrote. “We believe that pest management strategies are made in consideration of pest pressures, climate, landscape and numerous other factors … EPA released the report regarding soybean seed treatment without additional consideration of other crops or to USDA cautions about releasing a premature assessment of the costs and benefits of such seed treatments.”

Neonics, as they are also known, are used on soybeans and other row crops — like corn, cotton, and rice — as protection against aphids and other insects that suck the sap from plant leaves and stems. After the seed is coated with the insecticide, it is planted in the field and the neonics become incorporated into the plant cells themselves. That reduces or eliminates the need to spray the crops with insecticides, which may kill beneficial insects like honey bees and lady beetles. Neonics are also less toxic for humans and other mammals than many other insecticide classes.

In addition to the USDA’s Johannson, the American Soybean Association also disagreed with the EPA’s assessment.

But what about other crops?

Researchers from Mississippi State University did their own economic assessment, but instead of soybeans they focused their attention on rice and an insect called the rice water weevil (Lissorhoptrus oryzophilus), the most widely distributed and destructive early-season insect pest of rice in the United States. The results, which are published in the Journal of Economic Entomology, show that there are indeed economic benefits for many farmers who use neonic-treated seeds, but not all of them.

The researchers conducted an experiment at seven locations in the Mississippi Delta and observed rice water weevil populations on plants that had been treated with thiamethoxam, chlorantraniliprole, and clothianidin.

“All seed treatments and seed treatment rates reduced rice water weevil densities,” they wrote. “And all seed treatments yielded better than the untreated plots, but these differences were not always economical.”

The economic benefits depended on whether the rice fields were facing high or low weevil populations, they found.

“There was no observed yield or economic benefit from the use of an insecticidal seed treatment in areas of low pressure,” they wrote. However, “All seed treatments showed an economic advantage in areas of high weevil pressure.”

Determining an “economic threshold” — the pest density at which management action should be taken to prevent an increasing pest population from reaching an economically damaging level — is one of the key concepts of Integrated Pest Management. This study seems to confirm the validity of economic thresholds. If insect pressure is low, then the cost of the neonic-treated seeds may not be worth paying for; if insect pressure is high, then the money saved by investing in the treated seeds will probably be well worth it.

“The problem with rice water weevil is that it does not infest the field until it is flooded, about 3-5 weeks after planting,” said Dr. Jeff Gore, one of the co-authors. “Based on most of our research, we see an economic benefit of neonicotinoid seed treatments on 70-80 percent of the rice grown in the state annually. Unfortunately, we are not able to accurately predict which fields will have a significant infestation because of when infestations occur relative to when the crop is planted. As a result, we recommend a seed treatment on all of our rice in Mississippi.”

Read more at:

– Efficacy of Selected Insecticides Applied to Hybrid Rice Seed

Richard Levine is Communications Program Manager at the Entomological Society of America and editor of the Entomology Today Blog.