Insecticides Have Become More Toxic to Bees Over the Last 20 Years

Farmers might not be using as much insecticide as they used to, but those they do use appear to be more toxic today than they were 20 years ago. In some states, these chemicals are 121-times as toxic for bees in 2012 as they were in 1997, a study published in Scientific Reports found.

The U.S. uses millions of kilograms of insecticides every year to protect crops against bugs that may do them damage, but collateral victims—like the bumblebee—are targeted unintentionally. By some accounts, upwards of 40 percent of bee colonies are lost each year.

The new research compiles publicly available data collected by state and federal organizations, including the Environmental Protection Agency and U.S. Department of Agriculture. From this data, scientists were able to calculate annual estimates of "bee toxic load" at a county level every year between 1997 and 2012.

In this case, bee toxic load was defined as the number of lethal doses accumulated from all insecticides used in a particular county for agricultural purposes.

To get a clearer picture of what was going on, the team split toxic load into two, so that they could measure extent (the area of cropland treated) and intensity (chemical potency times application rate). The researchers also distinguished between contact toxic load (when an insect touches the insecticide) and oral toxic load (when an insect consumes the insecticide).

The results suggest little change in contact toxic load but a large increase in oral toxic load, which was around nine times higher in 2012 than it had been in 1997.

While application rates declined 64 percent over the time period, this was offset by increases in the amount of cropland treated (15 percent in 1997 versus 26 percent in 2012) and the potency of the chemicals used. According to the study, insecticides were 16 times more potent in 2012 than 1997.

"It is problematic that there is such a dramatic increase in the total insecticide toxicity at a time when there is also so much concern about declines in populations of pollinating insects, which also play a very critical role in agricultural production," Christina Grozinger, a professor of entomology and director of the Center for Pollinator Research, Penn State, said in a statement.

The most startling trends were noticed in the heartlands: in this case meaning the states of Iowa, Illinois and Indiana as well as parts of Missouri, Minnesota, Ohio, Kentucky, Nebraska and South Dakota.

The paper notes a 121-fold increase in oral toxic load, which the researchers attribute to the use of neonicotinoid seed treatments in corn and soybean plants. According to the study, more than 90 percent of cropland in the heartlands was used to grow these two plants.

A similar shift to corn and soy was seen in the Northern Great Plains where there was a 53-fold increase in oral toxic load—the second highest in the country. This area spans part of South Dakota, Nebraska, Colorado, Wyoming, Montana and Minnesota, and all of North Dakota.

Honey Bee
New research suggests the U.S. is using less insecticide than it used to but what it does is more potent. Toxic load for honeybees (pictured) has increased 121-fold in some states. Robert Alexander/Getty

Neonicotinoids (or neonics) are a class of insecticides highly toxic to bees with a similar chemical structure to nicotine. Among some of the most widely used insecticides worldwide, neonicotinoids are frequently used as a seed coating. This means the chemicals are absorbed by the crop as it grows from a seedling to a plant, and is distributed throughout its tissue. Some of the coating leaks into the environment.

"Several studies have shown that these seed treatments have negligible benefits for most crops in most regions," said Grozinger. "Unfortunately, growers often don't have the option to purchase seeds without these treatments; they don't have choices in how to manage their crops."

The study's authors point out some limitations to the research, including the fact that the information included in the datasets is not consistent. California, for example, doesn't collect much data on seed treatment, which could mean toxic loads have been underestimated. However, they hope it can help with future research and mitigation efforts.

"The indicator we use—bee toxic load—can be considered as an alternative indicator in cases where impacts to bees and other non-target insects is a concern," said lead author Maggie Douglas, assistant professor of environmental studies, Dickinson College. "Our work helps to identify geographic areas where in-depth risk assessment and insecticide mitigation and conservation efforts could be focused."