Bees are susceptible to many broad spectrum insecticides, but how are they getting exposed to a chemistry largely used for seed treatments? Christian Krupke, a field crops entomologist at Purdue University in Indiana, and several other researchers, have taken a closer look at how honey bees might be interacting with neonicotinoid insecticides, which are commonly used as seed treatments.
Based on the research of Krupke and his colleagues and from observations of researchers elsewhere, Iowa State University Extension entomologist Erin Hodgson provides the following information and recommendations for Iowa farmers who are planting insecticide treated seed this spring.
Neonicotinoids are a relatively new class of chemistry to control insects. They are now widely adopted because they are persistent and systemic in plant tissues. Most field crops in Iowa have a neonicotinoid seed treatment when planted. Common examples of neonicotinoids are clothianidin (Poncho), thiamethoxam (Cruiser) and imidacloprid (Gaucho). Active ingredient rates range from 0.25 to 1.25 milligrams per kernel (sold as 250 to 1,250 rates).
Public is more aware of pollinator health and the decline of bees in North America
Neonicotinoids are extremely toxic to bees. Lethal LD50 rates (the rate at which half of the exposed population dies) for clothianidin are 22 to 44 nanograms per bee for direct contact and 2.8 to 3.7 nanograms per bee for oral ingestion. In other words, a single corn kernel with a 1,250 rate of neonicotinoid seed treatment contains enough active ingredient to kill over 80,000 honey bees.
There has been an increased public awareness of pollinator health and the decline of bees in North America. Researchers have identified multiple contributing factors for honey bee decline, including: Varroa mites, disease-causing pathogens, habitat loss, malnutrition, the intensity of migratory pollination services and pesticides (Fig. 1).
So, how are bees being exposed to neonicotinoid insecticides and what can farmers do about it? Krupke and his colleagues at Purdue published a recent article reporting several potential exposure routes. Here is a summary of their findings:
* Bees, pollen and nectar were collected from an apiary during the corn planting season in Indiana. All dead and dying bees had traces of clothiandin, and stored pollen had high neonicotinoid levels.
* Soil samples collected from fields not planted with a seed treatment for two years still contained detectable levels of clothiandin.
* Dandelions collected from around field edges before planting had detectable levels of neonicotinoids (Fig. 2).
* Talc used as an additive for planting treated seed had extremely high levels of neonicotinoids. Planter exhaust expelling tainted talc could be coming in contact with bees or plants they forage.
* Corn pollen collected by honey bees later in the season was screened; half of the corn pollen samples analyzed had neonicotinoids.
The Purdue University research paper makes these conclusions in its summary: neonicotinoid exposure is likely a combination of direct contact; indirect contact with dosed weeds/crops, talc or soil; and through ingestion from pollen in dosed plants. This year, approximately 200 million acres of crop land will be planted with crops that are treated with neonicotinoids, 94 million acres planted to corn alone in the U.S.
"This means that some exposure is inevitable, but the following recommendations may help minimize the danger to honey bees during the planting season," says ISU Extension entomologist Erin Hodgson.
What farmers can do to minimize danger to honey bees during corn planting season
* Farmers should communicate with nearby beekeepers or apiaries about your intentions to plant. Visit the Iowa Department of Agriculture and Land Stewardship Sensitive Crops website for more information.
* Beekeepers should move hives away from production fields during the planting period if possible.
* Always use the recommended amount of talc to allow proper planting, removing this lubricant is not recommended.
* Do not clean planter equipment/hoppers near fields, especially around flowering plants.
"Because of the importance of pollinators and the prevalence of these insecticides in our cropping systems, there is a great deal of research on this topic in independent labs all over the world," notes Hodgson. "We will likely see more studies that explore the linkage between pollinator decline and pesticides in the near future, so stay tuned. For now, the best thing to do is minimize the high level exposures during planting as much as possible using the steps outlined above."