Erin Hodgson, Department of Entomology (ISU) and Christian Krupke, Department of Entomology (Purdue)
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. Common examples of neonicotinoids include: clothianidin (Poncho ®), thiamethoxam (Cruiser ®), and imidacloprid (Gaucho ®). Active ingredient rates range from 0.25-1.25 milligrams per kernel (sold as 250-1,250 rates).
Neonicotinoids are extremely toxic to bees. Lethal LD50 rates (the rate at which half of the exposed population dies) for clothianidin are 22-44 nanograms per bee for direct contact and 2.8-3.7 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).
Figure 1. Bees exhibit neurotoxic symptoms when dosed with neonicotinoids. Dying bees have trouble flying, collecting food and getting back into the hive. Photo by John Obermeyer, Purdue Extension Entomology.
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, and several others took a closer look at how honey bees might be interacting with neonicotinoids. They 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.
Figure 2. Dandelions are common pollen and nectar sources for insects. Photo by John Obermeyer, Purdue Extension Entomology.
Their paper makes the following 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 with corn alone. This means that some exposure is inevitable, but the following recommendations may help minimize the danger to honey bees during the 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. 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.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at email@example.com or phone 515-294-2847. Christian Krupke is an associate professor of entomology at Purdue University with extension research responsibilities.