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Are Pesticides Killing the Honeybees?

Are agricultural pesticides at fault for the decline of honeybee populations?

Probably not, at least not at the levels we use insecticidal seed treatments for corn and soybeans. That’s the opinion of Galen Dively of the University of Maryland, an entomologist and honeybee expert who advises crop chemical manufacturers, applicators, and farmers on appropriate use of crop pesticides. 

One area of emphasis is the appropriate use of neonicotinoids, a class of chemicals often used as corn and soybean seed treatments under names such as Gaucho, Poncho, or Cruiser. They control such early-season pests as wireworms, cutworms, and thrips. 

“Neonics are the most widely used insecticides in the world, with many uses, such as for control of termites and other insects,” says Dively. “There’s no question that, above certain levels, they can be highly toxic to bees. They interfere with an insect’s nervous system. Bees exposed to neonics can lose their ability to learn to forage and return to the hive.

“The issue is the level of exposure,” he adds. “At the levels we use them as seed treatments, there’s very low risk of harming bees.” 

How They’re Applied
Most neonics in corn and soybeans are applied before the seed is bagged. Such systemic insecticides not only protect the seed in the ground, but also are absorbed by the plant roots to protect the young plant. 

Later, neonic residue could end up on corn pollen or in soybean flowers, where bees could pick it up and carry it back to a hive.

Corn and soybeans aren’t the normal places where honeybees go for food, though. They prefer full-flowering crops such as fruits, some nut crops, or even weed patches and roadside ditches. When those flowers aren’t available, the bees will look for nectar elsewhere. 

“Lab studies have shown that exposure above 20 parts per billion (ppb) affects the bees,” says Dively. “While it can be difficult to extrapolate lab studies to the field, corn and soybean seed is treated at low enough rates that the pollen residue is below the 20-ppb threshold.

“When the neonic is applied to a field as a soil drench or a foliar spray, we find higher rates of residue, ranging from 30 ppb to 100 ppb,” he says. 

A study by Dively showed that the high range of neonicotinoid exposure (5 ppb) relevant for seed-treated crops had negligible effects on honeybee colony health. 

At higher doses, “the bees tend to avoid the contaminated food,” he says. “We know that sometimes we get delayed effects that can result in higher queen losses and weaker colonies going into the winter.”

The decline in bee populations is real, and farmers should be concerned, he continues. Honeybees are necessary for pollination of at least 130 food crops. While corn, soybeans, and wheat would not be on that list, some livestock forages do require bees for pollination, and there is the potential for drift of neonics from row-crop fields to neighboring fruit or vegetable crops where bees feed.

How to Help Honeybees
Here is Dively’s checklist of practices you can do to help bee populations.

  • Follow the application label directions when applying pesticides. Most insecticide labels now have a Pollinator Protection Box that may tell you not to treat crops in the midst of bloom, in certain weather conditions, or in midday when bees tend to travel and feed.
  • Avoid or restrict certain practices when applying neonics, including chemigation, drenches, and tree injections.
  • Use nonsystemic chemical products if you can.
  • Communicate with any neighboring beekeepers about your application timing and products.

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