'Best-selling' Insecticide Should Be Used Sparingly

Imidacloprid may not kill bees outright, but research suggests it can disorient and disable them to the point where colonies lose the ability to collect food (as seen here) and reproduce.

You may not know Imidacloprid by name, but chances are you've used it even if you're not a licensed pesticide applicator. New findings suggest the chemical could be adversely affecting honeybees, which are in decline across the globe.

Sold under the names Merit, Imicide and Marathon, the Imidacloprid is sprayed on shrubs, injected into trees and applied directly to pets for flea control. More dilute formulations are available to consumers. More potent formulas are sold to landscape professionals.

Prof. Eric Mussen of the University of California at Davis is recognized around the world as a leading bee expert. He has been interviewed extensively over the past two years about the world-wide decline of honeybees and related species.
Photos by Kathy Keatley Garvey, Department of Entomology, U.C. Davis

Smoking Bugs Out

Imidacloprid is a kind of synthetic nicotine that kills insects by impairing normal nerve function. The good news is that the bug killer is much less toxic that substances like diazinon, which was withdrawn from residential use in 2004. New concerns about its effect on bees and other wildlife, however, reminds pesticide applicators to follow application guidelines and to use integrated pest management practices to keep use at a minimum.

Bees Now a Concern

According to a recent label for Bayer Advanced Tree & Shrub Insect Control, the Imidacloprid-containing product should not be applied "to lakes, streams, rivers or ponds" because it is "toxic to aquatic invertebrates." The label, however, doesn't mention any effects on bees--although this has become a serious concern around the world over the past year.

In France and Brazil, its use is on hold because a growing body of evidence suggests that Imidacloprid may be contributing to the worldwide decline of honeybees and related species. For the past five years, bee experts have been baffled and concerned by a drop-off in honeybee populations that is hitting commercial beekeepers and may soon affect the price of fruits and nuts that depend on bees for pollination.

Asking an Expert

Imidacloprid application is still legal for commercially-licensed applicators in the U.S. But in the light of its possible effects on bees, it now makes sense to limit the use of Imidacloprid formulations and to follow a set of guidelines that will limit its impact on bees and other beneficial insects.

Eric Mussen of U.C. Davis' Honey Bee Research Facility is a leading bee expert who has been interviewed by major media outlets about the problem with bees--which scientists call Colony Collapse Disorder, or CCD. asked Mussen what he thinks about the emerging controversy.

"It is too soon to try to state that Imidacloprid is safe or unsafe around pollinators," Mussen replied. "However, it is safe to state that not enough research has been done done on a chemical that is becoming almost ubiquitous in agricultural and urban settings."

Given the uncertainty, it makes sense to use caution when applying Imidacloprid. At a minimum, applicators should always ask themselves three questions when dealing with the product.

Imidacloprid Use Checklist

  1. Am I (and my company) properly trained and licensed for pesticide application and are we following label instructions for Imidacloprid?
  2. Are we limiting application to late evening (when bees are inactive) and avoiding application to shrubs and trees where bees are actively foraging by day?
  3. Are we using best management practices that include fertilization, pruning, aeration and other practices that limit the need for Imidacloprid and other pesticides?

A guide to bee-safe insecticide use from Oregon State University is available at

Q & A - Professor Eric Mussen, Department of Entomology, U.C. Davis

We've read that beekeepers in France suspect that Imidacloprid may play a role in the reported decline of honeybees.

That is true. They even convinced the federal government to ban the chemical from use.

In your opinion, is there anything to the claim?

The published scientific papers are in conflict with themselves. It has been shown, without a doubt, that exposure to very minimal amounts of Imidacloprid can cause measurable changes in bee learning and behavior.

There are other papers that demonstrate that it is very difficult to determine an LD50 (median lethal dose) for the chemical. The values in print cover a very wide range.

Still other studies, where Imidacloprid was fed for weeks to colonies at sub-lethal doses, but way above what would be encountered in the field, had no short term or long term negative effects on the bees.

Our readers use various Imidacloprid formulations and we'd like to warn them of any side effects.

Our problem, here, is lack of information. How much Imidacloprid or major breakdown products should we expect from treated plants? Bayer says that the value for Imidacloprid is about 5 ppb in nectar from sunflower and canola. UCR claims to have found 550 ppb in nectar from Eucalyptus trees (red gums) that had a soil treatment (Merit) around the roots to control lerp psyllids. There are no studies about what we call honeydew. That is the sugary sap that passes through sucking bugs and sometimes is collected by honey bees. If the chemical is in a high enough concentration to kill the sucking bugs, then wouldn't it be pretty high in the honeydew?

It is too soon to try to state that Imidacloprid is safe or unsafe around pollinators. However, it is safe to state that not enough research has been done on a chemical that is becoming almost ubiquitous in agricultural and urban settings.

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November 19, 2017, 12:34 am PST

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