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Study Finds Imidacloprid Safe for Honey Bees at Realistic Exposure Levels

Honey bees were fed with imidacloprid-dosed pollen patties, like the one seen here. Photo by Galen Dively.

Honey bee colony declines are a major threat worldwide. Among the lineup of possible causes — including parasites, disease, climate stress, and malnutrition — many have pointed the finger squarely at insecticides as a prime suspect, especially at a class of pesticides known as neonicotinoids.

However, a new study from the University of Maryland shows that the world’s most common insecticide — imidacloprid — does not significantly harm honey bee colonies at real-world dosage levels.

“Everyone is pointing the finger at these insecticides,” said Galen Dively, emeritus professor of entomology at UMD and lead author of the study. “If you pull up a search on the Internet, that’s practically all anyone is talking about. This paper says no, it’s not the sole cause. It contributes, but there is a bigger picture.”

The study, which was published in the journal PLOS ONE, looked at the effects of imidacloprid on honey bee colonies over a three-year period. Insecticides in the neonicotinoid class are chemically derived from nicotine. In tobacco and other related plants, nicotine acts as a deterrent by poisoning insects that bite the plants. In fact, nicotine used to be commonly used as an insecticide, but it has fallen out of favor because it is highly toxic to humans and breaks down rapidly in sunlight. Neonicotinoids have been engineered specifically to address these shortcomings.

“Imidacloprid is the most widely used insecticide in the world. It’s not restricted because it is very safe — an order of magnitude safer than organophosphates,” Dively said, drawing a comparison with a class of chemicals known to be highly toxic to nearly all living things.

For the study, Dively and his colleagues fed pollen dosed with imidacloprid to honey bee colonies. The team purposely constructed a worst-case scenario, even at lower exposure levels. For example, they fed the colonies tainted food for up to 12 continuous weeks. This is a much longer exposure than bee colonies would experience in real-world scenarios, because most crops do not bloom for such an extended period of time.

Even at these longer exposure periods, realistic dosage levels of imidacloprid did not cause significant effects in the honey bee colonies. Only at higher levels did the colonies start to have trouble producing healthy offspring and surviving through the winter.

“A lot of attention has been paid to neonicotinoids, but there isn’t a lot of field data,” said Dennis vanEngelsdorp, an assistant professor of entomology at UMD who was not involved in the study. “This study is among the first to address that gap. It’s not surprising that higher levels will hurt insects. They’re insecticides after all. But this study is saying that neonicotinoids probably aren’t the sole culprit at lower, real-world doses.”

Dively and vanEngelsdorp both agree that a synergistic combination of many factors is most likely to blame for colony declines. Climate stress could be taking a toll, and malnutrition could be a factor as well. The latter is a particular concern for industrial bee colonies that are rented to large-scale agricultural operations. These bees spend much of their time eating pollen from one or two crops, which throws their diet out of balance.

“Except for the imidacloprid exposure, our test colonies were treated well,” said co-author David Hawthorne, associate professor of entomology at UMD and director of education at the National Socio-Environmental Synthesis Center. “They weren’t exposed to additional real-world stressors such as malnourishment or multiple pesticides. Colonies coping with these additional pressures may be more sensitive to imidacloprid.”

Dively, Hawthorne, and their colleagues found some evidence for at least one synergistic combination. At the highest dosage levels — 20 times the realistic dosage — colonies became more susceptible to Varroa mites, parasites that target honey bee colonies. A mite infestation can cause a whole variety of problems, including viral infections and an increased need for other pesticides to control the mites.

“It’s a multifactorial issue, with lots of stress factors,” Dively said. “Honey bees have a lot of pests and diseases to deal with. Insecticide exposure is one factor among many. It’s not the lone villain.”

Read more at:

Assessment of Chronic Sublethal Effects of Imidacloprid on Honey Bee Colony Health


  1. Now the work of undoing years of erroneous, false information from the beekeepers and anti-pesticide cabals can begin in earnest. As part of that work we should be telling people that two of the biggest toxicological stresses on colonies were found to be acaricides applied by beekeepers.

  2. “Imidacloprid Safe for Honey Bees at Realistic Exposure Levels”

    “neonicotinoids probably aren’t the sole culprit”

    The statements above are not nearly the same. Misleading title?

    • I own greenhouses. This seems like an interesting study but does it really provide any new insight?

      Studies have always shown that bees are not directly killed by imidacloprid residual. This was one of the original marketing points for this product. Imidacloprid is also widely used as a termiticide. It works on termites by interfering with their homing system. Bees have a very similar system to find their way back to their hives. It is my understanding that bees exposed to imidacloprid have a difficult finding their way home causing the hive to become depopulated or abandoned.

      Until a couple years ago I would use imidacloprid as a foliar spray. The amount required is extremely low to provide effective penetration. [Now my customers demand neonic free flowers so I no longer use this chemical.] Studies seem to indicate the biggest issue for bees with imidacloprid is when applied as a soil drench due to the long term residual in the soil and the small amount required to disrupt the bee’s homing system. Apparently imidacloprid applied directly to plant tissue [not roots] will only migrate within the material that it is contacting through xylemic activity. It is not translaminar. New growth is not protected after foliar application. This is not the case when imidacloprid is applied as a soil drench. It can then get into the flowers and any new growth and has a long residual in soil.

      Imidacloprid shows little toxicity to humans, so it is now widely marketed to home users. It is now often used for soil application to kill grubs and as a drench to trees for emerald ash borer (and other boring insects.) When applied to the soil it will often migrate to flowering crops or adjacent weeds.

      I am not a scientist, just a farmer. I watched my father apply imidacloprid to his apple crop for many years as a foliar spray. He would never spray this on the plants during bloom or just prior to bloom. We continue to support an abundant wild bee population and do not have to “in-source” honey bees for pollination. I do believe studies that indicate neonics can cause harm to bees by disrupting their homing instinct. If this happens with termites, it can probably happen to bees. This is just common sense.

      I will continue to watch this issue with a lot of interest. I do think it is worth noting that there are a lot of chemicals used in horticulture that have a negative impact on bees. This list does not only include insecticides. Chlorothalonil (aka Bravo, Fungonil,etc) is a widely used fungicide that is extremely toxic to bees when they come into contact with it while it is wet. I am also very curious about the impact of acephate (Orthene) which is very widely used in the horticulture industry. Acephate readly migrates into flowers. I can only speculate that this organophosphate is toxic to bees..

  3. Colluvial, according to a UMD press release, “To see significant negative effects, including a sharp decrease in winter survival rates, the researchers had to expose the colonies to at least four times as much insecticide encountered under normal circumstances.”

    At realistic exposure levels, they saw nothing. At FOUR TIMES those levels, they saw negative results. Hence the title.

    • I think colluvial has a valid point ET. The study authors agree that they have not tested the synergistic effects of real world honey bee diets (often impoverished), pest load and neonic impacts. Thus, it is misleading to say that real world levels of neonic use are harmless when the real world context has not been studied. What can be claimed is that real world levels of neonic use alone do not show harmful effects. We need to ensure clarity and context when reporting important findings.

      • Ann-Marie, the words “real-world doses” are used by Dennis vanEngelsdorp, one of the world’s leading authorities on honey bees and Colony Collapse Disorder, and by his University of Maryland colleagues. Regarding other factors, please re-read the third to last paragraph. That is addressed by Dr. Hawthorne.

    • I think the point remains ET that the title of the article does not reflect the real-world context that faces many bee colonies globally. Thus, a more circumspect summary of the UMD research could have been titled – Research finds that realistic neonicitinoid exposure levels in isolation do not adversely impact honey bees. This provides the necessary qualification that good science requires, and picks up on the points you note that are made by Dr Hawthorne.

    • Indeed and chronic exposure over even a longer bit of time will still accumulate a toxic effect.

  4. “It’s not restricted because it is very safe” REALLY?? You know that this is BANNED across the WHOLE of the EUROPEAN UNION COUNTRIES …right? So you are prepared to believe a single new ‘independent’ study… The US. has clearly eaten so much GM chemically produced frankenfood that you have all gone completely mad. Good luck with your future dead bees if you keep ignoring the facts and continue allowing the Petrochemical giants to rule your country.

    • Anna, “very safe” for humans compared to other classes of insecticide: YES, absolutely. Same goes for the GMOs you mentioned. They have been deemed safe by many scientific organizations, including the UK’s Royal Society of Medicine:

      Our “Frankenfoods” (Bt crops) have allowed us to eliminate millions of kilos of pesticide active ingredients, according to the U.S. Department of Agriculture and other organizations (Bt, by the way has been used for more than 50 years by organic farmers). Others (herbicide resistant crops) allow the employment of “no-till” farming (try Googling it), which prevents erosion and saves top soil. We do thank you for your concerns about our health, though.

    • Anna – the bans across the whole European union are not based on any scientific research or any real reason. They are based on misinformation and a total misunderstanding on what GM crops are. Your statement itself suggests that you have no basic understanding your self. I would be happy to talk to you about what happens in ag and how food is produced as I think that if you did actually meet some farmers and saw what they do and the food that produced you would not be so misguided.

    • Read the study very closely. Most of the colonies had to be requeened or repopulated quite regularly each study and each new season EVERY colony was replaced. Please ask yourself why colonies had to be replaced several times each year….

  5. Rachel Carson! Where are you?

    Read the history of DDT use and I think you’ll see a parallel — see for reference.

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