Fungicides and Bee Health

I have had a lot of questions/concerns regarding bee health and bloom fungicides sprays. Even though bloom is nearing its end, I thought I would try and answer a few questions I have received – and finally had some time to research and formulate an answer to these questions.

Do fungicides applied during bloom affect bee health?
The short answer is “We don’t know.” Research has shown that fungicides that are applied around or at bloom do adhere to the pollen, and are brought back to the hive during the process of pollination. The fungicides then inhibit the growth of different fungi within the bee hive, decreasing the the microbial diversity of the bee’s food source. As of the Almond Board of California 2009 Research Proceedings, it is unknown whether or not the fungi affected benefit or harm the overall health of the hive.

Which Fungicides are transported back to the hive?
Researchers at the Carl Hayden Bee Research Center in Tuscon AZ found five commonly used fungicides in the bee bread (food source) of hives placed in almond orchards. These fungicides include Chlorothalonil, Cyprodinil, Fenbuconazole, Iprodione, Boscalid, and Pyraclostrobin. The amounts of fungicide appear to vary by orchard and timing of application – BUT – this data was only based on hives placed in three orchards with two or three samplings- for now. Also, it is not known which fungicides target which fungi, if they negatively affect bee health directly, and if they have a negative impact on the entire hive by reducing the quality of the food source. The research group headed by Dr. DeGrandi-Hoffman will be looking into these questions through this year.

What about Pristine (Pyraclostrobin-Boscalid) applications?
Essentially, we don’t know exactly how this fungicide affects fungal growth in the hive. In vitro studies from Dr. DeGrandi-Hoffman’s lab indicates that the low rates of Pyraclostrobin-Boscalid that are comparable to the concentrations applied in the field resulted in the reduced growth of all 12 fungi the group isolated from the bee bread. It varied from slight to complete growth inhibition. Future research will hopefully determine if the concentrations found in the hive are able to decrease fungal growth and if the reduction of fungal growth negatively affects bee health.

How should we adjust our spraying to prevent harm to our hives?
Again, I am unsure. My best guess would be to talk with your beekeeper regarding a fungicide spray program, make fungicide applications when the bees are not active, apply the fungicides only by ensuring the spray tank is clean, and apply when pollen shedding is low (usually in the evening). For the most part, fungicides are required, especially this year, to maintain a profitable harvest – but so are bees. I hope future research will help guide our fungicide application practices to reduce unwanted damage to bees during pollination.

An article published on March 7th, 2009 in “The Western Farm Press” discussing previous/similar/more research findings by the Carl Hayden Research Center can be found here.

Information sourced from G. DeGrandi-Hoffman. 2009. “Determining the effects of fungicide contamination on nectar and pollen on honey bee colony health.” Almond Board of California Research Proceedings. Technical Report.

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2 thoughts on “Fungicides and Bee Health

  1. Commercial beekeepers from Texas have noted honeybee colonies exposed to fungicide used by almond growers fail to thrive. The expedient solution is to sell the contaminated hives for an attractive price to new beekeepers – or introduce new comb into all contaminated colonies. The former makes the commercial beekeeper a small profit; the latter, costs money. Guess which path commercial beekeepers are taking?


    1. That is interesting Britt. Thanks for the information. I would urge caution at some of your statements. Although research has suggested that some fungicides may negatively impact brood, not all fungicides have been shown to cause negative impacts on honey bee colonies. Many almond growers are moving away from these fungicides that are being implicated.

      With the amount of hive losses that have occurred, it is easy to point a finger and paint broad strokes. Only by researching these problems can we find the true cause. I urge you to contact the bee research group in Tuscon, UC Davis, Ohio State or elsewhere and see if there is any type of guidance that you can provide to the researchers who are working hard to figure out this problem.


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