Considerations for disease management
It was previously thought that fungicides and bactericides are harmless to honey bees and other pollinators, and in fact, most fungicides are still considered relatively safe, even while spraying when pollinators are present.
It was previously thought that fungicides and bactericides are harmless to honey bees and other pollinators, and in fact, most fungicides are still considered relatively safe, even while spraying when pollinators are present. However, about a third of the fungicides and bactericides used in horticulture production or landscape maintenance can be harmful to honey bees or native bees. Consult the table in Appendix 1 for a list of products that provide precautionary statements when spraying flowers on plants attractive to pollinators. Although these fungicides are rarely toxic enough to cause mortality due to contact or ingestion after spraying, they may impact bees in the following ways.
Very few fungicides or bactericides cause bee mortality with contact or ingestion, but eight (boscalid, captan, chlorothalonil, fosetyl AL, iprodione, manocozeb, neem oil, pyridaben + sulfer) have been reported to be harmful to honey bee larvae when consumed in pollen or bee bread. Seven additional are directly toxic to adults or are suspected for impacting honey bee colonies.
Nine fungicides disable the detoxification enzymes of insects, which can greatly increase the toxicity of certain insecticides to bees. In other words, when bees consume pollen or nectar with specific mixtures of fungicides and insecticides, the insecticides become more toxic because the bees can’t break down the active ingredients like they normally would. For example, when propiconazole is mixed with pyrethroid insecticides, it may increase the toxicity of the insecticide to bees. Also, when propaconizole and other DMI fungicides such as tebuconazole, myclobutanil and triflumizole are mixed with acetamiprid, the solution becomes fivefold more toxic to bees than acetamiprid by itself.
Natural defenses and bee bread
Several types of fungi grow in hives, such as Aspergillus, Penicillium, Cladosporium and Rhizopus, and they function as a natural defense against bee diseases like chalkbrood (Ascosphaera apis). These bee-defense fungi also play an important role in producing bee bread, a fermentation product of pollen. Bee bread is a critical protein source for bee larvae and adults. Closely related cousins to these bee-defense fungi also cause plant diseases such as leaf spots and post-harvest decline. Fungicides are implicated in reduction of fungal diversity in hives and reduction in bee bread, but specific active ingredient impacts have not been determined for bee-defense fungi.
Other than noted above, fungicides are usually considered to be safe for bees. Please see the table in Appendix 1 (end of this document) for a list of known impacts and those fungicides considered unlikely to impact bees. For non-pollinator attractive plants, products should be applied according to label directions to manage pathogens. For pollinator attractive plants, follow label directions along with the additional cautions below when plants are blooming. Fungicides applied before flowers open or after petals fall off are not expected to be harmful.
Read the next article in this publication series: Best Management Practices
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