Avoid spraying flowers with fungicides
At one time most fungicides were thought to be mostly harmless to honey bees and other pollinators.
April 13, 2016 - Author: David Smitley, MSU Department of Entomology; Diane Brown, Rebecca Finneran and Erwin Elsner, MSU Extension; Joy Landis, MSU IPM; Paula M. Shrewsbury, Univ. of Maryland Dept. of Entomology; and Daniel A. Herms, The Ohio State Univ. Dept. of Entomology
At one time most fungicides were thought to be mostly harmless to honey bees and other pollinators. However, recent research indicates fungicide brought back to the hive on contaminated pollen or on workers’ bodies interferes with the function of beneficial fungi in the hive. Several types of fungi, such as Aspergillus, Penicillium, Cladosporium and Rhizopus, grow in hives and the chemicals they secrete provide a natural defense against bee diseases like chalkbrood (Ascosphaera apis). They also play an important role in producing bee bread, a fermentation product of pollen which requires fungi.
Bee bread is a critical protein source for bee larvae and adults. Recent studies have shown bees exposed to fungicides do not produce as much bee bread in their hives. Some fungicides are more active against the fermentation fungi than others, but until more research is done, it is best to avoid spraying fungicides over open flowers of plants that are highly attractive to bees.
Fungicides applied before flowers open or after petals fall off are not expected to be harmful. Furthermore, certain fungicides can disable the detoxification enzymes of insects, which can greatly increase the toxicity of certain insecticides to bees (e.g., acetamiprid). Several studies have reported pollen contaminated with captan, ziram, iprodione, chlorothalonil and mancozeb may be harmful to bee larvae when they eat it.
Some mixtures of fungicides with insecticides may be more toxic to bees than the insecticide alone. When propiconazole is mixed with pyrethroid insecticdes, it may increase the toxicity of the insecticide to bees. Also when propaconizole and other DMI fungicides (e.g., tebuconazole, myclobutanil and triflumizole) are mixed with acetamaprid, the solution becomes fivefold or more toxic to bees than acetamaprid by itself.
Other than inhibiting beneficial fungi in the hive, and the six fungicides listed above as harmful when bee larvae consume contaminated pollen, fungicides are usually considered to be safe for bees.