Fungicides During Bloom — the Pollination Paradox

Fungicides During Bloom — the Pollination Paradox

This bumble bee is one of 80 species of wild bees found visiting Michigan tart cherry during bloom. (Photo: Emily May, Xerces Society)

Specialty crop growers use fungicides to protect against many serious plant pathogens, but they also must take into consideration the potential harm fungicides present to honey bees needed for pollination. To manage fungal diseases, fungicides are applied to the crops at strategic times throughout the growing season, including during bloom. When bees are foraging on blooming crops, they can become exposed to fungicides and other crop protectants.

In some instances, fungicide labels do not indicate what effect the products may have on bees, so growers may not be aware of their negative effects. Fungicide exposure can cause sublethal health effects that lead to weaker colonies, or even colony loss. Weaker and lost colonies can result in increased hive rental fees and less efficient pollination services.


Studies have linked fungicides to honey bee decline, and fungicides are the most commonly found pesticide in honey beehives. Because they stay in the hive for so long, exposure is complex. Honey bees are exposed to incoming crop protectants at the same time as they experience chronic exposure in the hive. Fungicide exposure alone can lead to a variety of sublethal health effects, but exposure to different combinations of chemicals can be more dangerous, as some of these combinations cause synergistic reactions, causing each chemical to become more toxic.

Because exposure is often complex and chronic, and because the health outcomes are varied and sublethal, it is very difficult to fully understand the risk of fungicides on honey bee health and the economic costs to beekeepers and growers.

Exposure Dangers

Even if you don’t directly rely on pollination, you can greatly impact bee health when making pesticide applications. Honey bees encounter a wide range of crop protectants throughout their active season and flight range. They can fly more than 6 kilometers away from their hive, covering an area of almost 20,000 acres.

As they visit plants on many different farms and landscapes in this range, they are exposed to chemicals through a variety of pathways, including direct spray, direct floral contact, contact with systemically treated plant materials, drift onto nearby floral resources or into the hive, and water contamination through runoff.

Foraging bees will carry resources and pesticides back to the hive, which means that bees that do not come into immediate contact with pesticides can still be exposed through stored hive products. The hive is made up of wax, which is lipophilic and can hold chemicals for years. Stored pesticides can remain in beehives for a long period, resulting in chronic exposure of a variety of chemicals.

Fungicide residues found in both hive pollen stores and in wax comb are significantly higher than residues in herbicides or insecticides. The accumulation and persistence of fungicides within a hive can mean that one exposure event can have long-term effects on the colony’s health and productivity.

Known Effects of Fungicides

A honey bee colony is a superorganism. Its health is dependent on an appropriate balance of individuals fulfilling various roles within the hive. While fungicides rarely kill an individual adult bee outright, they can cause a variety of health effects that can greatly disrupt the productivity and overall health of the entire hive.

Development. Fungicide contamination can cause severe health effects in developing bees (larvae and pupae). The queen must consistently lay eggs that develop into a healthy and high-functioning workforce for a colony to remain strong. Fungicides have been shown to cause decreased larval survival and abnormal development, leading to hive population loss. Reduced hive populations disrupt the social balance within the hive and, if severe, can lead to colony collapse.

Health and nutrition. When fungicides accumulate in a bee’s food sources, it can cause a negative impact on bee health and nutrition. Fungicide exposure can cause reductions in food consumption, beneficial gut bacteria, and protein or nutrient digestion.

Cellular Function. Honey bee cellular function can also be severely disrupted by fungicide exposure. Honey bees exposed to fungicides can experience lower energy levels, oxidative stress, and changes to hormones and enzymes. Perhaps the most dangerous effect on cellular function is the loss of the ability to regulate temperature, leading to an increased risk for hypothermia. 

Disease. Fungicide exposure can increase the prevalence and severity of honey bee diseases, as well as disrupt a bee’s ability to withstand and ward off infections. Fungicides have been shown to increase the probability of Nosema infection (a microsporidial disease), as well as reduce a bee’s ability to survive after becoming infected. Higher levels of other dangerous honey bee viruses have occurred after fungicide exposure, such as black queen cell virus and deformed wing virus.

Behavior and memory. Exposure to fungicides can have detrimental effects on honey bee learning, behavior, and memory. Fungicides have been shown to impair olfactory learning and memory, which may decrease colony survival and reduce its ability to adequately provide pollination services for a crop in bloom.

Multi-pesticide Exposure

Fungicides and other products typically labeled “safe for bees” can become very hazardous when they are combined with other chemicals. While fungicides can have health effects alone, their greatest risk is often when they work synergistically with other agrochemicals.

A phenomenon called “synergism” occurs when simultaneous exposure to multiple pesticides dramatically increases the toxicity of those pesticides. As honey bees forage, they are exposed to a huge range of chemicals that are then brought back to the hive and combined with chemicals already in the colony. These chemicals can work together causing synergistic effects including increased mortality, greater risk of disease and viruses, reduced digestion, decreased cellular function, and energy loss.

Although the mechanisms behind synergistic reactions aren’t always clear, fungicides are known to inhibit a bee’s ability to detoxify other chemicals they are exposed to. An inability to detoxify fungicides in the presence of other chemicals may ultimately be what makes fungicides and insecticide mixes so dangerous for bees.

This figure gives some examples of insecticide and fungicide combinations known to cause negative health effects in bees.


We often focus on active ingredients when we look at risk, but other ingredients also can affect bee health. Fungicide formulations often contain non-active ingredients, such as adjuvants and surfactants, that enhance the product’s effectiveness. These additives recently have been shown to be detrimental to honey bee health, despite being widely considered as having no biological effects.

The possible health effects of pesticide adjuvants and surfactants include an increase in mortality and abnormal development, higher rates of black queen cell virus, and learning and memory impairment.

Large Knowledge and Policy Gaps

Just because a product is not labeled as “toxic to pollinators” does not mean the product is “safe for bees.” Pesticide regulations and labels do not reflect true risks to bees. Pesticide warning labels have been focused on chemicals that are highly toxic to adults through direct contact, rather than the sub-lethal effects that many fungicides are known to have. Label restrictions focus on immediate death after acute exposure and are not found on chemicals like fungicides that cause sub-lethal effects.

We like to make decisions based on scientific risk assessments. For honey bees, this work has just begun. As we look further into pesticide risks, we see that different life stages of bees, such as developing larvae, respond differently to pesticide exposure than adult bees. We see that there are a variety of health effects, including sublethal effects on behavior, learning, immune function, disease susceptibility, digestion and absorption of nutrients, development, reproduction, and cellular function. We see different effects based on the different exposure scenarios and different product mixtures.

For a full understanding of a specific fungicide’s risk, we would have to study every application scenario in every life stage, examining for every health outcome, and in the presence of every potential combination. This is a long, daunting, and expensive process that will never be truly complete. We have to make management decisions before we will have all of the scientific answers and can confidently show that a product is safe.

We still have a long way to go before there will be an understanding of all the possible health effects of pesticides. For this reason, we can’t recommend any product as distinctly “safe for bees.”

Reducing Fungicide Exposure and Synergistic Effects 

Here are some quick tips to understanding and minimizing the effects fungicides can have on honey bees.

  • Don’t solely rely on product labels for all information regarding risks to pollinators. Just because a product is not labeled as “toxic to pollinators” does not mean the product is “bee safe!”
  • Avoid tank mixing fungicides and other pesticide types whenever possible. Especially avoid tank mixing fungicides with neonicotinoid insecticides and pyrethroids.
  • Space out treatments of different materials as much as possible to reduce the likelihood of combined exposures and synergistic effects.
  • Do not apply foliar pesticide applications during crop bloom. Do not apply systemic pesticides prior to the flowering period of your crop, as these products can accumulate in the flower’s pollen and nectar resources
  • Make applications at times when bees are inactive, such as after sunset, before sunrise, or when the temperature is below 50° This will allow for residues to dry, reducing bee exposure and the likelihood that residues will be carried back to the hive.
  • Be aware of where your product is going. Minimize drift, especially flowering weeds between rows or on orchard floors, and clean up completely any water that can contain pesticide residue.