Mold, should I be concerned?

It is early spring, and your beehive seems too quiet. You pop the lid only to find mold everywhere. It cloaks dead bees, top bars, and on the comb. There is no doubt in your mind: mold killed your bees. But did it? In truth, mold in a beehive is a result of colony death, not the cause of it. Fungi spores (the source of mold) are everywhere in the environment, waiting for the perfect conditions to germinate. In a beehive, those perfect conditions don’t normally exist until the colony is too weak to keep itself warm and dry. In most cases, by the time the mold starts to grow, the colony is already dead.

Fungi Spores

Fungi spores which grow into mold are fond of four things: moisture, food, moderate temperatures, and porous surfaces. Beehives can be a dream come true for many species.

• Moisture: Just like seeds, fungi spores need water to germinate and grow. Water can get into the hive in many ways, including humidity and leaks. But moisture in a winter hive comes mostly from the respiration of bees. This is natural and bees have dealt with this for thousands of years.
• Food: Like all plants and animals, fungi need a source of nutrients. Plenty of things in a beehive can provide nourishment, including pollen, nectar, dead bees, and other debris. Even the wood could be a source.
• Temperature: Fungi can grow over a wide range of temperatures, from just above freezing to around 120°F. But between about 70-90°F is best.
• Porous surfaces: Porous surfaces provide a nest of sorts, a place where the fungi can latch on and not be swept away .

Colony Health

Somewhere along the line, something goes wrong with your colony. It could be mites, queen failure, or a mouse infestation. For whatever reason, the size of the colony begins to diminish. As it gets smaller and smaller, fewer and fewer bees are available to maintain the temperature. The amount of circulation/convection inside the hive—warm air out, cool air in—which is driven by difference of the colony heat and cooler outside temperature, gradually decreases. When the colony becomes too small, the turnover of air is not great enough to keep the interior dry.

In fact, instead of circulating out, the moisture builds up inside the hive as it does in cold unheated abandoned homes. It condenses on cool surfaces including the frames and comb, where fungi seize the opportunity. As the colony shrinks the mold grows. The mold is merely a sign that things got out of balance and the bees were unable to maintain mold-free conditions. The mold didn’t kill the bees but merely took advantage of the environmental conditions created by the faltering colony.

You may wonder, are all fungi bad for the bees?  Should the inside of my hive be sterile and sanitized?  Would this help the bees when they are weak?  Let’s look at that idea even closer.  Could the bees be collecting or bringing in the fungi and if so, why?

Nutritional Imbalance

The collection of fungal spores by honeybees, Apis mellifera, can be classified as active or passive, the latter when spores are associated with pollen, nectar or honey dew. While low quality and shortage of pollen have been raised as hypotheses for fungal spore collection, the nutritional value of fungal spores for honeybees is not fully understood. It has been determined that consumption of diets that contained fungal spores increased the longevity of honeybee workers but had no significant effect on the development of their hypopharyngeal glands (HPGs) and ovaries. This demonstrates that fungal spores may have nutritional value for honeybees and that the consumption of fungal spores may compensate for nutritional imbalances of poor-quality pollen diets, a likely growing issue for many colonies in their diminishing multifloral environments.  Also, it’s been suggested that bees might be motivated to collect fungal spores due to (a) lack of floral resources, (b) the chemical composition of spores (i.e. nutrients, amino acids and steroids), (c) attractants such as colour or odour, and (d) because certain spores may resemble pollen grains.

Studies have also demonstrated that consumption of spores of B. cinerea, Cladosporium sp. and C. acutatum in association with pollen can extend the lifespan of honeybee workers but had no effect on the volume of the acini in the HPGs or activation of the ovaries. HPG development and ovary activation are known to be influenced by the quality and quantity of protein ingested by honeybee workers. Although fungal spores generally have a low protein content, spores could conceivably serve as a source of additional nutrients for bees when combined with pollen. This is supported by data showing that bees fed on pollen supplemented with spores survived longer than those that did not.

Fungi and Varroa

Entomopathogenic fungi (can kill or seriously disable insects) show great promise as pesticides in terms of their relatively high target specificity, low non-target toxicity, and low residual effects in agricultural fields and the environment. However, they also frequently have characteristics that limit their use, especially concerning tolerances to temperature, ultraviolet radiation, or other abiotic factors (nonliving environmental factors such as light, temp etc.). The devastating ectoparasite of honeybees, Varroa destructor, is susceptible to entomopathogenic fungi, but the relatively warm temperatures inside honeybee hives have prevented these fungi from becoming effective control measures. Using a combination of traditional selection and directed evolution techniques new strains of Metarhizium brunneum were created that survived, germinated, and grew better at beehive temperatures (95°F/35°C). Field tests with full-sized honeybee colonies confirmed that the new strain named JH1078 is more virulent against Varroa mites and controls the pest comparable to current treatments.

The primary mode of action for Varroa control is likely through mitospore (a small usually single-celled asexual reproductive body) adhesion and germination on the mite exoskeleton, followed by hyphal (filamentous branches) penetration through the exoskeleton and proliferation throughout internal tissues of the mite. These results indicate that entomopathogenic fungi are evolutionarily readily changeable and capable of playing a larger role in modern pest management practices.

Bees Spread Fungi

Honeybees, Apis mellifera, have been implicated as vectors (entity that is used to transfer the genetic material) of plant pathogens. As workers defecate outside the hive, transport of hives could give rise to biosecurity concerns if fungal spores remain viable following passage through the digestive tract. Although survival could be low, the large number of workers per hive implies a high probability of transmission of viable spores through honeybee feces. Hence, in the case of economically important fungal diseases, transported hives could be a source of inoculum (active material used in an inoculation)  and quarantine restrictions should be considered.

In healthy bee colonies, workers will defecate outside their hives. In theory, this could spread spores of plant pathogenic fungi over a typical forage range of 3 mi/5 km from the hive if workers feed on spores or infected food sources and the spores survive through their digestive tract. Such vectoring of generalist plant pathogenic fungi through feces could affect agricultural and natural systems as these species of fungi can cause disease in a large range of host plants, including almond, apple, citrus, stone fruit and strawberry. Furthermore, dispersal of viable spores would occur even if only a small percentage of spore-fed bees defecate, because of the large number of bees per hive, the amount of hives per unit area and the number of foraging trips made by each bee. As a single spore can induce infection in favorable conditions, if foragers and nurse bees feed on an infected food source in or outside of the hive, dispersal of viable spores through feces is a near certainty in common pollination scenarios where 100 or more hives are moved from one location to another.

Honeybees are known to disperse pathogens, such as the bacterium Erwinia amylovora (the causal agent of fire blight of apple and pear trees), from inoculum placed in hives to flowers, leading to the establishment of the bacteria on the surface of the stigmas and, consequently, infection. In addition, E. amylovora can remain viable in beehives and on the bee’s body for over 24 h, on pollen for 72 h and for a period of 36 h in the bee’s intestine. In Europe, these findings have led to the implementation of biosecurity restrictions associated with the transport of beehives to prevent the long-distance transport of the pathogen. Additionally, spores of Austropuccinia psidii, the causal agent of myrtle rust, survived in honeybee hives and on the body of workers for at least 9 days after they had been experimentally inoculated.  The period of time for which propagules of plant pathogens remain viable in the intestines of workers is critical as the duration of transport of hives between crops and regions is typically less than 24 h. In addition, there is potential for long range dispersal due to migratory beekeeping practices, which extend to distances of up to 2796 mi/ 4500 km in the USA and 868 mi/1400 km in Australia.

Fungal Benefits

Now that we see some of the negative results of bees and fungi, what are some of the known benefits? Bee–fungus associations are common, and while most studies focus on entomopathogens (as above), emerging evidence suggests that bees associate with a variety of symbiotic fungi that can influence bee behavior and health. It’s also been found that fungal communities differ across habitats, with some groups restricted mostly to flowers (Metschnikowia), while others are present almost exclusively in stored provisions (Zygosaccharomyces). Starmerella yeasts are found in multiple habitats in association with many bee species. Bee species differ widely in the abundance and identity of fungi hosted. Functional studies suggest that yeasts affect bee foraging, development, and pathogen interactions, though few bee and fungal classifications have been examined in this context. Rarely, fungi are obligately beneficial symbionts of bees, whereas most are facultative bee associates with unknown or ecologically contextual effects. Fungicides can reduce fungal abundance and alter fungal communities associated with bees, potentially disrupting bee–fungi associations.

Rarely, honeybees have been observed to directly collect spores of plant pathogens from plant surfaces, including the rust fungi Melampsora, Uromyces, and Zaghouania, powdery mildew in the genus Podosphaera, and other fungi within the genus Cladosporium. The purpose of this behavior is unknown, but it has mostly been observed at the end of summer, when floral resources are scarce. Honeybees will not consume pure fungal spores, but may mix these spores with provisions, potentially providing a nutritional benefit greater than pollen alone. Multiple plant pathogens infect plants and produce fungal spores that mimic pollen and although multiple bee species visit such mimics, whether they ingest these spores is unclear.  Fungi could also benefit bees by reducing the growth of pathogens or spoilage microbes in provisions through microbe–microbe competition, though evidence for this is mixed. Fungi may also affect bee immunity, as has been documented for bacteria in the corbiculate bee gut microbiome.

Moldy Frames

So, what do we do if we have moldy comb? The first thing to do is take the frames to a warm, dry place where any excess moisture can evaporate. These frames can smell bad so put them in a place where they won’t bother you. Separate any frames that are molded together and let them air dry. As they dry the mold growth will slow down and then stop.

Once the combs are dry you can store them. As your colonies build population in the spring, you can add these frames back in. The bees will clean and polish every cell in a matter of a few days. Worker bees always clean old cells prior to re-use anyway, so this is not an unnecessary burden on the bees. The bees are very thorough. After they are done, the combs can be used for brood or honey production. No taste or smell of mold will remain on the combs.

What Next

Moldy combs can look and smell horrible not to mention the disappointment of seeing you have a dead colony, but we should focus on what other factors may have caused this situation and determine how to address them.  We should also understand that colonies coming out of the spring take mold issues in stride, just as they would in nature. After the bees clean and polish them, the wax combs are as good as gold.

Regarding fungal spores as a solution to Varroa, this shows great promise especially for those of us that are anxiously looking for natural solutions but any such modification (strengthening) of a fungi (lets not forget COVID), that has had a lengthy coevolutionary develop alongside of bees, should be done with extreme caution.  It may take decades to truly understand how the advanced fungi will interact with honeybees, the soil, or any other insect for that matter.  We also know that we can’t trust that bees and mites won’t escape the lab such as we saw with Africanized bees escaping from a lab in South America.  What will happen if the bees are not also given the chance to advance as beekeepers jump to push this new solution into their colony .  Will the spore begin to attach bees?  Will this new fungi become the new Varroa? The history of past treatments should give us some pause.

We will never eliminate fungi spores from the inside of our hives and the bees are well adapted to address them and even use them for their own health.  What we rightly need to focus on if we see signs of mold are the conditions allowing large outbreaks to happen.

Reference Materials

• Physics for beekeepers: mold in a beehive
• Nutritional benefit of fungal spores for honey bee workers
• Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies
• Fungus fights mites that harm honey bees
• Survival and probability of transmission of plant pathogenic fungi through the digestive tract of honey bee workers
• Compartmentalization of bacterial and fungal microbiomes in the gut of adult honeybees
• Honey bee genetics shape the strain-level structure of gut microbiota in social transmission
• The role of the gut microbiome in health and disease of adult honey bee workers
• Bees just wanna have fungi: a review of bee associations with nonpathogenic fungi
• Don’t panic: how to handle moldy combs in your beehive