Viruses, bacteria and fungi are microbial pathogens that bug bees by Holly Holt, PhD

Bee diseases

Microbial pathogens, including viruses, bacteria and fungi infect bees. Many factors contribute to each bee's ability to withstand microbial infection. A bee's exposure to other stressors, like pesticides or poor nutrition, can determine whether an infection is mild or results in death. Alarming rates of colony losses have prompted unprecedented levels of scientific interest in honey bee pathogens. In contrast, native bee diseases remain largely unstudied despite concurrent declines in wild populations. To protect managed and wild bee populations, scientists need to investigate how diseases affect different bee species, how diseases are transmitted between bee species and how stressors change disease dynamics in bee populations.


There are approximately 20 known honey bee viruses.1,2 Viruses spread when healthy bees interact with infected nestmates or contaminated food. Viruses are also transmitted between drones and queens during mating and between queens and their offspring during egg laying. Hive pests, parasites and parasitoids may also carry viruses. Infected bees exhibit a range of symptoms depending on the virus and the severity of infection. Some viruses disproportionately affect certain honey bee castes and developmental stages.

Many honey bee viruses are widespread, and several are associated with colony collapse disorder. However, not all infections are symptomatic. Other stressors can determine how deadly viral infections are for colonies.3 For example, varroa mites are introduced parasites of honey bees.4 Varroa mites feed on the blood of immature bees. While feeding, mites spread viruses and immunosuppress their bee host, allowing viruses to replicate. Thus, varroa mite parasitization and associated viruses contribute to a significant number of the colonies lost each year in the US. IPM strategies that curb varroa mite levels also help control viral damage in honey bee colonies. Though, experimental antiviral colony supplements are in development,5,6 long-term solutions will likely involve the continued selective breeding for mite-resistant bees.4

Bee with Mites

Interestingly, many viruses that infect honey bees are found in other insect species.7 Deformed wing virus (DWV), for example, infects up to 20 other bee species,8 at least 8 insect orders and select arachnids.9 In fact, some interpretations of fossil records suggest that DWV circulated in prehistoric cockroaches.10 Yet, relatively little is known about the damage caused by honey bee viruses outside of honey bee hives. Experiments show that viruses can spread between bee species through shared habitat. For example, infected honey bees shed viral particles on flowers. Other bee species expose themselves and their nests by later visiting the same flower. Damaging effects of DWV have been observed in two bumble bee species, and “pathogen spillover" from managed honey bee and bumble bee colonies to native bee populations is a serious concern. Determining, when, how and why viruses are damaging in managed and wild bees (and other insects), and the best methods for controlling viruses requires further investigation.8,9


Two well-known bacterial pathogens cause European foulbrood (Melissococcus putonius) and American foulbrood (Paenibacillus larvae) outbreaks in honey bees.11 Both bacterial species attack and replicate in honey bee larvae. Infected larvae are reduced to gooey sacs that later harden into scales contaminated with bacterial spores. Bees or beekeepers that come in contact with these spores can spread the disease. Both types of foulbrood can be deadly for colonies, but American foulbrood is particularly damaging; unchecked infection can result in the destruction of entire apiaries.

There are IPM guidelines for the management of European and American foulbrood.12 Notably, antibiotics temporarily suppress both types of infection. However, antibiotics only work on actively replicating bacteria inside bees. Foulbrood spores contaminating apiaries can remain viable for decades, waiting to encounter a host. Once treatments wear off, infections usually reemerge. In North America, antibiotic overuse has contributed to the emergence of resistant foulbrood strains.13 Consequently, US beekeepers need veterinary supervision to treat colonies with medically important antibiotics.14 Unfortunately, burning all contaminated hives and equipment is the best long-term solution for controlling American Foulbrood.

Interestingly, while both American and European foulbrood attack European honey bees (A. meliifera), European foulbrood also attacks Asian (A. cerana) and Himalayan (A. laboriosa) honey bees. Another honey bee bacterial pathogen, Serratia marcescens, causes lethal septicemia in adult honey bees, and potentially brood. Various Serratia species attack up to 70 different insect species, but the role of S. marcescens in bee health has received little attention.11

Bacterial pathogens of native bees are largely undocumented, but a few pathogens have been described. For example, bacterial members of the Spiroplasma genus attack bumble, mason and honey bees. The bacterium Lysinibacillus sphaericus attacks stingless bees, which are important pollinators in neotropical zones. These early studies likely just scratch the surface of bacterial pathogens that infect native bee populations.11


Different fungal lineages are important insect pathogens. Several fungi infect economically important pests, and some fungi are even used as agricultural biopesticides. Fungi generally flourish in warm, humid environments, making bee nests, both above- and belowground, susceptible to fungal attack. Few pathogenic fungi of bees have received scientific attention, and several more pathogenic species likely await discovery. Two groups of important fungal pathogens in bees are discussed below.

Chalkbrood (Ascosphaera apis) is an important fungal pathogen of larval honey bees.15 Larvae that eat fungal spores are soon killed and enveloped in a white fungal growth. As the larval body slowly dries, the fungus produces infectious spores, changing the bee's body to a darker powdery brown or black. These desiccated bee remains are called chalkbrood mummies. Bees or beekeepers can spread the spores after handling mummies. Chalkbrood infection alone is rarely fatal to colonies, but it can reduce hive productivity or make colonies more susceptible to other stressors. Workers in hygienic lines of honey bees limit chalkbrood infection by identifying and removing infected larvae. Beekeepers can also reduce chalkbrood spread by not swapping frames between colonies. Other IPM tactics include sanitation of hive equipment and potentially the application of antifungal products.

Solitary cavity nesting bee species, including mason (Osmia spp.) and leafcutting bees (Megachile spp.), are also susceptible to chalkbrood infection caused by several related fungal species (Ascosphaera spp.).16,17 Mason and leafcutting bees are important pollinators in orchards and select crops like alfalfa, respectively. Commercial operations distribute solitary bees in artificially aggregated nests. Close nest proximity can facilitate pathogen spread, and best practices for controlling chalkbrood in managed solitary bees are still in development.18–20 Even less is known about the impact of chalkbrood on wild bee populations and whether commercial management promotes chalkbrood spillover to wild populations.

Microsporidia are a highly specialized group of fungal parasites that are transmitted through infectious spores.21 Three microsporidian species cause intestinal infections in European honey bees (Apis mellifera). Spores of Nosema apis were discovered in European honey bees in the mid-19th century, and the parasite was officially described in 1909.22 In contrast, N. ceranae was recently introduced to European honey bee populations from Asian honey bees (A. cerana). Though introductions may have occurred earlier, the first N. ceranae infections in European honey bees were reported in Spain and Taiwan in 2006 and 2007, respectively. Subsequent reports found that N. ceranae has a worldwide distribution.23 A third microsporidian species, N. neumanni, was recently discovered in Ugandan honey bees.24 N. neumanni's distribution and associated colony morbidity and mortality have not been determined. N. ceranae infections are generally more harmful than N. apis infections in European honey bees, but reported colony damage is highly variable. In most parts of the world, the chronic nature of N. ceranae infections weakens hives, reducing colony workforce and honey production and amplifying the effects of other stressors. Limited chemical controls exist, though new antifungal products are in development. Crucial studies are still needed to realize a complete Nosema IPM strategy in honey bees.21,25

N. ceranae has the potential to infect multiple native bee genera in North America, including bumble bees which already carry their own specialized microsporidian parasite, N. bombi. N. ceranae has been detected in bumble bees in England and China. Furthermore, N. ceranae now infects South American bumble bees, stingless bees and a social wasp. N. ceranae is also known to infect 4 honey bee species: A. cerana, A. mellifera, A. dorsata and A. florea. The importance and pathology of Nosema spp. infections in wild bee populations are understudied.26

How can you help limit pathogen spread in bees?

  1. Reduce managed and wild be stress by restoring bee habitat. Provide bees with food and nesting sites. Avoid spraying pesticides which can stress bees, making them more susceptible to infection.
  2. Use best management practices to maintain honey bee colonies. Learn how to recognize honey bee pathogens, parasites and pests. Use appropriate IPM strategies to control diseases and pests.
  3. If you elect to provide artificial nests for wild bees, follow best practices. At appropriate times, clean nest materials and purge parasitoids and pests.
  4. Read more about pathogens, parasites and pests of honey bees and the ecology of other pollinators in Pennsylvania: Quick guide to honey bee parasites, pests, predators and diseases; Honey bee viruses; Varroa mite IPM; American Foulbrood; Honey bee declines; Beekeeping practices; Spring bees: who are they and where do they live?; Bees in Pennsylvania: diversity, ecology and importance; Conserving wild bees in Pennsylvania.
  5. Participate in citizen science. (a.) Native bees: Help scientists locate ground nesting bees and study their diseases by contributing data to Bee Germs. If you decide to install an artificial bee nest or hotel, determine if there are ongoing citizen science projects that are requesting data from artificial bee nests. The Minnesota Bee Atlas is one example. (b.) Honey bees: Help define national trends in honey bee colony health and identify effective management strategies by participating in citizen science. You can provide data to programs like the Bee Informed Partnership. Also, if you live in Pennsylvania help scientists track the health of feral honey bee colonies by reporting sightings to Penn State.


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