Nosema Disease in Honey Bees: Symptoms, Causes, and Fall Treatment

Nosema in bees can be hard to recognize because the disease often shows very few outward signs. A colony may look a little weak in the fall, come out of winter far smaller than it should be, and by then the damage is already done. Nosema disease in bees is a gut infection caused by a microscopic parasite, and the only way to know how serious it is in your hives is to count the spores. 

This guide explains what Nosema is, how to spot it and test for it, what the research says about colony collapse, and what you can do about it this fall before winter arrives.

What Is Nosema Disease in Bees?

Nosema disease, also called Nosemosis, is an infection of the honey bee's midgut caused by microscopic parasites called microsporidia. Bees pick up the infection by swallowing spores. Once a spore reaches the midgut, it germinates and invades the cells that line the gut. 

Two species cause Nosema in honey bees: Nosema apis and Nosema ceranae. Penn State Extension notes that Nosema ceranae is now the predominant species. That matters because the two behave a little differently, and some of the signs beekeepers were taught to look for do not apply to both.

Nosema apis vs. Nosema ceranae

Nosema apis was first described in honey bees in 1909. Nosema ceranae was first described in 1994 in the Asian honey bee and was later found in European honey bees. For many years, N. apis was the only form beekeepers in the United States had to think about, particularly in northern areas where colonies spend long winters inside the hive. Nosema ceranae has since taken over in many places.

Under a microscope, the two are hard to tell apart without experience, and spore shape alone is not conclusive evidence of the species. Confirming the species takes a genetic test, which most beekeepers will not need. What matters in practice is knowing how many spores a colony carries.

What Causes Nosema and How It Spreads

Nosema spreads through what bees eat and what they leave behind. Infected bees pass spores in their droppings, and other bees swallow them, which is known as the fecal-oral route. Bees also pass the infection along by trophallaxis, the mouth-to-mouth food sharing that happens throughout a colony. Nosema spreads more rapidly when bees also have dysentery, because the infected droppings carry the spores.

How Can I Identify If My Bees Are Infected With Nosema?

Often you cannot, at least not by looking. The Texas Bee Inspection Service states that neither species causes a symptom that is unique to it. Instead, the signs are general ones that point to a colony that is struggling.

• Weak or shrinking colonies: Hives are less productive and carry lower populations.

• Slow spring buildup: Colonies come out of winter small and take a long time to recover.

• Heavier winter losses: More colonies die over winter than you would expect from their fall strength.

• Frequent supersedure: Colonies replace their queens more often than normal.

• Crawling bees and K-wing: Some beekeepers see bees crawling near the entrance or wings held at an odd angle. These are possible signs, but they are not specific to Nosema.

• Dysentery: Brown streaks on the front of the hive can point to N. apis, but dysentery has other causes, including poor quality feed, and N. ceranae is generally not associated with it.

These signs come with a caveat. In the Spanish colony study described later in this guide, N. ceranae infections did not produce crawling bees, dysentery, or queen supersedure, and the researchers report that no classic signs of nosema disease were evident. A colony with N. ceranae can look completely normal.

Some beekeepers pull the midgut from a bee and look for a pale, swollen gut. The extension.org guide calls this field test unreliable, so a normal-looking gut does not clear a colony.

The fall is a good time to notice weak colonies before winter. Blythewood's end-of-season inspection checklist walks through what to look at before you close hives up. Because these signs overlap with problems like Varroa and poor nutrition, the only reliable way to confirm nosema is to test.

How to Test for Nosema

The most reliable diagnosis is to look at the bees' gut contents under a microscope and count the spores. The Texas Bee Inspection Service describes crushing bees and counting with a hemocytometer as the most effective way to diagnose Nosema. You will need a compound microscope that reaches 400x magnification, a hemocytometer (a special slide with a counting grid), clean water, and a mortar and pestle or something similar for crushing the bees.

1. Collect a sample: Take about 100 adult worker bees, following a protocol such as the one from UF/IFAS. Foragers collected at the hive entrance are the better choice, because bees taken from inside the brood nest can understate how infected a colony is.

2. Crush the bees in water: Grind the bees, or just the abdomens for a cleaner sample, in a measured amount of clean water.

3. Load the hemocytometer: Place a drop of the mixture on the counting grid.

4. Count the spores: At 400x, count the spores in the squares the protocol specifies. Each protocol gives a formula that converts your count into spores per bee, and the formula depends on how much water you used, so follow it exactly. The Manitoba method, for example, counts five of the larger squares and multiplies by 250,000.

5. Compare the result to the action level: A count of 1 million spores per bee is the level most often used as the point to treat. The Texas service calls it the most accepted economic threshold, while Penn State describes it as anecdotal, so use it as a guide rather than a strict rule.

What Is the Impact of Nosema on Honey Bee Health and Colony Productivity?

Spores invade the cells of the midgut, the bee's main digestive organ, and the infection takes a toll on the bee. Nosema weakens a bee's immune system, which leaves it more open to other pathogens, and it reduces a bee's lifespan. N. ceranae in particular can suppress the immune system.

At the colony level, those effects add up. Bees that die sooner are replaced more slowly, so the population shrinks, fewer bees go out to forage, and honey production falls. Depopulation and low honey yields are among the signs of Nosema, and the Texas service lists lower populations, increased winter loss, and more supersedure.

Utah's 2015 pollinator health survey shows how often it turns up in struggling colonies. Adult bees were tested from 194 colonies that were weak, dead, or slow to build up, and Nosema was found at some level in nearly 24 percent of those in tree fruit areas and about 18 percent of those outside fruit and berry areas. The colonies were chosen because they were struggling, so these numbers do not show how common Nosema is in healthy apiaries.

Nosema also rarely acts alone. In that same survey, Varroa mite loads were above the recommended levels in every area. A mite plan from Blythewood's Varroa mite control collection helps protect colonies from one of the biggest stresses on their health.

What Are the Most Effective Treatments for Nosema in Honey Bees?

The short answer is fumagillin, an antibiotic that is mixed into sugar syrup. Penn State Extension lists it as the only treatment for Nosema disease in honey bees. Nosema treatment for bees works best when it is based on a spore count, because a colony with a low count may not need medication at all.

• Which species it targets: For Nosema apis treatment specifically, the label directions are written around N. apis, which the Fumagilin-B label describes as the target. The Higes study found it also controlled N. ceranae, and extension.org notes it is often still effective against both species.

• Rules and availability: The label says it is for veterinary use only, so check with your state apiarist, inspector, or veterinarian about how to buy and use it where you live. 

• Honey residue: The label says honey from the brood nest areas of treated colonies should not be harvested for human consumption, which is why timing matters.

• Reinfection is possible: Treatment does not give lasting protection. In the Spanish study, fumagillin cleared the infection, but the treated colonies became infected again about six months later, so keep testing after treatment.

• Management still matters: Vita Bee Health, the maker of Fumagillin-B, lists protein feed supplements, effective Varroa control, and regular comb changes as indirect ways to keep nosema in check, though it notes these are not always effective in harsh winters. Fresh frames and foundation make comb rotation easier.

How to Treat Nosema in Honey Bees This Fall

If a fall spore count is high and your inspector or veterinarian confirms treatment is appropriate, the label for fumagillin follows a simple order. Always follow the directions on the product you buy, because concentrations and syrup amounts differ between brands.

1. Test first: Do a spore count before you treat, so you know whether the colony needs it.

2. Remove the honey supers: The label says to feed medicated syrup for fall treatment after all honey supers have been taken off.

3. Mix fresh syrup and feed it promptly: Mix the syrup as directed and feed it within 24 hours, as the Fumidil-B label says. A good top feeder from the feeders and lids collection keeps this simple, and plain syrup supplies are in the syrup, protein, and supplements collection.

4. Let the bees finish it before winter: The label says the syrup should be fed early in the fall and consumed by the bees so residues do not end up in honey. It also says to feed additional unmedicated syrup to reach the colony's winter weight. If you are choosing between winter feeds, Blythewood's guide to candy board, fondant, and dry sugar can help.

5. Retest in the spring: Reinfection can happen, and some labels also describe a spring feeding, so test again in the spring and ask your inspector whether a second treatment makes sense.

Are There Natural or Organic Methods to Control Nosema in Bees?

There is no registered natural or organic treatment for Nosema. Research into alternatives is active, though, and basic hive management does make a real difference.

• Thymol: In a 2008 lab trial in Italy, bees fed thymol or resveratrol candy had significantly lower infection rates, and the resveratrol group also lived longer. A 2022 study found that thymol reduced spore loads in infected bees but caused problems in bees that were not infected, so it should not be used as a preventive.

• Good nutrition, with a caveat: Well-fed colonies cope better, but nutrition is not a simple fix. A 2015 PLoS ONE study found that caged bees eating some commercial pollen substitute diets had higher Nosema levels than bees eating wildflower pollen. The takeaway is that protein patties are not a cure, so choose supplements for nutrition and not as a Nosema treatment.

• Comb rotation: Infected bees pass spores in their droppings, so regular comb changes are one of the indirect steps worth including in your yearly routine.

• Varroa control: Keeping mite loads low removes one of the other stresses on the colony.

• Probiotics and gut supplements: These are popular for general gut health, but unless a product has research showing it lowers spore counts, treat it as general support and not as a treatment.

Natural approaches work best as part of prevention, and a spore count is still the way to tell whether any of them is working.

Conclusion

Nosema is easy to miss because it rarely announces itself. The best protection is a routine of fall inspections, a spore count on colonies that look weak, and a plan for what to do if the number is high. Medication, where it is allowed and appropriate, works best alongside good comb management, mite control, and strong winter stores.

Blythewood Bee Company carries the supplies that support that routine. You can find fresh frames and foundation for comb rotation, feeders and lids, syrup, protein, and supplements for fall feeding, and the full Varroa mite control range to keep colonies strong going into winter.

Frequently Asked Questions

What is the difference between Nosema apis and Nosema ceranae?

Nosema apis was described in 1909 and is more associated with dysentery. Nosema ceranae was first described in the Asian honey bee in 1994, is now the predominant species, and often shows no dysentery at all.

Does Nosema cause dysentery?

It can with Nosema apis, but dysentery has many other causes, including poor quality feed. Nosema ceranae is generally not associated with it, so a clean-looking hive does not rule nosema out.

How many spores is too many?

About 1 million spores per bee is the level most often used as the point to treat. It is a widely used guide rather than a strict rule.

Can you treat Nosema in the fall?

Yes. The fumagillin label describes fall feeding after the honey supers are removed, with the medicated syrup consumed before winter. Confirm the rules for buying and using it with your state apiarist, inspector, or veterinarian first.

Can I harvest honey from a colony treated for Nosema?

The fumagillin label says honey from the brood nest areas of treated colonies should not be harvested for human consumption. Follow the label timing so residues do not end up in honey you plan to eat or sell.