Lesson 61: Nosema

A gut parasite of adult bees: what it does, how to recognize it, and what (little) you can do

Nosema disease (also called nosemosis) is a disease of adult bees, caused by tiny single-celled parasites that live in the bee's gut. Nearly every colony carries some Nosema, just as nearly every colony has some varroa mites. Most of the time it's a background problem. It becomes a concern when a colony is under stress.

What causes it

Two closely related parasites cause Nosema disease:

• Nosema apis, known in honey bees since 1909;
• Nosema ceranae, originally a parasite of the Asian honey bee, first found in our honey bees in 2007. It is now the more common one in the United States.

(Scientists have proposed renaming them Vairimorpha, but the new name hasn't been universally accepted, so you'll see both.)

They're microsporidia, related to fungi. They form spores that survive outside the bee.

How it spreads

• A bee swallows spores: from feces of sick bees, or from contaminated water, honey, bee bread, royal jelly, or flowers.
• The spores sprout in the bee's midgut, multiply inside the gut cells, and produce new spores, which pass out in the bee's feces.
• Healthy bees don't defecate inside the hive; they wait for cleansing flights. Sick bees may defecate inside the hive and on the front of it, contaminating combs and food. House bees cleaning it up swallow the spores, and the cycle continues.
• Spores can also pass between colonies at shared flowers and water.

What it does

• Infected bees can't digest and absorb food well. They eat more but stay malnourished.
• Nurse bees make less brood food.
• Bees age faster and die younger; with N. ceranae, they start foraging earlier and lose their way home more often.
• In queens, it can lead to supersedure.
• The colony result: smaller populations, slow spring build-up, less honey. N. apis hasn't been seen to kill colonies outright; N. ceranae can weaken colonies over time. Nosema is considered a stress disease: it matters most when combined with other problems like poor nutrition, mites, pesticides, and long periods of confinement in bad weather.

The signs

• Dysentery: brown fecal streaks on the front of the hive and on combs and frames. This is the classic sign, but mainly of N. apis. N. ceranae usually shows no obvious signs.
• Bees crawling weakly in front of the hive.
• Slow spring build-up and a dwindling population.

Dysentery isn't always Nosema. Long confinement without cleansing flights, syrup that's too thin, or too much moisture in food can also cause it.

Confirming it

You can't diagnose Nosema by eye. Options:

• A lab test: send a sample of bees (in alcohol; lesson 40) to a USDA ARS or university lab. Molecular tests confirm Nosema and tell which species.
• A microscope: beekeepers with a microscope can count spores in crushed bee abdomens. (Online guides show how.)

When is it worth treating? No firm threshold is known. About 1 million spores per bee is often mentioned, but that's based on experience, not research.

Managing Nosema

Management first (the same things that keep colonies healthy in general):

• Keep colonies strong and well fed, with good forage or feed; keep varroa low (lessons 54–56).
• Choose sunny, well-ventilated hive sites; damp, cold conditions seem to favor Nosema.
• Replace old comb regularly (lesson 53); spores build up in it.

Medication:

• Fumagillin (sold as Fumidil-B) is the only approved treatment for Nosema in honey bees, and it's used against both species. It's mixed into sugar syrup and fed in spring and/or fall by label directions. It can't be used when honey supers are on.
• It only kills the active infection, not the spores left on combs and equipment, so colonies can be reinfected. Researchers say the right dose still needs more study. Follow the label exactly.
• Beekeepers differ on whether to use it at all. Older guides recommended feeding it routinely every spring and fall. Current guidance treats Nosema as a stress disease and points to management and testing first. Other products (herbal or feed supplements) have been studied; results vary.

Cleaning equipment: heat (120°F for 24 hours) and gamma irradiation kill spores but aren't readily available to beekeepers. Freezing combs for a week lowers the survival of N. ceranae spores.

What to remember

• Nosema = a gut parasite of adult bees; nearly all colonies have some. It's a stress disease.
• N. ceranae is now the common one and often shows no signs; N. apis can cause dysentery streaks.
• Effects: short-lived, malnourished bees, slow build-up, less honey.
• Confirm with a lab or microscope. Dysentery alone doesn't prove Nosema.
• Manage with strong, well-fed, low-mite colonies in dry, sunny sites; fumagillin is the only approved drug. It follows the label, and it doesn't kill spores.

Quick check

1. How does Nosema spread inside a colony?
2. You see brown streaks on the front of your hive after a long cold spell. Is it definitely Nosema?
3. What does fumagillin not do?

Show answers

1. Sick bees defecate in the hive; house bees cleaning up swallow spores, which multiply in their guts. 2. No. Long confinement, thin syrup or moist food can also cause dysentery; a lab or microscope test is needed to confirm Nosema. 3. It doesn't kill the spores left on combs and equipment, so reinfection is possible.

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Adapted from: Penn State Extension, Nosema Disease of Honey Bees (updated February 2026) and A Quick Reference Guide to Honey Bee Parasites, Pests, Predators, and Diseases; UGA Bee Program, Honey Bee Disorders: Microsporidian Diseases.

Adapted from university Extension sources. Experienced Georgia beekeepers are reviewing these lessons. See something that doesn't match your experience? Tell us.