Winter Nosema in Honey Bees: Symptoms, Risks, and What You Can Actually Do When It’s Too Cold to Treat

Nosema often shows up in winter when bees can’t take syrup. Learn the real symptoms, risks, and practical steps you can take to support colonies until spring.

Winter Nosema — What You Can Do When It’s Too Cold to Treat

What Is Nosema? (Beginner + Technical Overview)

Nosema is a gut infection caused by Nosema ceranae or Nosema apis. These are spores that can last inside the hive for some time. The main symptom is honey bees with dysentry. Or, in other words, when you are wondering why your honey bees have diahreah this is the culprit.

For those who want to understand it a little better, here is the technical explanation.

Nosema is a microsporidian parasite — a fungus‑related, spore‑forming gut pathogen.

There are two species in honey bees:

• Nosema apis — the older, classic version

• Nosema ceranae — now dominant in North America

Nosema is a microsporidian parasite — a fungus‑related, spore‑forming gut pathogen.

There are two species in honey bees:

• Nosema apis — the older, classic version

• Nosema ceranae — now dominant in North America

How Nosema infects bees

• Bees ingest spores through contaminated food, water, or feces

• Spores germinate in the midgut

• They inject their genetic material into gut cells

• Infected cells rupture, releasing more spores

• Spores spread through the colony via trophallaxis and fecal contamination

What Nosema does inside the bee

• Damages gut lining

• Reduces nutrient absorption

• Shortens lifespan

• Weakens immune function

• Impairs queen egg‑laying

• Increases colony stress and mortality

Why winter makes Nosema worse

• Bees cannot take cleansing flights so spores accumulate

• Cold & moisture are ideal conditions for spore survival

• Bees live longer in winter so there is more time for infection to build

• No fresh pollen coming in creates weakened immune response

• Clustered bees spread spores more easily

Why treatments don’t work in winter

All known treatments require ingestion. Honey bees cluster in winter to keep warm. Clustered bees do not take syrup. Therefore, no treatment can reach the gut until temperatures rise.

Why Nosema Shows Up in Winter

• Cold weather prevents cleansing flights

• Bees defecate inside the hive, so spores spread more rapidly

• The combination of moisture and stress create a higher spore loads

• Winter bees live longer, so there is more time for infection to build

• A lack of natural forage weakens immune response

Symptoms of Winter Nosema

• Dysentery on the front of the hive (more common with N. apis)

• Spotty or shrinking brood nest

• Bees crawling or trembling near the entrance

• Slow spring build‑up

• Queen supersedure attempts

• High winter mortality without obvious starvation

What You Cannot Do in Winter

• You cannot feed syrup (bees won’t break cluster to drink it)

• You cannot deliver fumagillin or essential‑oil treatments (these are administered in syrup)

• You cannot “cure” Nosema until temperatures rise

• You cannot open the hive repeatedly without causing more stress

Winter Nosema management is about reducing stress, not treating the infection.

What You Can Do in Winter (Evidence-Based Actions)

Improve Ventilation

• Add an upper entrance or notch

• Use a moisture quilt or absorbent material to absorb excess moisture in the hive

• Tilt the hive forward, so moisture doesn’t pool in the bottom of the hive

Reduce Moisture

• Moisture is more deadly than cold

• Wet bees, become chilled bees, which are more impacted by Nosema

Provide Dry Sugar (Mountain Camp)

• Prevents starvation in stressed colonies

• Absorbs moisture

• Does not stimulate brood rearing

Keep Entrances Clear

• Bees need cleansing flights on warm days

• Snow, ice, or dead bees block airflow and exit routes

Minimize Disturbance

• Every inspection increases food consumption

• Stressed bees have higher Nosema loads

What to Do in Early Spring (When It Warms Up)

Confirm Nosema With a Sample

How to Confirm Nosema (and Why You Can’t Rely on Visual Symptoms)

Nosema can’t be diagnosed by looking at bees, comb, or hive entrances. The only reliable way to confirm it is through microscopic examination of a bee gut sample. This is the same method used by labs, inspectors, and researchers.

To check for Nosema, you’ll need a small sample of adult bees and a microscope capable of 400× magnification.

Collect the right bees

• Gather 25–50 adult bees, ideally older foragers or bees showing abnormal behavior.
• If the colony is dead, collect bees from the cluster or top bars, not the bottom board.

1. Prepare the sample

• Place the bees in a clean plastic bag.
• Crush the abdomens thoroughly.
• Add 1 mL of water per bee (for example, 25 bees = 25 mL water).
• Mix until the solution is uniform and milky—this releases spores from the midgut.

Examine under a microscope

• Place a drop of the homogenized solution on a slide.
• View at 400× magnification.
• Nosema spores appear as bright, oval, rice‑grain‑like structures.

Count spores (optional but helpful)
Using a hemocytometer, labs quantify infection by calculating spores per bee. General interpretation:

• <1 million spores/bee → low infection
• 1–5 million → moderate
• >5 million → heavy
• >10 million → severe, colony likely compromised

These numbers aren’t treatment thresholds—they’re indicators of stress and risk.

---

Why Visual Diagnosis Is Unreliable

Many beekeepers try to diagnose Nosema by sight, but visual symptoms simply don’t match up with actual infection levels. Here’s why:

1. Dysentery doesn’t mean Nosema

Brown streaking on the hive is not diagnostic. Dysentery can be caused by:

• Long confinement
• Moisture issues
• Fermented honey
• Stress
• Nosema apis (sometimes)
• Nosema ceranae (rarely)

Ceranae—the dominant species today—usually does not cause dysentery at all.

2. Nosema ceranae often has no visible symptoms

Colonies can carry millions of spores per bee and still appear:

• Active
• Queenright
• Normal in brood pattern
• Normal in flight behavior

This is why ceranae infections are so often missed.

3. Many unrelated issues mimic Nosema

Weak colonies, crawling bees, spotty brood, and sudden collapse can all be caused by:

• Starvation
• Viral infections
• Pesticide exposure
• Queen problems
• Chilled brood
• Poor nutrition
• Moisture stress

None of these symptoms are specific to Nosema.

4. You can’t see spores with the naked eye

Nosema spores are microscopic. No amount of looking at bees, comb, or hive entrances can confirm infection.

5. Even experienced beekeepers guess wrong

Studies comparing visual diagnosis to microscopic confirmation show very low accuracy. Most “Nosema” diagnoses made by sight alone turn out to be something else entirely.

---

The Bottom Line

You cannot diagnose Nosema by sight.
Only a microscope can confirm it.

If you want certainty—especially before treating or making management decisions—a simple 25‑bee gut sample gives you the real answer.

In the spring

Feed Light Syrup + Pollen Sub

• Only when bees are flying

• Supports gut repair and brood rearing

Replace Old Comb

• Nosema spores persist for months

• Rotate out dark brood comb

Requeen if Colony Is Struggling

• Nosema weakens queens

• New queens improve spring recovery

Consider Fumagillin (With Caveats)

• Historically Fumagiin is effective for N. apis but delivers mixed or poor results for N. ceranae

• Must be delivered in syrup

• Not a winter option

Preventing Nosema in Future Winters

• Maintain strong fall nutrition

• Reduce moisture before winter

• Avoid late‑season syrup feeding that increases humidity

• Keep colonies well‑ventilated

• Replace old comb regularly

• Manage mites (immune stress increases Nosema susceptibility)

Going forward

While realizing your hive has Nosema is not good news, it is encouraging to realize that a strong hive should be able to recover from this diagnosis. Don’t try to cure it in the winter time- instead try to reduce stress as much as possible. Once spring comes, then is the time to cure Nosema.