Nosema

What it is

A microsporidian parasite of the adult bee gut. It shortens lifespan, reduces the colony's ability to raise brood, and interferes with nutrition. It does not kill bees dramatically; it makes them less effective and shorter-lived, which shows up as a colony that never quite builds.

Why it is hard to see

There is no cappings sign, no brood sign and usually no smell. A heavily infected colony can look like a colony that is simply underperforming, which is why it is often blamed on the queen.

Dysentery streaking on the hive front is sometimes associated with it and is not diagnostic on its own; a colony confined for a long winter on poor stores can streak without nosema.

When it shows

Late winter and early spring, when bees have been confined and unable to take cleansing flights. That is the same period when a colony is spending stores fastest and a poor build-up is most costly.

Diagnosis

Guessing at nosema and treating for it is how people spend money on a problem they do not have.

Management

Most of what helps is general good practice rather than anything nosema-specific. Strong colonies, good fall nutrition, dry hives with proper ventilation, and replacing old comb, since spores persist on it.

Requeening helps for the same reason it helps with several other things: a brood break and a younger, better-laying queen. And a colony that can take cleansing flights on mild winter days does better, which is another argument for siting hives where they get winter sun.

Two species, and the advice changed

Older writing describes Nosema apis: a winter and early-spring disease of temperate climates, associated with visible dysentery streaking the front of the hive, and largely self-limiting once bees can fly freely again.

Nosema ceranae is now the more commonly found of the two across much of the world. It behaves differently. It is not seasonal in the same way, it is often present without dysentery, and its effect shows up as colonies that dwindle quietly rather than as a mess on the landing board.

That change is why sources disagree so sharply here. Guidance written around apis and guidance written around ceranae describe what look like two different diseases, and both are in circulation.

You cannot diagnose it by looking

This is the honest core of the page. Dysentery is not diagnostic: it is caused by any excess of undigestible matter and appears in colonies with no nosema at all, typically after a long confinement on poor stores. And ceranae frequently produces no dysentery.

Confirmation requires a microscope, from a sample of about thirty older bees, crushed and examined for spores. Many state apiary programs and university labs will run it, and several national services accept posted samples. A club with a microscope is the cheapest route.

Until you have that, a dwindling colony is a symptom with a long list of causes, and mites and queen failure sit higher on it than nosema does.

What you can actually do

Reduce the stress that lets it matter. Good stores, a dry hive, ventilation that prevents condensation dripping on the cluster, and the earliest possible cleansing flights.

Comb is the reservoir. Spores persist on comb and are transmitted when bees clean cells, so heavily contaminated comb from a collapsed colony should not be recycled into a healthy one. Irradiation is used commercially; at small scale the answer is to retire the comb.

The antibiotic historically used for nosema is no longer generally available for this purpose in the United States, and that absence is not something to work around with an unapproved substitute. It leaves management as the intervention, which is less satisfying and is what the evidence supports.

References