Honey Bee Diseases: A Field Guide to What Kills Colonies, and What Just Looks Like It

Honey Bee Diseases: A Field Guide to What Kills Colonies, and What Just Looks Like It

Honey bee diseases get diagnosed wrong constantly, and usually in the same direction. A beekeeper finds spotty brood, or a dwindling cluster, or dead bees on the landing board, reaches for the disease that matches the picture, and misses the mite population that caused it.

This guide is built around that problem. For each condition: what it looks like, what actually causes it, and whether a mite wash should be your first move.

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The quick reference

The one that does most of the killing

Varroa destructor is a parasitic mite that feeds on the fat body of adult and developing bees — not hemolymph, as was believed for decades, but the organ that runs immunity, detoxification and winter reserves. Ramsey and colleagues settled that in 2019, and it explains why mite-damaged colonies die months after the damage is done.

You will rarely see the problem coming by looking. Mites hide under cappings, bees carry them on their undersides, and a colony can go from a comfortable count to a collapse in eight weeks of late-summer exponential growth. The only reliable way to know is to wash a sample. How to do a mite wash takes five minutes and settles the question.

Everything below this line should be read with that in mind.

The virus that rides with it

Deformed wing virus existed in bees before varroa arrived, at low levels, doing little. The mite changed that by injecting it directly and amplifying it. Crumpled wings and stunted abdomens are the visible cases. The invisible ones — bees that look normal and die weeks early — are what empty a hive in January.

DWV is not treated directly. It is treated by controlling the mite that carries it, and the timing of that control matters more than the product. Deformed wing virus in honey bees goes into what it means to find it.

The ones that are their own disease

American foulbrood is the serious one. A spore-forming bacterium that kills capped larvae, leaves sunken and perforated cappings with a ropy brown mess inside, and produces spores that survive decades. It is not a mite problem, it is not a stress problem, and in most states it is reportable. If you suspect it, get it confirmed and follow your state rules. American foulbrood: signs, test and what to do.

European foulbrood and chalkbrood are the stress diseases. Both exist at low levels in many healthy colonies and flare when the colony is weakened — by cold, by poor nutrition, by a failing queen, and very often by mites. Fix the stress and they usually clear. European foulbrood and chalkbrood covers telling them apart.

Nosema is a gut parasite that gets blamed for a great deal it did not do. Dysentery on the front of the hive is not diagnosis; only a microscope is. And a colony fighting mites and nosema together does markedly worse than the sum of the two. Nosema in honey bees.

The pests that arrive when the colony is already losing

Small hive beetle and wax moths are opportunists. A strong colony polices both. When you find slimed comb or moth tunnels through a brood nest, the beetles and moths are the second thing that went wrong, and the first thing was usually a colony too weak to guard its own comb. Treat the pest, and then ask why the colony was weak. Small hive beetle and wax moths in beehives.

Robbing belongs here for a different reason. A robbed colony is being finished off, and its mites go home with the robbers. One collapsing hive in a yard becomes a mite delivery service for every other hive within flight range. Robbing bees: how to recognize and stop it.

The diagnostic habit that fixes most of this

• Wash before you diagnose. Whatever you think you are looking at, get a mite count first. It is the one number that reframes everything else.
• Look at the brood pattern, not just the bees. Spotty brood with sunken cappings is foulbrood. Spotty brood with an otherwise healthy colony and a high mite count is varroa.
• Read the empty hive properly. Stores untouched, tiny dead cluster, few dead bees on the floor, a scatter of mites on the bottom board: that is a mite collapse, not cold and not absconding.
• Treat the mite on the calendar the mite runs on. Winter bees are raised in August. Damage then shows in January. The treatment schedule lays out when.
• Use the broodless window. One oxalic acid vaporization when the colony has no capped brood reaches every mite in the hive. It is the highest-leverage disease prevention in beekeeping. When is my hive broodless.
• Get foulbrood confirmed. AFB is the one condition on this page where the answer is not a mite wash. Do not guess with it.

Related reading

• Varroa mite life cycle: why timing beats dosage
• Varroa mite treatment guide: every option compared
• What is oxalic acid?
• Does oxalic acid vaporization actually work?

Sources

• Ramsey, S.D. et al., “Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph”, PNAS 116(5):1792–1801, 2019
• Rosenkranz, P., Aumeier, P. & Ziegelmann, B., “Biology and control of Varroa destructor”, Journal of Invertebrate Pathology 103:S96–S119, 2010
• Bee Informed Partnership, annual colony loss and management surveys

Happy Beekeeping from the Lorob Bees Team

General beekeeping information, not a substitute for the product label. Always read and follow the label of any product you use in a hive.