European Foulbrood and Chalkbrood: The Two Stress Diseases, How to Tell Them Apart, and How to Fix the Stress
Two brood diseases scare new beekeepers more than they should, and for the same reason: they look terrible. European foulbrood (EFB) turns larvae into twisted yellow-brown smears; chalkbrood turns them into hard white mummies that pile up on the landing board. Both are "stress diseases." They show up when a colony is weak, hungry, cold, or fighting mites, and they usually fade when the stress is removed. Here is how to identify each one, and what to do.
The short version
EFB: larvae die before capping, twisted in the cell, yellow to brown, sour smell, rubbery scale that lifts out. Chalkbrood: hard white or gray "mummies" in cells and at the entrance. Neither has a chemical cure worth relying on. Fix them by feeding, requeening with hygienic stock, improving ventilation, and getting mites under control with a wash and an oxalic acid vaporizer, because a varroa-stressed colony is the one that gets both.
Telling the Brood Diseases Apart
| European foulbrood | Chalkbrood | American foulbrood | |
|---|---|---|---|
| Cause | Bacterium Melissococcus plutonius | Fungus Ascosphaera apis | Spore-forming bacterium Paenibacillus larvae |
| When larvae die | Before capping, usually within 5 days | After capping, as pre-pupae | After capping |
| What you see | Larvae twisted or corkscrewed in the cell, yellow then brown, "melted" and deflated; tracheae visible | Hard mummies: white at first, turning gray or black; rattle in the cell; found at the entrance and on the bottom board | Sunken greasy perforated cappings, coffee-brown goo |
| Rope test | Little or none (may stretch about ½ inch) | None; it's solid | Ropes ½–1 inch |
| Smell | Mildly sour | Little | Foul |
| Scale | Rubbery, lifts out easily | Chalky mummy, falls out | Hard, black, glued in |
| Reportable? | Some states | No | Yes, everywhere |
If anything ropes out an inch and smells foul, stop and read our American foulbrood guide; that one is a different conversation.
European Foulbrood
M. plutonius infects larvae in their first day and multiplies in the gut, competing with the larva for food. A well-fed larva in a strong colony usually outgrows it and nurse bees remove the ones that don't. A hungry larva in a weak colony doesn't. That is why EFB flares in cool wet springs, during dearths, in colonies with poor queens, and in colonies carrying high varroa loads, and why it often clears on its own when a strong nectar flow starts.
What works: feed 1:1 syrup and pollen; add a frame of capped brood and bees from a healthy hive; reduce the entrance; requeen with hygienic stock; rotate out old dark comb; treat varroa. In a stubborn case, a shook swarm onto fresh foundation with the old comb burned is the most effective non-drug fix: in a 50-colony UK trial, reinfection the next season was 4.8% with antibiotic plus shook swarm versus 21.1% with antibiotic alone. Oxytetracycline is the only FDA-approved antibiotic for EFB, it requires a vet prescription or Veterinary Feed Directive, it can't be used before or during a honey flow, and the Honey Bee Health Coalition calls it a last resort because it prevents rather than cures and the underlying stress remains.
Chalkbrood
The fungus is everywhere; spores survive on comb for years. Larvae get it when they eat spores and then get chilled, usually because a small cluster can't cover the brood on a cold damp night. The mummies you find at the entrance are actually a good sign: hygienic workers pulling them out before the spores mature.
What works: there is no registered chemical treatment for chalkbrood, and nothing you can spray. Improve ventilation, tilt the hive slightly forward so water runs out, move it into full sun, don't spread a small colony across more boxes than it can keep warm, feed if it's hungry, rotate out the oldest comb, and requeen with hygienic stock, because susceptibility is heritable and a queen change often clears it within a brood cycle. And treat mites: a varroa-weakened colony can't keep its brood warm.
The Common Thread Is Stress, and the Common Stress Is Mites
Extension lists for both diseases name the same risk factors: weak population, poor nutrition, cool damp weather, and varroa. A colony losing bees to deformed wing virus is a colony that can't feed its larvae, can't heat its brood nest, and can't clean out sick cells. That is why our answer to a brood disease call is always "what did the mite wash say?" Keep the count under 2 per 100 with oxalic acid vaporization on the right schedule and most EFB and chalkbrood problems never start.
Tools We Use (and Recommend)
- InstantVap Oxalic Acid Vaporizer: the cordless, battery-powered oxalic acid vaporizer we use in our own yards to keep mite loads low, which is the single best defense against every problem on this page. See the Lite, Compact, Original and Turbo. Lorob Bees is the primary US seller, with stateside customer service and a warranty we stand behind.
- Mite Monitoring: the Easy-Check mite wash. How to do a wash in five minutes.
- Oxalic Acid: EPA-registered EZ-OX.
Happy Beekeeping from the Lorob Bees Team
Keep reading: Explore all Lorob Bees beekeeping guides — varroa, oxalic acid vaporization, pests and diseases, all in one place.
Sources
- UF/IFAS, Identification and Treatment of European Foulbrood (ENY174), rev. 2026
- Delaware Dept. of Agriculture, European Foulbrood, 2023
- Bee Health eXtension, Shook Swarm and OTC Antibiotics for European Foulbrood Control
- UF/IFAS, Chalkbrood Recommendations (ENY116), rev. 2026
- Minnesota Dept. of Agriculture, Chalkbrood Disease
- Honey Bee Health Coalition, Identifying and Mitigating Foulbrood, 2019
General beekeeping information, not a substitute for the product label. Always read and follow the label of any product you use in a hive.
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