Every beekeeper will eventually open a hive and see something wrong with the brood. Maybe the cappings look sunken and greasy. Maybe there are scattered empty cells where solid brood should be. Maybe there is an unfamiliar smell. These are the moments that separate beekeepers who lose colonies from those who save them. Brood diseases are among the most destructive problems in beekeeping, and your ability to catch them early — sometimes within a single inspection — can make the difference between a recoverable situation and a total loss.

This guide covers the major brood diseases you are likely to encounter, how to tell them apart, and what to do when you find them. Some of these conditions are manageable with simple interventions. Others require immediate, decisive action to protect not just one colony but your entire apiary.

Why Brood Health Matters More Than You Think

A healthy queen can lay 1,500 to 2,000 eggs per day during peak season. If even a fraction of those larvae are killed by disease before they emerge as adult bees, the colony's population growth stalls. Worse, some brood diseases are highly contagious — they spread through robbing, drifting, shared equipment, and even the beekeeper's own hive tool. A single infected colony left unchecked can spread disease across an entire beeyard within weeks.

Regular brood inspections are your first line of defense. Every time you pull frames, you should be reading the brood pattern. A strong, healthy colony will show a tight, solid pattern of capped brood with very few empty cells scattered throughout. When you start seeing irregular patterns — what beekeepers call a "shotgun" brood pattern — it is time to look more closely at individual cells.

A good brood pattern is not just a sign of a good queen. It is a sign that larvae are surviving. When the pattern deteriorates, always investigate the brood itself before blaming the queen.

American Foulbrood: The One You Cannot Ignore

American foulbrood (AFB) is caused by the spore-forming bacterium Paenibacillus larvae, and it is the most serious brood disease in beekeeping. The spores are extraordinarily resilient — they can remain viable on equipment and in wax for more than 50 years. There is no practical way to decontaminate wooden equipment once it has been exposed to AFB spores.

The disease targets larvae after they have been capped. Infected pupae die and decompose into a brown, ropy mass inside the cell. The visual signs are distinctive once you know what to look for:

  • Sunken, greasy cappings — Healthy capped brood has slightly convex, dry-looking cappings. AFB-infected cells have cappings that appear wet, darkened, and concave, often with small perforations where nurse bees have begun to uncap and then abandoned the cell.
  • The ropy test — Insert a matchstick or toothpick into a suspect cell and slowly withdraw it. If the contents stretch into a brown, mucus-like rope 2 to 3 centimeters long before snapping, that is a strong positive indicator for AFB. No other brood disease produces this characteristic ropiness.
  • Dried scales — In advanced cases, the decomposed larva dries into a hard, dark scale that adheres tightly to the lower cell wall. These scales are packed with billions of spores and are almost impossible to remove. Tilt the frame so light hits the bottom of the cells at an angle — the scales become visible as dark, tongue-shaped residue.
  • Foul odor — The name is accurate. Advanced AFB infections produce a distinctive sour, rotting smell that experienced beekeepers can detect before even pulling frames.

If you suspect AFB, stop working other hives immediately to avoid cross-contamination. Contact your local bee inspector or state apiary program — in many jurisdictions, AFB is a reportable disease and inspection is required by law. The standard recommendation in most regions is to kill the colony and burn all equipment, including frames, boxes, and bottom boards. This sounds extreme, but the spore load in an AFB-infected hive makes reuse genuinely dangerous. Some beekeepers irradiate equipment with gamma radiation where facilities are available, but burning remains the most accessible and reliable option.

Never attempt to salvage equipment from an AFB-infected hive by scraping or bleaching. The spores survive these treatments and will infect the next colony housed in that equipment. When in doubt, burn it.

European Foulbrood: Serious but Manageable

European foulbrood (EFB) is caused by Melissococcus plutonius and, unlike AFB, it attacks larvae before capping — typically when they are 4 to 5 days old. The bacterium competes with the larva for food in the royal jelly, essentially starving the developing bee. EFB is a serious disease, but it does not form persistent spores, which makes it far more manageable than AFB.

The signs of EFB differ from AFB in important ways:

  • Discolored larvae in open cells — Healthy larvae are pearly white and curled in a C-shape at the bottom of the cell. EFB-infected larvae turn yellow, then brown, and often twist into unnatural positions or appear deflated. You will see these in uncapped cells, which is a key distinction from AFB.
  • Shotgun brood pattern — Because larvae die at different stages, the brood pattern becomes highly irregular. You will see empty cells, cells with dead larvae, and healthy brood all mixed together on the same frame.
  • No ropiness — The matchstick test will not produce the characteristic rope of AFB. The dead larvae may be slightly sticky but will not stretch. The consistency is more watery or pasty.
  • Sour smell — EFB can produce an acidic, vinegar-like odor, distinct from the heavier putrid smell of AFB.

Treatment for EFB depends on severity. Mild cases often resolve on their own during a strong nectar flow, when the colony's population is expanding rapidly and nurse bees can clean out infected cells. Requeening can also help, because the brood break during queen replacement gives the colony time to remove diseased larvae. In more severe cases, oxytetracycline (Terramycin) has historically been used as a treatment, though regulations vary by country and region. In some areas, a veterinary prescription is now required for antibiotic use in bee colonies. Check your local regulations before treating.

Chalkbrood: The Fungal Problem

Chalkbrood is caused by the fungus Ascosphaera apis and is one of the easier brood diseases to identify visually. Infected larvae are killed by the fungus and mummified into hard, chalky pellets that are white to dark gray in color. You will often find these mummies on the bottom board, at the hive entrance, or still lodged in their cells.

The disease tends to appear in spring when colony populations are still building up and brood nest temperature regulation is inconsistent. Chalkbrood thrives in cool, damp conditions. Colonies that are too small to adequately cover their brood area are particularly vulnerable. You may also see flare-ups after a cold snap interrupts the normal warming of the brood nest.

There is no approved chemical treatment for chalkbrood, but it rarely requires one. Management strategies are usually sufficient:

  • Improve ventilation — Ensure your hive has adequate airflow. A screened bottom board, a slightly propped inner cover, or an upper entrance can all reduce the moisture that favors fungal growth.
  • Reduce brood area — If the colony is too small to keep all the brood warm, remove empty frames or use a follower board to consolidate the nest. The bees need to be able to maintain 34–36°C across all the brood.
  • Requeen — Some genetic lines are more susceptible to chalkbrood than others. Hygienic behavior — the ability of worker bees to detect and remove diseased larvae — varies significantly between queens. Requeening with stock from a hygienic line often resolves chronic chalkbrood problems within one brood cycle.
  • Replace old comb — Fungal spores accumulate in old wax. Rotating out dark, heavily used comb and replacing it with fresh foundation reduces the spore reservoir in the hive.
If you see a few chalkbrood mummies during a spring inspection but the colony is otherwise strong and growing, do not panic. Monitor it over two to three inspections. A strong colony in warm weather will usually clean it up without intervention.

Sacbrood: The Viral Infection

Sacbrood is caused by sacbrood virus (SBV) and affects larvae in the final stages before pupation. Infected larvae fail to shed their last larval skin and die with their heads raised toward the cell opening — a distinctive "Chinese slipper" posture that is the hallmark of the disease. If you carefully remove an affected larva from its cell with tweezers, you will find it encased in a tough, fluid-filled sac that gives the disease its name.

Sacbrood is generally self-limiting in strong colonies. The virus is present at low levels in most apiaries, and outbreaks tend to coincide with periods of stress — poor nutrition, queen failure, or after other diseases have weakened the colony. There is no chemical treatment. Management focuses on maintaining strong, well-fed colonies and requeening if the problem persists across multiple brood cycles. Colonies with good hygienic behavior will detect and remove sacbrood-affected larvae before they become a significant problem.

Diagnostic Tools and Field Tests

Visual inspection is the foundation of brood disease diagnosis, but several tools can increase your confidence in the field:

  • The rope test — As described above, this simple matchstick test is the most reliable field method for distinguishing AFB from other brood diseases. Practice it on healthy brood first so you understand what normal dead larvae look like versus the characteristic AFB rope.
  • Holst milk test — Mix a small sample of suspect larval remains with a teaspoon of powdered milk dissolved in water. If AFB spores are present, the solution will clear within 10 to 20 minutes as the bacterial enzymes break down the milk proteins. This is a useful confirmatory test if the rope test is ambiguous.
  • Lateral flow devices — Commercial AFB and EFB test kits are available that work similarly to home pregnancy tests. You crush a sample of suspect larvae in a buffer solution and apply it to a test strip. These kits cost roughly $5 to $10 per test and give results in about 3 minutes. They are highly accurate and worth keeping in your bee bag during inspection season.
  • Laboratory analysis — When in doubt, send samples to your state or national bee laboratory. Package a section of suspect brood comb (approximately 10 by 10 centimeters) in a paper bag or newspaper — never plastic, which promotes mold growth during shipping. Most government labs provide brood disease diagnosis at no cost to beekeepers.

Prevention: Building Disease-Resistant Management Practices

The best approach to brood disease is preventing it from gaining a foothold in the first place. Several management practices significantly reduce your risk:

First, practice good apiary hygiene. Scorch your hive tool with a smoker or torch between hives. Do not share frames between colonies unless you are certain of their disease status. If you acquire used equipment, inspect it carefully for AFB scales before putting bees in it — or better yet, scorch the interior of all used boxes with a propane torch until the wood is lightly browned.

Second, maintain strong colonies. Most brood diseases gain a foothold in stressed, weakened colonies. Ensure adequate nutrition, especially in early spring when natural forage may be limited. Do not allow colonies to become overcrowded, which causes stress, but also do not spread brood across more frames than the colony can adequately cover and warm.

Third, rotate your comb. Old, dark brood comb accumulates pathogens, pesticide residues, and fungal spores over time. A good practice is to replace 3 to 5 of your oldest brood frames each year, cycling them out of the center of the brood nest. Mark your frames with the year they were introduced so you can track their age.

Fourth, select for hygienic behavior. If you raise your own queens or choose where to purchase them, prioritize genetics with proven hygienic traits. Hygienic bees detect diseased or parasitized brood and remove it before it can sporulate or spread. This trait is measurable through freeze-kill assays and is one of the most valuable defensive characteristics a colony can have.

Finally, keep records. Note any disease signs you observe during inspections, even mild ones. Patterns often emerge over time — a colony that shows chalkbrood every spring may benefit from requeening, while scattered EFB across multiple hives might indicate a shared equipment contamination issue. A simple notebook or digital record for each hive will serve you well across seasons.

Brood diseases are an unavoidable part of beekeeping, but they do not have to be devastating. The beekeepers who manage them successfully are the ones who inspect regularly, recognize the signs early, and respond with appropriate action — neither panicking at a few chalkbrood mummies nor ignoring the warning signs of something far more serious. Build the habit of reading your brood carefully at every inspection, and you will catch problems when they are still small enough to solve.