European Foulbrood and Chalkbrood: The Recoverable Brood Diseases

Open a brood frame with EFB and the first impression is wrongness in the open cells rather than in the cappings. Healthy young larvae lie in a neat C-curl at the bottom of the cell, glistening pearly white, floating in brood food. EFB larvae are twisted out of that curl, lying at odd angles, along the side of the cell or corkscrewed across it, and they lose the pearly wet look. Color runs from off-white to a dull yellow, then tan, then brown as they decay. On many, the tracheal system shows through the body wall as a pale branching network, an unnerving detail that is close to diagnostic when you see it.

The mechanism explains the look. Melissococcus plutonius is swallowed with brood food, multiplies in the larval midgut, and competes with the larva for the food it needs, so the larva starves and dies, most often at four to five days old, before its cell is ever capped. Because death comes early, the colony's nurse bees can usually clean the cells out, and the queen relays them, which produces the scattered, patchy brood pattern that first draws your eye. In a bad case, some larvae survive to capping and die after, which puts a few sunken cappings on the frame and starts the AFB anxiety.

Odor is variable. Classic descriptions call EFB sour, and a heavily infected frame can smell distinctly off, but plenty of EFB colonies smell like nothing in particular. Do not rule EFB out on a clean-smelling frame, and do not diagnose AFB on a smell alone. The tests that actually separate them are the rope and the scale, covered below.

EFB behaves less like an invader and more like an opportunist. The bacterium is widespread, and colonies carry it at low levels without visible disease; what turns a carrier into a case is stress on the brood-rearing system. The recognized triggers are consistent across extension guidance and the eXtension bee health program: a spring buildup that outruns the incoming nectar and pollen, a dearth in the middle of brood rearing, a cold snap that shrinks the cluster off the edges of the brood nest, poor or one-dimensional forage, and the general disruption of colonies being moved, which is why EFB is a recurring headache in migratory pollination.

The practical reading of that list: EFB is telling you something about the colony's food and stability, not just about a bacterium. A colony that gets hit is often one that expanded fast on stimulative feeding and then hit a wall, or one whose nurse bees are stretched too thin across too much brood. Where the beekeeper contributed, spreading the brood nest too early in the season is the classic own goal, the disease is partly a management verdict.

This is also why EFB famously "clears up on the flow." Once a strong nectar and pollen flow arrives, brood food is abundant, larvae outpace the bacterium, and cases fade. That is real, and it is not a reason to do nothing: a colony that recedes on the flow and relapses in the next dearth is a colony that needs a queen change, not another season of waiting.

Requeen. This is the standard management response and it does two jobs at once. A new queen means a brood break, a period with no young larvae for the bacterium to colonize, which lets the workers clean out the infected cells. And it brings new genetics, ideally stock selected for hygienic behavior, which detects and removes diseased brood. If you raise your own, a compact system like the APlayfulBee queen rearing kit ($19.71) makes a scheduled requeening habit affordable; the full process is in the requeening guide and the stock question in the queen biology guide.

Feed if the flow is not there. EFB is a brood-food problem as much as a bacterial one. Syrup and a pollen substitute during a dearth remove the nutritional stress that let it flare; the mechanics are in the syrup feeding guide.

Strengthen or consolidate. A colony too weak to cover its brood keeps chilling the edges, and chilled brood invites both foulbrood and chalkbrood. Reducing the colony to the space it can actually hold, or combining two weak colonies as described in the combining guide, is often better medicine than anything else on this list.

Rotate comb. Old brood comb accumulates pathogens and residues. Culling the darkest two frames per box per year, replacing them with fresh foundation, is a background practice that pays off across every disease on this site. Frame and foundation options are in the frames guide.

On antibiotics. Antibacterial treatment has a history in EFB management, and its limits mirror AFB: it can clear symptoms without ending the colony's carrier status. Under current US rules the relevant antibiotics require veterinary involvement rather than over-the-counter purchase, so this is a conversation with a veterinarian and your state apiary program, not a shopping decision. We publish no product names, doses, or timings for it.

This is the question the whole page exists to answer, because the responses diverge completely: EFB is usually requeened and fed, AFB is reported and its combs destroyed. Three checks settle it in the yard.

1. When did the larva die? EFB kills before capping, so you are looking mostly at open cells with visibly wrong larvae. AFB kills after capping, so you are looking at sunken, greasy, perforated cappings with brown liquid inside. Mixed pictures happen, so use the other two checks.

2. Does it rope? Stir a toothpick into the remains and withdraw slowly. AFB draws an elastic brown thread, commonly 20 to 30 mm or more. EFB is watery, granular, or slightly pasty and usually breaks almost immediately. A long clean rope is the strongest field indication of AFB there is, and the full protocol is in the AFB guide.

3. Does the scale come out? This is the check most beekeepers skip and the one most likely to save them a panic. EFB scale is rubbery and loose, and lifts out of the cell in one piece on a toothpick. AFB scale is hard, dark, and glued to the lower cell wall so tightly that you tear the cell before it comes free.

Chalkbrood is the easiest brood disease to recognize and the least likely to end a colony. Ascosphaera apis spores are eaten by larvae with their food, germinate in the gut, and the fungus overgrows the larva after its cell is capped, consuming the body and replacing it with a mycelial mass. What is left is a hard, chalky white mummy roughly the shape of the larva that filled the cell. When two mating types of the fungus meet, the mummy fruits and turns gray to black, and those dark mummies are the spore-loaded ones.

You typically diagnose chalkbrood without ever pulling a frame, because undertaker bees carry the mummies out and drop them on the bottom board or push them off the landing board, leaving a scatter of white pellets in front of the hive that look like little chunks of styrofoam. Tip a frame and you may hear the loose ones rattling in the cells. On the comb itself, expect a spotty brood pattern with mummies sitting in uncapped or partly opened cells.

Severity ranges from a handful of mummies in a spring hive, which is close to normal and needs nothing but a note in the log, to a chronic case that hollows out the brood nest and stalls a colony's buildup all season. The dividing line is usually a combination of moisture, chill, and genetics, which is exactly where the management levers are.

The pattern extension programs describe, including the University of Minnesota bee health program, is consistent: chalkbrood flares in cool, damp conditions and in colonies whose brood nest is being chilled. That is why it is largely a spring and early summer disease in most of the US, and why it concentrates at the edges of the brood nest where the cluster's grip is weakest.

Four contributors do most of the work. Moisture: a hive that sits in a low spot, in deep shade, on wet ground, or under a lid that condenses all night gives the fungus what it wants. Chill: a colony too small for its boxes, a beekeeper who supered too early, or brood frames spread apart in a cold snap. Colony weakness: fewer nurse bees means less thermal control and slower removal of mummies, which is a feedback loop. Genetics: susceptibility varies substantially by stock, and some queen lines simply produce chalkbrood-prone colonies year after year.

Notice that none of those four are addressed by anything you can pour into a hive, which is a relief, because there is no approved chemical treatment for chalkbrood in the United States. The disease is managed by changing conditions and changing genetics.

Take moisture out of the hive. This is the highest-leverage change, and it is a hardware problem with hardware answers. A BeeCastle hive quilt box ($59.99) sits above the inner cover and holds absorbent material, pulling condensation out of the airspace instead of letting it rain back onto the cluster. A ventilated or notched inner cover does related work by giving humid air a route out: the two-pack DoubleFill beehive inner covers ($27.99) is the cheap way to get a proper inner cover onto hives that shipped without one. Both matter twice, because the same moisture that drives chalkbrood in spring is what kills clusters in winter, as covered in the winterizing guide.

Fix the site. Morning sun, good air drainage, and a stand that keeps the hive off wet ground do more than any equipment. If your hives sit in shade over damp soil, move them; the criteria are in the placement guide.

Do not give a small colony a big house. Match boxes to bees, use a follower board or a reduced setup for a struggling colony, and resist the urge to super early. A colony that can cover its brood keeps it warm, and warm brood does not get chalkbrood.

Requeen from resistant stock. For chronic cases, this is the fix. Chalkbrood susceptibility tracks strongly with genetics and with hygienic behavior, so a new queen from a line selected for hygiene often ends a problem that three seasons of fiddling did not. Raising your own from a good mother colony with the APlayfulBee queen rearing kit ($19.71) is the low-cost route once you have identified which of your colonies never gets it.

Rotate out heavily contaminated comb and scrape mummies off bottom boards. Spores persist in comb and hive debris for years, so removing the worst frames shortens the exposure while the other changes take effect.

Print this, tape it inside the lid of your toolbox, and use it before you post a photograph to a forum. Sacbrood is included because it is common, harmless in the long run, and constantly mistaken for something serious; it is a virus, covered in the virus guide.

Two shortcuts carry most of the value. If the remains rope a long brown thread, treat it as AFB until an inspector says otherwise. If the scale lifts out of the cell in one piece, it is not AFB.

Sacbrood is caused by a virus, and its signature is unmistakable once you have seen it: the larva fails to pupate, fluid collects under the unshed skin, and the whole larva lifts out of the cell intact like a small water balloon, head dark and curled upward toward the cell opening. Dried, it forms a brittle, gondola-shaped scale that comes out easily. Sacbrood is usually a minor, self-limiting problem that fades as the season improves, and a persistent case is a requeening candidate rather than an emergency.

Chilled brood is not a disease at all: dead brood in patches, often at the outer edges of the nest, following a cold snap or a beekeeper who spread frames apart. It can look grim and it resolves on its own.

Parasitic mite syndrome is the great imitator: spotty brood, larvae dying at various stages, sometimes with a semi-ropy quality that alarms people, alongside a dwindling adult population and deformed wings. If a brood frame looks wrong and you have not counted mites, count mites first; the case is made in the virus guide and the timing in the varroa guide.

Stonebrood, caused by Aspergillus fungi, is uncommon and produces hard, stony mummies that are often greenish or yellow when sporulating rather than chalk white. It deserves one caution the other diseases do not: the fungi involved can affect people, so handle heavily affected combs with care and speak to a clinician if you have respiratory concerns.

The management levers for EFB and chalkbrood overlap so heavily that a single set of habits covers them.

Keep colonies strong and appropriately housed. Bees cover brood; brood that is covered stays warm and well fed. That one sentence prevents more brood disease than any product.

Control moisture year-round. A quilt box, a proper inner cover, a hive stand that lifts the boxes clear of wet ground, and a site with morning sun. The same equipment appears in the stands and wraps guide for the same physical reason.

Feed through dearths rather than watching the colony grind. Nutritional stress is the trigger EFB waits for.

Requeen on a schedule, from stock that does not get sick. Two seasons is a reasonable default for a backyard operation, and the mother colony to raise from is the one that has never shown you a mummy or a twisted larva.

Rotate comb. Two dark frames per box per year, replaced with fresh foundation, keeps pathogen load and residue load from compounding.

Log what you see. A note that says "six chalk mummies on the board, hive 2, April 14" is what turns a vague impression into the pattern that identifies which queen to replace.

Most of what beekeepers report as an outbreak is a normal spring hive. A handful of chalk mummies on the landing board in April, with a strong colony and a good brood pattern, is a note in the logbook. A few sacbrood larvae in a building colony is a note in the logbook. Some scattered brood in a hive that just got moved is a note in the logbook.

Escalate when the pattern is persistent, when the colony is failing to build, or when the signs point at AFB. Persistent chalkbrood into summer means requeen. EFB that does not fade on the flow, or that returns every dearth, means requeen and re-examine your feeding. Any long brown rope, any glue-pot smell, any scale you cannot remove from the cell means stop and call the state apiary inspector, as laid out in the AFB guide. And a colony that fails outright over winter gets an autopsy, not a guess, which is what the deadout diagnosis guide is for.

The rest of the library rounds out the picture: nosema and dysentery covers the adult-bee gut problems that get blamed on brood disease every spring, and bee viruses and deformed wing covers the mite-driven collapse that causes more colony deaths in the US than every bacterium and fungus on this page combined.