Chalkbrood Disease in Honey Bees
Symptoms and Treatment for Chalkbrood
Of all the disease names, chalkbrood paints the most vivid picture. Even if you’ve never seen chalkbrood symptoms, white chunks of mummified brood are hard to mistake for anything else. You may discover them on the landing board where they have been left by worker bees, or you may see a pattern of white cells right in the brood nest.
Chalkbrood disease is caused by a fungus called Ascosphaera apis. The disease is spread by reproductive spores that can enter a hive attached to pollen, robbing bees, or tools that were used in an infected hive. Once in a hive, the spores are accidentally ingested by the larvae. Larvae that are three-to-four days old are particularly susceptible to infection.
An Unfortunate Meal
Once the chalkbrood spores are eaten, they pass into the larval gut where they germinate and grow into adult organisms. If the spores are not eaten, they cannot germinate or infect the brood. However, the spores can remain viable for long periods—up to fifteen years—patiently waiting to be swallowed by an unsuspecting larva.
After a spore germinates in the larval gut, it quickly grows a mycelium—an intertwined network of filaments that behave like roots. Each filament grows out from the main body of the fungus in search of water and nutrients that are used to fuel the organism and produce more spores.
The chalkbrood organism grows so many of these filaments (often called hyphae) that the brood cell becomes completely stuffed with them. The filaments steal all the nutrients from the larval host, killing it in the process.
A developing honey bee spends roughly five-and-a-half days as a larva before its cell is capped. If the larva becomes infected on day three or four, it doesn’t show any symptoms right away. In fact, most chalkbrood-infected larvae appear normal at capping time, so the doomed larva is capped along with healthy larvae. But beneath the cap, the fungus continues to harvest nutrients from the baby bee.
The mycelium grows so prolifically that it eventually forms a mummy, a hardened mass in the shape of the brood cell. Eventually, the white color gives way to dark gray or even black. The color change signals the maturity of a new crop of spores and the death of the parent organism. The spores are now ready to spread throughout the hive.
Sometimes the honey bee workers detect a problem and remove the cell capping to reveal the mummy. Sometimes the bees leave the mummies in place, and sometimes they remove them and deposit them on the bottom board or cart them outside and dump them. If the bees remove the mummy while it is still white, the spread of the fungus is minimal. But if the mummy has already turned dark, moving it spreads spores throughout the hive—just what the fungus needs to produce another generation.
How to Recognize Chalkbrood Disease
Sometimes the bees do not uncap the infected larvae at all, especially if there is more chalkbrood than the workers can handle. If you suspect your bees left mummies under the caps, you can shake the frame and listen for a rattle. This telltale noise demonstrates just how hard and dry the mycelium has become!
Infected cells are most often seen on the outer edges of the brood nest. The cell caps may be intact but flattened, they may have small holes, or they may be partially removed. If the disease is severe, the remaining brood may appeared scattered and sparse, due to the removal of so many mummies.
Treatment for Chalkbrood
Since chalkbrood is caused by a fungus, conditions that favor fungal growth should be avoided. As a general rule, fungi like damp, dark, and cool conditions.
At the same time, the colony must be able to keep itself reasonably warm and free from temperature fluctuations. These conditions may seem at odds, but bees are excellent temperature regulators as long as they are not wet.
To achieve good ventilation, place hives away from low, damp, or swampy areas. Provide direct sunlight, at least for part of the day, and use screened bottom boards so moisture can drain from the hive. Depending on local conditions, an upper entrance or a screened inner cover can also keep moisture at bay.
Other Management Techniques
Because spores spread so easily, do not use tools that were used in an infected hive. Clean them thoroughly with alcohol or a propane torch before reusing them. Also, avoid transferring combs from an infected hive to a clean hive, and use robbing screens to discourage drifting bees. If you live in an area with high rates of infection, consider buying queens known to produce hygienic workers that will remove mummies before spore production.
As with any disease, bees are most able to resist chalkbrood infection when they are healthy and well fed. Assure your bees have plenty of good forage and use supplemental feeding in times of dearth. Well-fed bees are better able to regulate hive temperature and more adept at removing mummies. If the colony is weak, it cannot do either.
As a matter of course, beekeepers should replace old
blackened brood combs on a regular basis. Whether you have disease symptoms or
not, old combs contain more pathogens then new ones. If you replace 20-30% of
your oldest combs every year, you can lower your incidence of many brood
diseases as well as nosema disease. In addition, comb replacement
reduces pesticide accumulation.
Recovery is Normal
You are most likely to see chalkbrood in late spring when brood rearing increases but temperatures are still cool and erratic. If the colony doesn’t have enough workers to keep the nest adequately warm, you may see mummies at the perimeter of the brood nest where it is coolest. A cold rainy spell after a period of rapid egg-laying can easily initiate symptoms of disease.
In most cases, a small outbreak of chalkbrood will clear up on its own when summer temperatures keep the hive both warmer and drier. Although chalkbrood rarely kills a colony on its own, reduced vigor can weaken the bees, making them more susceptible to other pathogens such as nosema. A warm, dry hive is the best prevention.
If your bees had chalkbrood disease in the past, what have you done to prevent its recurrence?