Bee disease is an unfortunate part of beekeeping that all beekeepers must deal with. The good news is that with some proven precautionary steps, your colonies can be protected from disease. The main diseases you should be concerned with are the following:

  • American Foulbrood
  • Varroa Mites
  • Tracheal Mites
  • Nosema Disease

Each disease has a specific treatment and treatment time. When treating your colony with medicine, always follow the manufacturer's directions on doses and times to avoid any problems.

Below is a lesson is composed of excerpts from the research booklet “Honey Bee Diseases and Pests 2016” written by Marla Spivak and Gary S. Reuter for the Department of Entomology at University of Minnesota-St. Paul, Minnesota. You can read the full booklet here.

You can find updates to this information, and other important information contained in that booklet on the University of Minnesota’s Bee Lab website:

This booklet is updated every two years and was last updated in 2016.


A responsibility of every beekeeper, and one of the principles of productive beekeeping, is to keep all colonies in a "disease-free" condition. American Foulbrood (AFB) American foulbrood (AFB) is the most damaging brood disease. It is highly contagious among bee colonies (not to humans). Although it is not commonly observed in colonies, if left unchecked it will cause colony death. More over, AFB can spread to neighboring colonies within 3-5 miles, causing their demise. It is extremely important to be able to identify and control this disease if found.

American foulbrood is caused by the bacterium, Paenibacillus larvae. The bacteria infect young larvae, which die after the cells are capped. Infected pre-pupae turn brown, gooey and smelly, and sink to the bottom of the cell (Figure 2). The dead brood then dries into a characteristic hard scale on the bottom of the cell. The colony will eventually die from disease, but the infectious scales remain in combs and honey taken from diseased colonies.

American Foulbrood

• Discolored sealed brood (pre-pupae), which when stirred with a toothpick, will "string-out" like glue when the toothpick is withdrawn slowly from the cell
• Characteristic foul odor.
• In advanced cases, the cappings over the older brood are perforated with one or more small holes, are sunken, and have a greasy appearance. The brood pattern will be very scattered.

• Maintain strong colonies with young, prolific queens, and inspect brood regularly.
• Replace old combs regularly (see page 5). Old brood combs contain the disease spores, the source of AFB infection. The most effective way to eliminate AFB spores is to eliminate the infected combs by burning them.
• Use caution when buying used equipment, exchanging brood combs between colonies, and feeding bees honey from an unknown source. Do not move frames from a diseased colony to a healthy colony. Do not feed extracted honey from a diseased colony or from an unknown source to bees at any time.
• Some bees have a genetic predisposition for disease resistance. Colonies bred for hygienic behavior are able to detect, uncap and remove diseased brood from the nest before the disease reaches the infectious stage. Hygienic bees have a natural behavioral defense against AFB and may never show symptoms of the disease. It is important when purchasing queens bred for hygienic behavior to ask if the queens mated predominately with drones from surrounding hygienic colonies. This will ensure that the queen’s colony (her worker bees) will have the genetic ability to quickly uncap and remove diseased brood.

• If symptoms of AFB are found, the following steps should be taken:
• Shake, Requeen and Burn.
1. Shake all the bees off the infected comb and hive them into brand new equipment with frames containing only foundation (no drawn comb).
2. Introduce a queen bred for hygienic behavior into the colony. The queen should remain caged for 3 days.
3. Isolate the colony well away from the infection source, and feed them light sugar syrup (1:1) to encourage them to draw out new comb. The bees will have no larvae to feed for at least 6 days; feeding them syrup during that time allows the disease spores to be passed out of the bees’ digestive systems.
4. Burn all frames, combs, honey and brood from the original diseased colony. If you want to reuse the bottom boards, top boards and supers, they must be scorched with a torch. To do this, all interior surfaces must be heated until charred black.
5. If it is too late in the season to salvage the bees, you may need to burn them also. This seems drastic but it is the most responsible way to eliminate the spread of this infectious, highly devastating and antibioticresistant disease. NEVER leave diseased equipment out where other bees can rob from it. • We do not recommend that beekeepers use antibiotics in their bee colonies to treat AFB. It is better to shake, requeen and burn.
• Be aware that the antibiotics kill only the actively growing bacteria. They do not kill the spores. These spores remain in the comb and boxes, and are a source for re-infection when the antibiotic is stopped. Replace and burn all combs that had any signs of AFB.
• If you do use an antibiotic you must use only antibiotics registered for use in bee colonies, and follow the labeled instructions precisely. Use the correct dosage, timing and method of application. Treat colonies only during nonhoney flow periods to avoid contaminating honey. If you treat during a honey flow, remove all supers while treating and wait 30 days after stopping treatment to put supers back on the colony.


Nosema is a fungal disease affecting the adult bees. Nosema is a single-celled microsporidian (fungal microorganisms) that lives in the gut of bees. The spore stage of this disease is passed in the bees' feces. Infected bees may defecate within the hive, and the nest cleaning behavior of the bees spreads the disease. Nosema can cause problems during winter months when bees are confined within the hive for long periods.

In the past 10 years, a new species of Nosema was detected in bees around the world. This new species, called Nosema ceranae, has almost entirely displaced the old species, Nosema apis, throughout the U.S. Researchers are currently trying to understand the symptoms and problems associated with Nosema ceranae.

We know that Nosema ceranae infection levels naturally rise and fall in a beehive over the year. Levels are highest from April through June, then drop naturally until late fall when they begin to rise again through winter.

• No readily observed symptoms.
• Severe infection may lead to problems with the queen, colony dwindling, increased winter loss and reduced honey production. Prevention:
• Maintain strong colonies with young, prolific queens.
• Replace old combs regularly. Old brood combs contain the source of infection, the disease spores. The most effective way to eliminate the spores is to eliminate the infected combs.
• Learn how to diagnose your own bees. Diagnosis will require a compound microscope with magnification up to 400X and a hemacytometer (blood cell counter).

• Research is showing that Fumagilin, the medication sold to treat Nosema, may exacerbate the disease if used improperly.
• We urge beekeepers to stay informed about this, and all diseases, through beekeeping associations and beekeeping trade journals.

Tracheal Mites

Tracheal mites (Acarapis woodi) are microscopic mites that live and reproduce inside the tracheal (breathing) tubes of adult honey bees and feed on bee blood. High infestation levels of mites in bees used to cause serious damage to bee colonies, especially during the winter months, but honey bees in most of the U.S. have developed natural resistance to these mites. We have not had serious infestations of tracheal mites in colonies in Minnesota for many years, so we encourage beekeepers not to worry about them.

Symptoms and Diagnosis:
• No readily observed physical symptoms.
• The bees from colonies that die in fall or winter from tracheal mites are found in piles in front of the entrance. Ample honey remains in the colony after the colony dies. Currently in Minnesota, most colony deaths with these symptoms are due to varroa mites, not tracheal mites.
• For accurate diagnosis, collect a sample of 25-50 bees from each colony or 100-200 bees from each apiary for laboratory examination. Collect bees of foraging age from frames at the outer ends. Dissect Individual bees and observe the tracheal tubes for mite infestation. You can learn how to dissect your own bees, or contact your local beekeeping association for help.

• Maintain strong colonies with young, prolific queens. Bees have developed natural resistance to tracheal mites and survive well with low tracheal mite infestations. We recommend that beekeepers in northern climates do not treat, as most colonies will not require treatment.

Varroa Mites

Varroa mites (Varroa destructor) are the most serious problem for honey bees, and are the leading cause of colony death over the winter. It is very important that you keep track of, and control, mite levels in your colony to keep your colony alive.

Varroa mites are visible with the unaided eye. These mites infest and feed on the blood of both adult and immature stages of bees. An adult mite enters the cell containing an older larva and is sealed within the cell when the workers cap it over with wax. The mite then lays eggs (the first egg will be a male, the rest will be females), which mature and mate within the sealed cell. Usually 1- 2 mated daughter mites, along with the mother mite, leave the cell when the bee emerges as an adult. When infestation levels are high, the mites cause extreme damage and death to honey bee colonies.

When a colony is dying from mites, symptoms may include being able to see mites on adult bees, spotty brood patterns, numerous uncapped cells and pupae being removed from cells. At advanced stages of infestation, the adult population dwindles and larvae are abandoned and die and decompose. Some call this larval death parasitic mite syndrome. The mites may transmit bee viruses from pupa to pupa during feeding, which may be the main reason for colony death. Some bees may tolerate continued low infestation levels, and some stocks may have mechanisms of defense against mites, which gives them a level of resistance to this pest.

Symptoms and Diagnosis:
• Mature varroa mites are reddish brown and can be readily observed on white drone and worker pupae. With a trained eye, they also can be observed on the thorax or between the abdominal segments on adult bees and on the bottom board of the colony.
• Emerging bees that were parasitized may be deformed, weakened and have suppressed immune systems and shorter life spans.
• It is easy to sample a colony to determine the level of varroa mites. See the instructions on the following pages and color poster.

• Purchase or rear queens from lines of bees bred for resistance to the mites. We define "resistance" as the ability of a colony to live with mite infestation without treatment for a longer period of time than unselected lines of bees. In other words, it will take more time for resistant bees to require treatment compared to unselected lines. The lines bred for resistance may still require treatments but at less frequent intervals.
• Currently, the Russian line of bees and bees bred for VSH and hygienic behavior demonstrate resistance to the varroa mite. Sample your colonies for mites in early spring through fall. If large numbers are found, consider using one of the treatments below.

Economic Thresholds of Varroa Mites:
An economic threshold (or action threshold) is defined as the number of mites that should trigger management action to prevent damage to the colony and economic loss for the beekeeper. This threshold is relatively easy to determine for insect pests in a crop, but very difficult to determine for mites in a bee colony.
When there are many colonies within 3-5 miles of each other, such as in many cities, or a commercial beekeeping operation, the threshold is lower due to movement of mites on robbing and drifting bees among colonies (horizontal transmission). When colonies are relatively isolated (i.e., there are no other colonies within 3-5 miles), the threshold may be higher. Highly infested colonies often produce large honey crops before they collapse from mite infestation in the fall.

The following thresholds are guidelines based on our experience in Minnesota. If colonies have above 2-3 mites per 100 adult bees in May, mite levels usually become very high (e.g., over 10 mites per 100 bees) by late summer as the colony grows and mite populations increase. This increase in mites is especially evident in areas where there are many beekeepers such as in cities, or where there are many commercial beekeeping apiaries. It is wise to treat colonies with over 2-3 mites per 100 adult bees in May before the honey flow.

If colonies have over 4-5 mites per 100 adult bees in late August or early September, it is critical to treat them to reduce transmission of mites to other colonies and to help reduce colony death during late fall and winter. It is very important to do a final mite sample toward the end of October when there is no brood in the colony to determine if the treatment was effective, or to ensure mite levels did not rise due to horizontal transmission.

Sampling for Mites

• We strongly urge all beekeepers to monitor mite levels in all colonies at a minimum in early May, in late August or early September, and again in October.
• Our research has found that you can obtain a reliable estimate of the number of mites in the colony by dislodging the mites from a sample of 300 adult bees collected from a frame of brood in the brood nest (not from a honey super). Sampling fewer than 300 bees does not give a good estimate of the mite level.
• There are various ways to collect 300 bees from a brood frame. Refer to the Instructional Poster on Center Plate.
• We recommend the “Powdered Sugar Roll” method to dislodge mites from bees.
• Collect 300 bees in a container with a screened lid, as shown in the Instructional Poster. Generously coat the bees with powdered sugar and let the coated bees sit for at least one minute. Then vigorously shake the mites through the screened lid into a white container for one minute. Add water to the white container to dissolve the sugar, which will allow you to better see the mites to count them.
• Divide the total number of mites by 3 to obtain number of mites per 100 bees.
• If you find 12 mites on 300 adult bees, divide 12/3 = 4 mites per 100 adult bees. This will tell you that the colony level infestation is 4 mites per 100 adult bees.
• Our Bee Squad program at the University of Minnesota Bee Lab has developed a Varroa Mite Testing Kit that has everything you need for the powdered sugar roll test method put together in an easy-to-carry bucket. These kits are available at bee suppliers. A list of suppliers can be found on the Bee Squad website (
• We also developed a mite sampling “Gizmo” to help you collect 300 bees or you can make your own Gizmo from instructions on our website. Alternatively, you can use a 100 ml measuring cup, which will hold approximately 300 bees. We recommend using a square scoop, such as one that is sold to hold juice boxes. Mark 100 ml on the inside with a permanent marker. Run the scoop down the comb so the bees fall into the cup (rather than trying to scoop them upward into the cup). Sharply rap the bees so they settle at the 100 ml mark. You can verify you are collecting about 300 bees by freezing a couple samples long enough so they are not moving, and counting the chilled bees.
• The following are the two best methods to dislodge mites from a sample of adult bees.
1. Powdered Sugar: This sampling method was developed at the University of Nebraska. It has the advantage of not killing the bees. A canning jar with a two-piece lid is used to collect the sample. Before the sample is collected, prepare the jar by making a screen from #8 mesh hardware cloth the size of the center of the lid. Retain the metal ring that comes with the two-piece lid, and discard the center portion. Put the ring with the 8-mesh screen over the jar containing the sample of bees. Add 2-3 Tablespoons of powdered sugar to the jar through the screen to thoroughly coat the bees. Roll the jar sideways to distribute the sugar on the bees. Allow the jar to sit for a minimum of one minute – this is very important. Do not put jar in the sun. Then invert the jar and shake vigorously over a white container to recover the mites. The bees will remain in the jar, and the mites and sugar will pass through the screen to the white container. With this method, you can dislodge from 90- 95% of the mites from the adults.
2. Alcohol Wash: This is the most accurate method, but it kills the sample of bees. Collect the sample of bees in a jar already filled with alcohol (rubbing alcohol or blue windshield washer fluid is fine for this). The following can be done immediately or brought home to do later. The alcohol from the sample jar can be strained through a 1/8" screen into a white bowl. Shake the screened bees in the alcohol to dislodge the mites into the alcohol. This method will recover 95-100% of the mites on the adult bees. If you count the number of bees in the sample, you can determine the percent infestation of mites on adult bees.

Sampling for Mites Using a Sticky Board
This method of sampling for mites involves placing a "sticky board" (for example, cardboard coated with Vaseline) on the bottom board of a colony under a screened bottom board. Monitoring the number of mites that naturally fall and adhere to the board over 3 days. The mites that adhere to the board may be alive or dead at the time they fall. The sticky boards are good for general monitoring, but not good for quantifying how many mites are in a colony. There is no accurate way to relate the number of mites on a sticky board to mite infestation in the colony (i.e., mites per 100 adult bees). We strongly urge beekeepers to sample mites using powdered sugar or an alcohol wash so there is a common basis for comparison of mite levels.

Treatment Decision Examples:
• If you sample for mites on adult bees in early May and find a colony level infestation of 2-3 mites per 100 adult bees it is wise to treat now, without treating at this time, mite levels will increase rapidly through the summer, causing colonies to die in late summer or early fall. Sample again in late summer to determine if you need to treat your colony immediately after honey is harvested (late August).
• If you sample for mites in late August or early September, when there is still a lot of sealed brood remaining in the colony, and find 4-5 mites per 100 adult bees, we strongly recommend you treat your colony. It is best to treat as early in September as possible.
• Sample your colony 2-3 weeks after treatment to ensure the treatment worked. Do not allow your colonies to become a source of mites for healthy colonies within 2-3 miles from you.
• If you keep records of mite levels in spring through fall every year, you will be able to determine your own treatment thresholds specific to your area and management style.

• If you choose not to treat your colony for varroa mites when mite levels are over 4-5/100 adult bees in late summer, there is a very high probability your colony will die over the winter.
• There are effective organic treatments on the market that are based on natural, rather than synthetic compounds.
• It is very, very important to sample for mites 2-3 weeks AFTER treatment to ensure the treatment was effective. If it was not, you may need to use a different product. It is every beekeeper’s responsibility to not allow your colonies to become a source of mites to healthy colonies located 2-3 miles from you.

Treatment Options
Screened Bottom Board: Mites drop naturally off of adult bees and from the combs during the course of the day. Normally, they can crawl back onto the bees and return to the brood nest. If you use a screened bottom board (commercially available), the mites drop through the screen and land on the bottom board beneath it. However, the mites cannot return to the brood nest because the space (at least ¾") between the bottom board and the screen is too great for them to cross. This method is not very effective alone, but you can eliminate 10-20% of the mites by using screened bottom boards.

Drone Brood Removal:  Varroa mites prefer to reproduce on drone pupae because the development time of drones is longer than workers allowing the mites to produce more offspring on drone pupae. This management practice relies on providing bees extra drone comb and removing the drone brood containing mites from the colony before it emerges.
1. To do drone brood removal, provide a colony with a comb of commercially-purchased drone foundation or an empty frame with no comb or foundation. The bees tend to build drone comb on empty frames. Watch your colony carefully, and when the bees have sealed the majority of drone brood on that frame, remove it and freeze it to kill the mites. After freezing for 36 hours, you can put the frame back in the colony and the bees will remove the dead drones and start again. Be sure to remove the drone brood BEFORE the drones emerge or you will be propagating mites rather than controlling them.
2. This method is very labor intensive, but can remove a substantial number of mites during the summer. The efficacy will depend on how diligently you remove drone brood.

Thymol (Oil of Thyme) Apiguard®
• Thymol is a botanical oil. One thymol product registered for use is Apiguard®. This product contains thymol formulated in a slow release gel. The most effective range of ambient temperatures for proper vaporization of Apiguard® is 60-80°F (max 105°F).
• You will need to close any screened bottom boards and upper entrances during treatment. Peel back the cover of the Apiguard® container and place it upright on top of the frames in the top box (there should be no supers on the colony).* Put a ring (1.5” high) or an empty box on top, then replace the cover of the colony. The ring provides needed space above the container for effective delivery of the Apiguard®. After 2 weeks put a second treatment on the same way. If any gel remains in the first treatment, leave the container in the colony, otherwise remove the empty container. Leave the second container on for 4 weeks (total 6 week treatment time).
• Label requirement: Remove Apiguard® from the hive prior to putting on honey supers to prevent contamination of the honey.
• This treatment is not labor intensive, but efficacy will depend on ambient temperature and the continuous, complete volatilization of the product over the treatment period. It is important to sample colonies after treatment to determine how effective the treatment was.

Thymol (Oil of Thyme) ApiLife VAR®
• Thymol is a botanical oil. This thymol product, registered for use, is ApiLife VAR®. This product contains thymol and minor proportions of menthol, eucalyptol and camphor. In our experience, it is not as effective as ApiGuard®. This product is formulated as an evaporating tablet. The most effective range of ambient temperatures for proper vaporization of ApiLife VAR® is 59-69°F.
• Take one tablet from a bag and break it into 4 pieces. Place pieces on the top bars of the top brood box, in the corners of the brood nest (not necessarily the corners of the box). Avoid placing pieces directly above the brood nest. After 7-10 days, replace with a fresh tablet broken into pieces as above. Repeat procedure again, 7-10 days later (3 treatments total). Leave the last tablet on for 12 days and then remove any residual product from the colony.
• Label requirement: Remove ApiLife VAR® tablets from the hive at least 30 days prior to harvesting honey to prevent contamination of the honey. • This treatment is not labor intensive, but efficacy will depend on ambient temperature and the continuous, complete volatilization of the product over the 21-30 days of treatment. It is important to sample colonies after treatment to determine how effective the treatment was in controlling the mites.

Formic Acid
• A new formulation, called Mite-Away Quick Strips®, is now registered for use. Formic acid is an organic acid, and is considered an organic treatment. We are finding this to be a very effective treatment when used correctly.
• See and the label for complete instructions, and for storage and disposal instructions. • Formic acid is corrosive. Wear protective gear according to the instructions on the label. You will notice that the bees react strongly to the formic acid for several days, and it may cause queen loss and some brood damage.
• Daytime ambient temperatures during the treatment period must be between 50-85 °F. Colonies will require adequate access to fresh air during treatment; so keep the bottom hive entrance, and any additional holes fully open. Remove the Quick Strips from the clear pouch, separate them, leaving paper wrap intact. Place two strips across the top bars of the frame over the bottom box so they lay flat and across the full width of the hive body, with approximately 2 inches between the strips and 4 inches between the ends of the brood chamber. Follow the instructions on the label for placement of the strips. Do not disturb the colony for 7 days. Check for mite levels 1-2 weeks after treatment, and allow one month between applications.

Oxalic Acid
• Although recently approved by the EPA, oxalic acid, an organic treatment, has been used to control mites in colonies since 2010 in Canada and for more than 20 years in Europe.
• Oxalic acid treatment has a low risk of hive product contamination and mite kill can be over 90%. Oxalic acid is applied in a liquid (trickle method described here) or by vaporization. Neither application method will kill mites in sealed brood.
• Oxalic acid should be used when the colonies have little or no brood in order to be most effective. It is most effective to use it when bees are loosely clustered and should be applied when temperatures are between 30-55 °F. Depending on weather conditions after treatment, it might be difficult to test mite levels after an oxalic acid treatment.
• Using a scale to measure the oxalic acid, prepare a solution by dissolving 35 grams of oxalic acid dihydrate (minimum 97% purity crystals) in 1 liter of lukewarm sugar syrup (light syrup which is 1:1 solution). Make sure the entire 35 grams is mixed it. It takes a lot of stirring for it to mix well. Mix only the amount you will use in one day. Dispose of any leftover at the end of the day. Mix new solution each day.
• Smoke the bees down prior to application. Use a syringe or other applicator to apply the solution directly to the bees in between the frames. Five milliliters (ml) should be trickled in between each frame on the bees for a total of 30 ml in a five frame nucleus, 40 ml in a single deep colony or 50 ml (maximum dose per colony) in a two or three deep colony. If you miss and place the solution on top of the frames, the bees will not remove it and your treatment will not be as effective.
• Only apply oxalic acid once a year if needed. It can be an effective part of a varroa management program, but overuse could harm bees. Do not treat weak or starving colonies. Do not treat when honey supers are in place.
• Protective gear should be worn per label guidelines.
• For instructions for the vaporization method, consult with the manufacturers of the devices.
• For instructions on treating colonies, read the Oxalic acid label.

• Another new synthetic miticide was recently registered for use in bee hives: Apivar® (Amitraz plus inert ingredients) N’- (2,4-demethylphenyl), N- [[(2,4-dimethylphenyl) imino] methyl]- Nmethylmethanimidamide. It is not an organic product, and it is likely that mites will develop resistance to this product, as they did to Apistan® and CheckMite+®. We do not recommend this treatment for use by backyard beekeepers.
• You must wear chemical resistant gloves. It can be fatal if absorbed through the skin.
• Follow label and use 2 strips per brood chamber, hanging strips over the combs so that bees can walk on and contact the strips. After 14 days remove the strips. All strips must be removed at least 2 weeks before a honey flow.

University of Minnesota Bee Lab

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