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beehive collapse over winter

Why Did My Bees Died Over Winter? Conducting A Hive Autopsy

Mar 18, 2023

The hardest part of beekeeping is handling the death of a hive, but when a hive perishes, the beekeeper has to put aside their emotions and figure out the cause of death before the evidence is lost.

Why would you want to examine a bunch of dead bees? 

The two main reasons for wanting to determine the cause of your hive's death is that you want to learn from your mistakes so you can increase your hive's survival rate next year and to make sure it is safe to reuse the honeycomb in the spring. 

It is important to determine the cause of your beehive's death so you know whether it is safe to use the honeycomb in another hive next spring.

Watch my YouTube video about determing the cause of the your hive's collapse over winter.

Stumped? If you're still struggling to figure out the cause of your hive's death over winter, leave it in the comments and I'll get back to you as soon as I can.

The top culprits to causing a honeybee hive to die during winter are:

  1. varroa mite infestation
  2. starvation
  3. not enough ventilation and/or condensation dripping over cluster
  4. nosema
  5. tracheal mites
  6. animal attack
  7. failing queen

Let's take a look at each one of them, so you know the signs of a hive loss due to each culprit and figure out how to prevent this from happening again. 

#1 Varroa Mite Infestation

The #1 killer of honey bee hives in the winter is varroa mites! Many beekeepers think varroa mites aren't the problem because they treated their hives once or twice over the spring and fall. Maybe they treated before they closed the hive up for winter, but did they do a mite test? Did they know how many mites were in their hive? Most importantly, did they test their hives for mites in the late summer/early fall when the winter bees are pupating? 

A little bit of background on what a honey bees do in order to survive winter

In mid/late summer, the queen bee lays eggs for the winter bees. These bees have different fat deposits, different hormone profiles and are the only ones designed to live for months. When these bees are pupating, it is usualy late summer/early fall, a time when varroa mite levels are usually very high. Robbing happens during this time of year, bringing in varroa mites on the backs of these robber bees. You won't see all of these mites because over 90% of them are hiding in the cells with the pupating winter bees, weakening the bees and giving them viruses.


If winter bees have viruses, they won't be able to form a strong cluster and insulate the hive.


Signs Your Hive Died from a Varroa Mite Infestation over Winter

  1. Your hive died in the fall or early winter.
    Hives weak from varroa usually perish not long after the temperatures go below 40F. Since the winter bees are weak, they cannot form a strong cluster to stay warm and they die fairly early in the season.
  2. Varroa on the bottom board. 
    A lot of dead mites on the bottom board can be a sign of an infestation. Usually you want to do a count and see how many mites per 100 bees you see, but since that isn't possible, look to see if you see a lot of mites on the bottom board.
  3. Unhealthy, capped pupae present
    In a healthy hive, the remaining pupae emerge in the fall. If you are seeing capped cells where pupae never emerged or cells with a hole in the capping, that is a sign your hive had a varroa infestation. In a healthy hive, the pupae emerge on their own. 
  4. Your hive was large in the summer and seemed healthy
    Big hives with a high population in the summer often have a high varroa mite population come late summer. Lots of bees means lots of mites and once the bee population goes down. However, if the beekeeper does not treat this time of year, the mite population will remain high, leaving your hive overun with viruses and mites. Hives that swarmed or requeened in the summer had a break in their brood cycle and so have a considerable lower level of mites in the hive.
  5. Lots of honey in the hive
    With lots of honey in the hive, it is safe to assume that the hive did not starve. 
  6. Mite feces seen in the hive
    Mite feces, known as guano, can be seen on the edges of the hive cell walls and is white. Do not confuse this with crystallized honey. You'll see this strictly on brood frames, not honey frames.
  7. A tiny cluster of dead bees
    Tiny clusters of bees is a sign that the winter bees were weak and could not survive. A tiny cluster is a cluster that is the size of a softball or smaller.
  8. Spotty brood
    Along with sick-looking brood, if you seeing spots of capped brood, that is a sign of weak winter bees.
  9. No Varroa Mite Management in the summer or fall
    Whether you treat your hives for mites or not, it is still important to know what your hive's mite levels are. Without conducting a mite test, the beekeeper will not know if a hive is infested with mites. You cannot look for mites on your bees or put a sticky board under your hive to get an accurate reading. You have to take a sample of bees in order to get an accurate reading.
  10. Affected 75% of more of your hives
    If only one hive out of 3 or more died over winter, then varroa mites might not be there cause. However, if you lost all of your hives or only one survive, then chance are that you need a better varroa mite management plan for next year.
  11. Dead bees in cells with their head sticking out and proboscis sticking out of their mouth
    These dead bees are bees that died while hatching, too weak to fully hatch they starved to death in their cell. Since varroa mites attack the brood, weak brood that cannot hatch is a sign of a varroa mite infestation. 

Watch my video on how to do a mite test. It's easy to make and only takes 5 minutes.


How to Prevent a Varroa Mite Infestation

Test one hive in your apiary every month with an alcohol wash.

If your hive has a high mite count (3 of higher), it is recommended that you treat your hives to lower the mite levels. Usually, the beekeeper will treat in the spring (before the bees start bringing in a lot of honey), late summer (after you harvest your honey for the year) and once more in the fall before the hive is closed up for the winter. 

See my video about when to treat your hives and treatment options for you to use.


#2 Starvation

Lack of food is another common cause of a honey bee hive to die over winter. The best food for your bees is the honey that they gathered, but the beekeeper can also add dry, white sugar or make candy (also known as fondant) and put that at the top of the hive, under the lid. 



You do not want to feed your bees a liquid sugar syrup during the winter!


Signs your hive died from starvation are:

  1. No food in the hive
  2. Bees died with their heads in the cells (see photo below)
  3. Cluster is surrounded by empty frames and although there is food in the hive, it is far from the cluster.
    This is common with top bar hives when the cluster starts in the center and moves to one side of the hive. They starve, not able to break the cluster and access the honey on the opposite end of the hive. 

Photo above: dead bees most likely died from starvation

How to Prevent Your Bees from Starving over Winter

Make sure your hive has ample food. If you have a langstroth hive, you want your honey to form a bell shape in the hive with the largest amount of honey at the bottom, and gradually decreasing the amount of honey in the upper boxes. 

You also want to make sure there is a super full of dry, white sugar or fondant above the upper most box in the hive, under the lid.

After a couple of months, start checking on the hive to make sure they have enough food. On warmer, sunny, low wind days, add more food under the lid if needed.

An easy way to see if your hive needs food is by lifting one end of the hive to see how heavy it is. As the weeks go by, you'll notice the weight of the hive decrease.

For most of the U.S., 50lb of honey in the hive is sufficient. In the colder, northerm states, 60-70lb is needed. In the warmer, southern states, 35lb is usually sufficient. The best way to figure out how much food to leave your bees is to leave them all of their food the first winter and weigh the frames before you close the hive up. In the spring, during your first inspectino, see how much food is left. You can also take an in-person beekeeping class in your area, sign up for my online beekeeping class which includes mentorhip and we can work together on figuring out how much food to leave your bees or find your local beekeeping association and attend a meeting or ask on their facebook page.

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Make sure to keep track of how much honey you put in your hive each fall and how much is left in the spring to see how much food your bees ate over winter.


#3 Nosema

Nosema is a parasite caused by a fungus. Nosema affects the bees' diggestive tract and will kill a hive over winter. 

Signs of nosema:

  • Brown/yellow streaks on the outside walls of the hive
  • Send a sample of dead bees for testing to a bee lab
  • Population was strong in spring but was smaller in summer


You cannot prevent a hive from getting nosema. It is an unfortunate thing when it happens and one reason why beekeepers should have more than 1 or 2 hives in their apiary.

If you think your hive had nosema, it's important to send the bees to a lab for testing or examine bees under a microscope. 


If your hive had nosema, do not resuse the comb! You have to dispose of the comb or you can infect your other hives.


#4 Tracheal Mites

Another cause of honey bee hive loss that is out of the beekeeper's hands is tracheal mites. The only way to know for sure you have tracheal mites is by sending a sample of bees to a lab for testing,

Signs Your Hive Has Tracheal Mites:

  1. Hive was healthy in spring and then smaller in the summer
  2. Dead bees spread all over the hive, not in a cluster
  3. Bees trembling outside, in front of the hive


Unfortunately, there is no way to prevent this from happening. The good news is that this isn't a common cause of hive loss that you should be seeing across all your hives or see year after year. It is a problem the beekeeper will come across occasionally.


#5 Animal Attack Killed Your Bees

An obvious one, but signs an animal killed your hive are:

  1. Hive was knocked over
  2. Animal feces inside hive
  3. Comb was eaten

It's important to check on your hives (not open the hive, just stand near the hives to observe them, coming and going or listen to the buzzing by putting your ear on the wall of the hive). Sometimes the beekeeper will blame a rodent or animal for destroying a hive, but the hive was already dead. The animal is just taking advantage of a free meal. The sooner you realize a hive has collapsed and can examine it, the greater the chancec you'll figure out the correct cause of death.


  1. Use mouse guards over the lower entrances
  2. Put up an electric fence to keep bears and larger animals away

I've occasionally had a hive tipped over by a cow or wild boar. I do not put fences around my hives because it is a rare occurance.


#6 Poor Ventilation/Too Much Moisture

People often think they need to keep their bees warm, but if the bees are healthy, they produce their own warmth. What the beekeeprs needs to do is make sure there's ventilation.


Signs your hive died from poor ventilation/moisture:

  1. Wet inside the hive
  2. Mold in the hive (not on the cluster)
  3. Entrances blocked

How to Prevent Your Beehive from Getting Attacked by Animals

Bees need a way to leave the hive and go on a cleansing flight on warmer days as well as fresh air. It's best to have two forms of ventilation in your hive over winter. There should be at least one entrance, usually the main entrance to your hive all summer, just reduced in size with an entrance reducer and a secondary entrance in case the first entrance gets blocked. This secondary form of ventilation can be an upper entrance or a screened bottom as your bottom board. See my blog post Is An Upper Entrance Bad for Bees to help you decide where you should put your sources of ventilation for your bees and how to help your bees stay warm over winter. 

There will be condensation in your hive over winter and that is a good thing! Bees need moisture in the hive. However, you want this condensation to drip down the inside walls of the hive or to drip onto the backup sugar feed at the very top of the hive. You do not want condensation dripping onto the cluster, it can kill your bees. To prevent this, you will want to put a piece of insulation directly under the lid (such as a moisture board) to reduce the amount of condensation building up under the lid as well as something to absorb any condensation dripping off of the lid. Having dry, white sugar under your lid or fondant is sufficient and also good for the bees because it will help the bees eat this sugar. Just make sure you continue to refill it as needed because the bees will be eating it over winter. some people use a quilt box which is a box full of moisture abosrbing material such as wood shavings. This is another option. However, some people find that the wood shavings can get soaked fairly quickly and having a box full of wet wood shavings is just as bad. If you choose to use a quilt box, be prepared to check on it often.

Check on the hive weekly and scrape away dead bees accumulating on the bottom board. You do this by scraping the board with your hive tool. Do not open the hive! You can put the hive tool in through the lower entrance. Make sure the lower entrance is clear for the bees to come and go.


#7 Failed Queen

A failed queen is not as common as the other causes of a colony to collapse, but still can happen. 

Signs Your Hive Died Over winter Because of a Failed Queen

  1. Queen cells on the brood frames
  2. Colony was alive by late January but no brood when it collapsed
  3. Brood may have been spotty in the summer
  4. Queen is over 2 years old

Ways to Prevent a Failed Queen Over Winter

You can requeen hives with a queen that is 3 years or older to prevent the possibility of her not laying come late winter. Personally, I do not pinch my queens based on age. I only requeen if the hive is struggling and a new queen is the only way to get the hive to be healthy and heavily populated again. However, it is up to each beekeeper to make this decision. Don't let anyone else tell you what is wrong or what is right!

Notice that I didn't include cold weather!

Cold weather alone does not usually kill honey bees. Honey bees can withstand very cold temperatures. In order for bees to survive cold weather, they need three things:

  1. Good health

  2. Lots of food

  3. Insulation

The bees make their insulation. As a beekeeper, you can provide them extra insulation in the form of bee cozies or wrapping the hive with roofing paper, but these things are not necessary. Did you know that Russia and Ukraine are some of the biggest honey-producing countries in the world? My great grandfathers had bees on their farms in rural Poland and Ukraine and they did not have bee cozies to keep their hives warm. The bees vibrated their flight muscles and produced their own heat as they have been doing for many years.

As a beekeeper, your job isn't to make the hive as warm as possible, it's your job to make sure they are healthy and have lots of food! 

So let's get to the list of culprits for your hive loss.

In Conclusion

It's never easy to figure out a hive's cause of death. If it was obvious, we would have done something sooner to prevent it! Most likely, you'll never know without a doubt the cause of death. All you can do is take detailed notes as best you can, talk with other beekeepers about methods they use to successfully overwinter their bees, and try to use better techniques next year.

As always, you can ask me! 

Sign up for my online beekeeping class. It includes mentorship so you can email me directly questions you have. Include photos and/or videos and we can troubleshoot together!

Click here to enroll today!


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