Why didn’t my bees make it through the winter?

This is a question we are asked multiple times in the spring. As a result, I have had the opportunity to not only observe my own hives, but to also look at a lot of hives belonging to other beekeepers confused and saddened by what they think is the sudden loss of their bees. Was it not enough food, were they not warm enough, was there too much moisture in the hive, did they swarm and leave? The list goes on and on, and in the majority of the hives the loss is really related to varroa mites. When I say varroa mites, I hear responses such as, “I didn’t see any,” or “I treated, it couldn’t have been.” My next question is, “Did you sample for mites, and what were your mite loads?”

I am a firm believer in sampling for mites to determine how many mites you have. You don’t know what you don’t know. Use a screen bottom board or better yet a sugar roll or alcohol wash. Sample frequently.

Assuming your hive was of sufficient strength and had adequate food, let me show you why your hive died. To do this I am going to take advantage of an interactive bee/varroa population model put together over several years by Randy Oliver. (You can find it at ScientificBeekeeping.com). To use your bee numbers in the model, sample for mites and plug-in the actual numbers for your colonies. For the sake of today’s discussion, the model below shows a hive started with new spring package bees that are treated for mites in early September yet ultimately fail. Let’s look at why and how this happens so frequently.

Mite treatment was effective at lowering mite numbers in September.  As fall continues, immigration of bees from untreated and collapsing hives in the area (often called mite bombs), and subsequent robbing of these colonies brings mites back into your hive. You quickly have too many mites, the virus load increases and although the hives still seems strong the health of the colony is actually declining.  If it is a warm fall, you may find your hive being robbed as it weakens and numbers decline, or you may open your hive in January and find few if any bees. This is a result of sick bees fleeing the hive even in very cold weather (in an assumed effort to protect the colony). Pretty soon the colony is so small it cannot function and dies. It takes at least one generation of new bees to recover from high mite/virus loads, something that doesn’t happen when winter sets in.

Figure 1 Note the mite treatment the first week of September. Note the mite immigration numbers. These parameters were adjusted for what we typically see in the Magic Valley, In an area isolated from other bees, it will be less.

This could also go another way, if we had a cool fall which shut down bee activity in early October, then little influx of mites would happen and this hive would survive just fine. Let’s adjust the numbers and show what happens to this same hive the following year assuming it survived the winter.

Figure 2 No mite treatment shown in this second-year hive. Hive crashes late summer or early fall.

This hive starts out in the spring looking good. It takes off and before you know it, you are adding honey supers to this colony and mentally calculating all the honey you’re going to get. And then, nothing is happening, they seem to have stopped working, or they’ve disappeared completely. Did they lose a queen? Did they swarm? What happened? In my experience, once they cross a particular threshold of mites, and mite transmitted viruses, the colony stagnates and then comes crashing down. Even with treatment, a new queen and lots of TLC, they just don’t bounce back very quickly.  10 years ago, I would tell people that if they didn’t treat for mites they would lose their bees by the second year. Here you can see why. Fast forward a decade to an increase in mite viruses, and it is more likely you will lose them the first winter without adequately timed treatment. The key to preventing your colony from dying due to mites is monitoring and treating based on that monitoring. You need to know how and when various treatments are effective.

Now that you know why your bees died, you can take action to monitor and treat in a way that not only will help prevent winter related mite kills in your bees but will help keep other bees in the area safer as well.

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