The Beekeeper’s Companion Since 1861
icon of list

Cover Story

Why Did My Honey Bees Die Last Winter?

- August 1, 2016 - Meghan Milbrath, PH.D. - Michigan State University Extension - (excerpt)

large brood nest frame dead honeybees

Getting Ready for Winter 2016-2017

Beekeepers in northern climates lost a lot of colonies last winter (2015-2016). Some trends are starting to emerge. One of these trends is a specific type of colony death. In Michigan, I’ve received so many calls describing the scenario below, that I can describe the dead-out before opening the hive, or before the beekeeper describes it over the phone. While I may impress some with these predictive powers, the frequency of these types of losses indicates a real epidemic that is affecting honey bee colonies in northern states.

Characteristics of the common early winter death in northern states:

  1. The colony was big and looked healthy in the fall
  2. A lot of honey is left in the top supers
  3. The cluster is now small, maybe the size of a softball
  4. There are hardly any bees on the bottom board
  5. Near or just below the cluster is a patch of spotty brood — some fully capped, and some with bees dying on emergence (heads facing out, tongues sticking out).
  6. If you look closely in the cells around the brood, you will see white crystals stuck to the cell walls, looking like someone sprinkled coarse salt in the brood nest.


  1. You don’t have records showing that varroa was under control.

Sound familiar?
We see this classic set of symptoms over and over in the states with a proper winter. A big colony that seems to just shrink down and disappear. Many people want to use the term colony collapse for this type of death, and while collapse is a good descriptor of what happens, this is not true colony collapse disorder. This is death by varroa associated viruses.

How does It happen?

  1. The big colonies—While beekeepers are often surprised that their big colonies are the ones that are gone first, it makes perfect sense in terms of varroa growth. Since varroa mites reproduce in capped brood, the colonies that made the most brood (i.e. got the biggest) are the ones most at risk of having a high population of varroa. Colonies that swarmed, or didn’t take off, or even fought a disease like chalk brood are less at risk from high varroa populations, because they didn’t consistently have large amounts. You should have good notes indicating cluster size going into winter, but even if you don’t, you can see the large circle of food eaten by a large cluster.
  1. Lots of Honey—Lots of honey means that the colony died fairly early. Colonies with high levels of varroa, tend to die fairly early in the season (before February), leaving lots of honey behind. Once the bees are stressed and in cluster, the viruses take their toll very quickly. In some cases the colony will even abscond in fall, or be dead before winter really hits.

The colony in the first photo had a third deep box that was filled with capped honey, indicating that the bees died early, and starvation was not the culprit.

  1. Small cluster—Varroa levels peak right when the winter bees are getting formed. The bees that emerge from varroa-infested cells are weakened, and more importantly, are riddled with viruses. Varroa mites are notorious for carrying deformed wing viruses (DWV), but are known to transmit many more. When bees are close tight in a winter cluster, the viruses can spread very quickly. In our colony, the cluster was only the size of our hand —some bees had their heads stuck in the cells, trying to stay warm, others had fallen between the frames.
  1. No bees on the bottom board—When a colony starves, the bees just drop to the bottom board, and you end up with a pile of dead bees in the hive. When bees get sick with viruses and other pathogens, however, they often will fly away. Sick bees by nature leave the colony to die in the field, an act designed to prevent pathogen transmission in the colony. When most bees are sick, they either …