Research on the Dynamics of Death: Varroa Mites in Dying Honey Bee Colonies

In February 2023, our Director of Research and Education, Dr. David Peck, had a paper published based on a research project that he did as a graduate student at Cornell University. The paper, coauthored with Auburn University Professor Michael L. Smith, is titled "Dynamics of honey bee colony death and its implications for Varroa destructor mite transmission using observation hives." Little was known about precisely how a failing bee colony dies, and whether a dying colony could be a source of varroa mites for other colonies nearby who may pick the mites up when robbing the failing colony. This small study attempted to discover more about exactly how things go wrong when a colony is in serious trouble.

Experimental setup

The four experimental colonies were housed in 4-frame glass-walled observation hives, with the four frames stacked vertically so that all frames and bees could be seen at any time. Each colony had honey, pollen, brood, and both worker and drone comb available to them at the start of the experiment. At the bottom of each hive, a very thin version of a screened bottom board allowed varroa mites to fall onto an oil-covered plastic sheet so they could be counted. 

The bee and mite populations were monitored regularly, as was the amount of comb devoted to worker and drone brood and to storing food resources. In order to watch how a colony dies, each of the four observation hives were rendered "hopelessly queenless" by first removing the queens, and then by destroying all emergency queen cells a few days later. 

Hopelessly queenless

The hopelessly queenless colonies could not create new queen cells, so some of the workers activated and grew their ovaries and started laying unfertilized eggs. These laying workers provided the struggling colony with something to do while they were waiting to die - they produced drones from those unfertilized worker-laid eggs, and the drones matured and were taken care of by the colony. Despite the great efforts to produce them, only about 3% of these new drones survived to adulthood. Though other colonies in the area had already evicted their drones for the year, these colonies maintained these new drones until the bitter end, presumably in the hopes that one might mate with a very late-season queen and spread the dying colony’s genes into the next generation. The same can be seen in autumn in colonies that are in the middle of requeening - they’ll continue housing and feeding drones (even drones from other colonies) until the colony has successfully requeened.

Not-so-sudden death

It came as no surprise that many of the bees died soon after the loss of the queen. An average of 50% of the starting bee population were dead or gone by day 25 of the experiment. Interestingly though, the colonies still persisted for much longer than that, typically 2-3 months. 95% of the bees were dead by day 74 of the experiment, but even then the few remaining bees held on as long as they could. 

The slowly dropping worker population received a small boost from a few newly produced drones, so varroa mites in the colony had plenty of adult bees to feed upon until the last bee died. Three of the four colonies died when they starved to death, and may have been able to last longer with additional food reserves. The last of the three colonies survived until the supplemental heat in the observation hive room was turned off, and was dead within 24 hours. Therefore, the arrival of freezing weather in temperate climates may ultimately determine the survival of such failing colonies. 

Varroa

The heart of this project was studying how varroa mites navigated the death of their host colony. Past research (by Dr. Peck and others) has shown that varroa mites can spread from sick and dying colonies to other colonies by robbing. In all colonies, live mites remained in the colony until 200 or fewer bees remained. In one colony the mites died 13 days before the last bee, in another the last mite and last bee died on the same day, and in the other two the mites actually outlived the bees by around two days! These mites could have left on drifting bees, or could have jumped onto robber bees if any had found the dying colonies before winter weather arrived. 

Perhaps the key point of this study was that mites hang on to the bitter end of a colony, and that as a colony dies (whether due to queenlessness, due to varroa, or due to anything else) the mites in that colony pose a risk to other bee colonies within the foraging range of nearby colonies. 

Take-home messages

Thanks to the sacrifice of these four colonies, we learned that dying colonies don’t all die immediately, but instead linger on for some time. These dwindling colonies, if queenless, invest in drone production as a last-ditch effort to spread their genes. Since new drones are produced, mites are perhaps able to slightly increase their population as they cling to the last remaining bees, hoping to be carried off on a drifter or to jump onto a robber bee arriving from a hive nearby. Allowing colonies to collapse and die (from mites or any other ailment) may expose healthy colonies nearby to an unnecessary risk of picking up additional mites. Far better is to keep your mite populations under control and to know when to "give up" on a dying colony so that they don’t just slowly die in your apiary.