Colonies huddle in tight thermoregulated clusters to survive winter. High Varroa mite levels further add to the stressors that may lead to devastating overwintering losses. Several Varroa mites are shown on the honey bee in the photo above.
Beekeepers hold their breath as the cold starts to bite wondering if their bees will make it through the winter in good condition.
Honey bee colony losses of around 15 percent are typically expected following the winter season, but in recent years losses have been between 22 and 34 percent.
A honey bee colony is a dynamic social structure containing 40,000-60,000 or more bees depending on colony maturity, the time of year and the health of the hive. Even a healthy colony can lose up to a thousand bees per day during the summer, because they are constantly being replenished.
Wintering bees are physiologically different than their summer siblings, and are more suited to surviving for several months confined in the hive by cold weather. If the health of winter bees is compromised, the entire colony may not survive the winter period.
Honey bee colonies are highly adaptable and can survive long, cold winters and extended periods when there is no natural forage. But colony health can be negatively impacted by many factors including parasites, pathogens, predators, severe weather events, habitat loss, nutritional deficiencies and hive management practices.
Since the Varroa mite, a blood-sucking parasite that vectors diseases, first became established in North America during the latter half of the 1980s, beekeepers have been playing catch-up in their efforts to manage their hives. Just a few years ago, the National Honey Bee Health Stakeholder Conference identified Varroa as the “single most detrimental pest” of honey bees and other scientists have shown that it remains a leading cause of colony failure.
Adult winter bees that were infested with Varroa as immature bees within the brood cells do not fully develop the physiological characteristics of a long-lived winter bee. This makes them less likely to withstand the grueling environmental stresses associated with winter conditions and survive until the spring.
“Since 2013, beekeepers in the United States have been doing better at reducing winter honey bee colony losses. Part of this success comes from better management of the Varroa mite. However, during my 2015 hive evaluations, I was disturbed to find the vast majority of hives contained mite infestations well above the threshold level of concern,” said Dick Rogers, manager of Bayer Bee Health and Integrative Apiculture Research.
Samples taken from hives containing three Varroa mites per 100 bees suggest that the colony is in trouble. While three mites may not seem like a big deal, remember that a typical colony may contain 40,000 bees – and that equates to more than a thousand parasites, which weaken bees through their feeding and disease transmission activities. In fall 2015, a majority of the hives Bayer and other experts examined exceeded seven mites per 100 bees, a level that is almost certain to result in colony failure this winter.
What’s causing this? When honey bees are doing well, large infestations of Varroa mites are never far behind. In addition, mites may have developed resistance to Apivar® (amitraz) strips that beekeepers use to keep mite infestations in check.
There is hope. Experts with the Honey Bee Health Coalition released a new Varroa Management Guide recently which offers beekeepers practical, effective methods of monitoring and controlling this invasive pest.
At Bayer, we are seeking and testing new varroacides, as well as more efficient delivery systems to better manage infestations.
Winter normally is a stressful time for colonies, but high mite infestations make this year’s situation particularly challenging.