Winter Bees and Varroa Mites

You often hear the advice to control varroa mites in the late summer to protect your winter bees. It makes sense to keep mites suppressed throughout the year, but why is there such a particular emphasis on protecting winter bees? Explore the factors that go into overall winter bee survival rates—and see how varroa mite testing and treatment during summer can affect colony outcomes.

Can bees survive the winter?

Western honey bees (Apis mellifera) were originally tropical organisms that, over time, adapted to life in temperate regions. Because of this, bees can withstand long periods with no forage and lethally cold weather. These adaptations range from behavioral ones (extreme resource hoarding and winter cluster behavior are two examples) to physical differences in winter bees, which we explain further below.

Though bees have adapted for winter survival, many factors can affect winter honey bee survival rates and colony losses, including:

• Exposure to cold, moisture, or humidity
• Starvation
• Weak colony
• Disease
• Pests, such as mice
• Parasites, such as varroa mites

How long do winter bees live?

A queen bee stops laying eggs when temperatures cool — somewhere between October and December. To compensate for this lack of egg-laying, winter bees have longer lifespans than spring and summer bees: While worker bees only live about six weeks during spring and summer, winter bees live up to four or five months. This extra-long life helps the colony survive the winter. Longer lives mean a large number of the bees live from fall through spring, helping the spring population buildup.

Fat bodies and their crucial role in winter bees

Looking beyond lifespan, there is a significant physical difference between winter bees and their summer sisters: adult winter bees' bodies are crammed with fat bodies. The term "fat bodies" refers to the internal organs of bees that function somewhat like our liver. In addition to playing important roles in maintaining immune function, detoxifying pesticides, and thermoregulation, the fat bodies' principal role is as a reservoir of fats, proteins, and the source of vitellogenin. Put simply, fat bodies store and release nutrients and energy.

Vitellogenin plays a crucial role in extending the lives of winter bees and reducing physiological stress. Normally, fat bodies only occur in large quantities during the pupal stage and, for summer bees, during the short period when they are young nurse bees. The increased amount of fat bodies is what gives winter bees the capacity for longer life.

Randy Oliver provides a side-by-side example of a forager compared to a winter bee.

What do varroa mites eat?

For many years, it was believed that varroa mites fed on honey bees' hemolymph, much like how ticks or fleas feed on mammalian blood. Recently, the work of Dr. Sammy Ramsey changed that understanding. Ramsey demonstrated that the mites were feeding on the bees' fat bodies¹, not their hemolymph.

Varroa mites devastate honey bees by spreading lethal viruses, but we now also understand that varroa's appetite for fat bodies also inflicts damage at particularly crucial points in winter bees’ lives. Varroa mites that are reproducing under the cappings with developing winter bee pupae consume a large amount of the pupae's fat bodies, leaving the surviving bees weakened — including affecting the development of proper fat bodies in adult winter bees, reducing their winter lifespans and leaving colonies short when spring arrives.

Sammy Ramsey's award-winning 3-minute presentation on his groundbreaking work on varroa mites and what they eat. Can't get enough of Dr. Ramsey? Here's a link to an article on his work in the journal Entomology Today. There are spectacular pictures!

Winter bee survival depends on summer mite control strategy

Not only do the overwintered nurse bees have to make food for the new larvae from their bodies, but they need a large enough population to shelter and keep the brood warm in later winter and early spring. Lack of physical reserves in the bees' bodies, or a diminished population, forces the colony to restrict the pace of the build-up. A delayed build-up can result in colony loss because, while winter bees live a long time, they don't live forever. This may explain the situation where a colony that appears to be strong in mid-winter can be dead by March.

How to protect your future winter bees against varroa mites

Well-timed late summer mite treatments can allow winter bees to escape damage while they are pupae, but the adult bees are still vulnerable during fall and winter. No treatment is 100% effective, so mites can build up again — especially in a strong hive that is still making a lot of brood. Even after treatment, a colony may be exposed due to drifting and robbing. A colony treated in summer may be strong and healthy enough to rob honey from untreated hives nearby in the fall, and they may even bring home a new round of mites from those robbed hives².

Relying only on a late summer treatment could leave the door open to additional damage to adult wintering bees. As the bees settle down into their winter quietness, any remaining varroa mites are now cooped up with a veritable buffet of their preferred food: the fat bodies of bees. And in contrast to the summer, all the winter bees are loaded with fat bodies!

In addition to your summer treatments, we recommend monitoring for varroa and, if levels are above the threshold of 6 or more per 300 bee sample, treating in August and September to protect your winter bees while they are developing. You may wish to treat again in the fall to continue to knock the mites back and protect the last rounds of winter bees that are emerging. And no matter what, plan on doing one final treatment with oxalic acid (either as a vaporization or a dribble) when the colony enters the brood pause. This coup de grace will help you knock out any mites that otherwise would be dining on your bees all winter long.

Can I treat for varroa mites in winter?

While varroa mites require brood to reproduce, they may overwinter on adult bees — and they can live and feed on adult bees for a span of up to 3 months. Not all varroa mite treatments are suitable for use during the winter, but there are some wintertime varroa mite treatment options you can consider. The best time to treat winter bees for varroa is the summer beforehand, but if winter treatment is necessary, the best options are Apivar and oxalic acid treatment. As with any mite treatment in any season, ensure you pay attention to all requirements for use — including temperature requirements, removal of supers, and safety guidelines.

Because varroa mites feed on the fat bodies of winter bees, helping your colony overwinter and thrive next spring and summer depends on a strong mite control strategy ahead of the winter season. Consider your mite monitoring and treatment strategies early, and stay on schedule, for better hive outcomes when the weather begins to warm again. For more beekeeping tips and expert advice, explore our Beekeeper Guide.

¹https://www.pnas.org/doi/10.1073/pnas.1818371116

²https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0218392