Where Do Campus Bees Go in the Winter?
Eric Malcolm of the UMD Bee Lab sheds light on how honey bees plan and perform remarkable feats of survival during the coldest time of year—and what we can do to help them.
When the temperature drops at the University of Maryland, certain corners of the campus fall eerily silent. The thousands of bees that buzz loudly in the spring and summer seemingly disappear without warning—so where exactly do they go when it’s cold?
“Pollinators overwinter in different ways. Wasps and bumblebee colonies typically die off, and the queens will overwinter underground and restart colonies in the spring. Many of our native bees will go dormant and overwinter as larvae or pupae underground, in hollow stems or the like,” said Eric Malcolm, apiary manager and extension educator for the UMD Bee Lab in the Department of Entomology. “Our honey bees actually stay here on campus and overwinter as a colony. They’ve developed a way to survive during the winter that doesn’t require leaving their hive at all.”
European honey bees, the type found in UMD’s official hives located by the Research Greenhouse, have developed a survival strategy so effective that it allows them to maintain a tropical climate inside their hive even on Maryland’s coldest nights. Instead of dispersing to survive the chill alone, honey bees gather together in layers inside their hive to create what’s called a winter cluster. Inside their cluster, the bees work together as a single superorganism that generates its own heat, enabling the bees to collectively survive temperatures that would kill them individually.
“When the temperatures fall to less than 45 degrees Fahrenheit, the bees start coming together and form a ball shaped cluster,” Malcolm explained. “This living, breathing ball of bees can maintain up to a cozy 91 degrees Fahrenheit inside the hive, even when it’s freezing outside.”
Lila Wilson, a senior mechanical engineering major and president of UMD’s student-led Beekeeping Club, says that the bees’ winter miracle doesn’t happen by chance. It’s the culmination of an entire year’s worth of preparation—a collective effort where individual bees work tirelessly for a future they may never see.
“Winter is tough for them,” Wilson said. “From just freezing to death to starving to death, there’s a lot for them to overcome during the cold seasons.”
How bees beat the cold
The hard work begins the moment honey bees emerge from the previous winter. Throughout warmer months, bee colonies explode in size—from 15,000-20,000 bees in early spring to 50,000-70,000 at peak season.
Malcolm noted that honey bees do not simply expand their in-hive population when spring comes; they’re also diligently stockpiling for the future. Every “forager bee” brings back nectar, which “house bees” then convert into shelf-stable honey by dehydrating the nectar’s water content. A colony of healthy honey bees can fill box after box (or “super”) with honey, storing far more than they need to survive the period where no nectar or nutrition is available from the environment.
“Honey bees are one of a few insects that will store an overabundance of food,” Malcolm said. “In Maryland, our honey bees can produce and store well over 60 pounds of honey in their hive as their food source, depending on local conditions that could be significantly more.”
By September and October, as flowers become scarce, the bees switch gears. The queen bee reduces egg-laying, triggering a planned population reduction. The drones—which do no foraging, cleaning or nursing—are evicted from the colony, as their reproductive services are no longer needed. The summer worker bees are replaced by winter bees, whose bodies are better tailored for enduring the winter.
When temperatures drop to around 45 degrees Fahrenheit, the remaining colony comes together to form a spherical cluster of about 15,000 to 20,000 individuals. “Heater bees” vibrate their flight muscles to raise their body temperatures to up to 115 degrees Fahrenheit to protect the queen and a small amount of brood from the cold temperatures deep within the cluster’s core. The worker bees only consume enough calories to keep their hive toasty.
“Theoretically, you could reach your hand down inside the hive during the winter and find it very warm,” Malcolm noted. “As winter progresses, the whole cluster moves upward through the hive, consuming the stored honey.”
On warmer days above 45 degrees Fahrenheit, the bees occasionally venture out of their cluster for “cleansing flights” after weeks of holding in waste. Then, as the temperatures drop again, the cluster reforms and the bees settle back into their usual winter routine: generating heat, consuming their food stores and waiting for spring to begin the cycle all over again.
How humans can help
But even the most prepared bees sometimes need extra support.
“There’s always going to be a challenge that our pollinators have to face,” Malcolm said. “One of the biggest problems now is dealing with Varroa destructor, these pinhead-sized mites that feed on honey bees’ fat bodies and spread harmful viruses.”
The UMD Bee Lab protects the campus colony by testing mite levels monthly, treating the bees when counts exceed 2 mites per 100 bees. Malcolm noted that mite populations could effectively double each month if left unchecked, causing significant damage to the population, in addition to risk of reinfestation from honey bees from surrounding areas.
“I’ve seen dead [winter bees] just inches from their stored food, too weak from mite-vectored diseases to reach their food,” he said. “Besides threats like queen failure or condensation in hives that can freeze them, starvation is a risk for bees, especially during warmer winters when elevated temperatures lead to higher bee metabolism, making them burn through their food stores faster. In winter, sick bees will also leave the cluster, sacrificing themselves to avoid draining colony resources.”
To boost the campus hives’ food supply, Malcolm and Wilson feed the bees a sugar-water solution throughout the fall, resorting to “sugar boards” (hardened sugar-water mixes) and even baker’s fondant. With these extra calories to burn, bees have a better chance at fighting off disease and staying warm in their clusters during the cold.
Humans can also support pollinators by planting native flowers. Malcolm transformed his own backyard at home into what he calls a “bee haven” including clustered mountain mint, asters, goldenrod and other plants that bloom for long periods or during times of year when forage is scarce. On a summer day, his mountain mint plants host entire ecosystems: honey bees, native bees, butterflies, wasps, even praying mantises and jumping spiders. Then, late-season bloomers like asters provide critical nutrition for bees before winter, while early spring flowers like crocuses give bees their first pollen after months of stored food.
“Humans have created ‘green deserts’ all over the country. Many of the problems bees face are not from competing bees or necessarily even weather, but habitat loss from human activity,” he explained, referring to urban areas and spaces like green lawns devoid of flowering plants. “Just creating your own ‘bee haven’ with a few native, pollinator-friendly plants can really make a difference in helping your neighborhood bees.”
