Colony Size Drives Honey Bees’ Overwinter Survival

By Meredith Swett Walker

When the temperature drops and the days get shorter, honey bees don’t hibernate—they huddle. Meanwhile, worker bees produced in the fall are plump and have longer lifespans than their spring counterparts. These winterized workers form a “thermoregulatory cluster” around their queen. Powered by honey stores, they shiver their muscles to produce heat, keeping temperatures at the center of the cluster around a comfortable 21 degrees Celsius (C). Still, winter is a stressful time for honey bee (Apis mellifera) colonies. In the United States around 30 percent of colonies don’t survive until spring.

In Europe, overwinter survival of honey bee colonies is generally better, and evidence shows that honey bee populations may be “locally adapted” to regional climatic conditions. For instance, honey bees originating in Greece are less likely to survive a winter in Finland than are local honey bees. But honey bee populations in North America are a lot more “geographically scrambled.” That’s because commercial beekeepers in North America routinely move bees great distances. Colonies providing pollination services are moved around the U.S., and queens are shipped great distances by breeders. So, locally adaptive genetic traits may not necessarily stay in the place where they are advantageous.

But could North American beekeepers boost overwinter survival of their hives if they used bees from closer to home? In a new study published in December in the Journal of Economic Entomology, Mehmet Döke, Ph.D., along with colleagues at Pennsylvania State University and Embu University College in Kenya, examine whether U.S. honey bee stocks may be locally adapted to their regional climate and have higher overwintering success in the climates where they were bred versus elsewhere.

The researchers looked at overwinter survival of honey bee colonies in central Pennsylvania, where winter temperatures dip down to –4 degrees C. They compared survival of colonies headed by queens bred in the northern U.S. (Vermont or West Virginia) versus colonies those headed by queens bred in the southern U.S. (Texas and Florida). They established 60 colonies at three different apiary locations near the Penn State campus and maintained them using standard bee keeping practices. The researchers tracked numbers of adult bees and brood and size of food stores, as well as net colony weights (combined weight of adult bees, brood, and food stores). In addition, they looked at DNA microsatellites to determine if northern bees differed genetically from southern bees and if bees from different breeders within a region differed. The landscape around each of the three apiary locations was also assessed to determine differences in floral resources.

About 30 percent of the colonies in the experiment did not survive the winter, but there was no difference in overwinter survival between bees bred in the north versus the south. This was unexpected and Döke says the most surprising finding was how little northern and southern bees differed genetically.

“The extent of genetic differentiation between the queens from four different providers in distant parts of the U.S. was nowhere near as great as I would expect,” he says. “There was virtually no difference within region—i.e., the two northern stocks were the same and the two southern stocks likewise—and a small but significant difference between the regions, north versus south. Given the distance between our providers, I had hoped to find more of a difference than what we saw.”

A recent study examined honey bee overwinter survival in central Pennsylvania, where winter temperatures dip down to –4 degrees Celsius. Here, technician Bernardo Niño visits one of the three apiary locations the researchers near the Penn State University campus. (Photo credit: Mehmet Döke, Ph.D.)

In a study on honey bee overwinter survival, researchers in Pennsylvania used a portable platform scale to weigh colonies. The weight of the boxes, frames, bottom board, inner cover and top were subtracted to determine the net weight of the colony (adult bees, brood, and food stores.) (Photo credit: Mehmet Döke, Ph.D.)

The best predictors of overwinter survival were colony weight and the number of worker bees in October, just before winter weather begins. There appeared to be a threshold colony weight, with colonies weighing less than 20 kilograms (or about 44 pounds) having low survival and colonies weighing more than 30 kg (or about 66 pounds) surviving at high rates (about 94 percent). Döke and colleagues suggest that colonies with more workers might be able to form more effective thermoregulatory clusters and use their food stores more efficiently.

Colony weight and survival also differed among the three apiary locations. Assessing landscape effects on overwinter survival was not the main question in the study. And, because only three apiary locations were used, the effect of landscape could not be statistically tested, but the difference between the apiaries was suggestive. The apiary location with the highest colony weights (averaging about 31 kg) and best overwinter survival (90 percent) was surrounded by a mix of agricultural and natural landscapes, while the other two locations were surrounded by a mix of forest and developed land providing less abundant and diverse flowers. The authors suggest that higher abundance and diversity of flower resources may support larger numbers of worker bees in a colony, which increases the likelihood of overwinter survival.

So, what does this mean for beekeepers? Döke says there’s no one-size-fits-all recommendation because the goals of backyard versus commercial beekeepers (and all the beekeeping operations in between) differ greatly. But, if beekeepers want to maximize overwinter survival, Döke recommends building strong colonies by using quality queens and controlling Varroa mite parasites, as well as tracking colony weight during the summer. If colonies are underweight, keepers may be able to supplement them with pollen and sugar syrup to boost worker numbers before winter comes. Or, small colonies can be combined to increase the odds of overwinter survival.

“Honey bees are going through a rough time,” says Döke. “We keep hearing about large-scale beekeepers of many decades closing shop due to profitability issues as a result of high loss and replacement rates over the winter.” Still Döke is optimistic that, with continued research and informed management practices, overwinter survival—and the health of the beekeeping industry—can be improved.

Meredith Swett Walker is a former avian endocrinologist and wannabe entomologist. She now studies the development and behavior of two juvenile humans in the high desert of western Colorado. When she is not handling her research subjects, she writes about science and nature. Find a sampling of her work at www.magpiescicomm.com.