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Honey Bee Heat Warms Up Fellow Pollinators for Early-Season Blooms

closeup view of corner of a hivetop incubator on top of a honey bee hive box. the incubator is a wide, shallow box about two inches tall, topped with a wood panel. in the picture, near the corner of the incubator is a hole about a half-inch in diameter, and several Osmia lignaria bees are gathered on the outside of the box near the hole.

An incubator that draws excess heat from a honey bee hive warms up managed Osmia lignaria bees so they can pollinate early-blooming fruit trees such as cherry, apple, and almond. A new study shows the hivetop incubators are effective, with little effect on the honey bee hive temps below. Shown here is a hivetop incubator atop a honey bee hive, with a small exit hole from which O. lignaria bees can be seen emerging.

By Paige Embry

Paige Embry

Paige Embry

Honey bees (Apis mellifera) are the go-to pollinator for early-blooming fruit trees like cherries, apples, and almonds, but they aren’t the best pollinator for these crops. That title belongs to Osmia lignaria, often known as the blue orchard bee or BOB.

In the chilly days of early spring, BOBs fly more hours than honey bees and go out when it’s colder. They carry pollen, dry, in hairs on the underside of their abdomen where it may easily rub off when they flop into flowers, while honey bees carry pollen in tidy packets on their hind legs. BOBs are also flitters, moving from tree to tree rather than just working one plant like a honey bee often does—promoting the cross-pollination needed for some of these trees.

Lindsie McCabe, Ph.D., with long light brown hair and red wire-rimmed glasses and wearing a fuschia hooded windbreaker, smiles at the camera, next to a blue plastic box hanging from a tree. the box is open on the side facing the camera, and inside are two bunches of small wooden tubes, many of which appear  capped with dirt at the ends.

Lindsie McCabe, Ph.D., is a postdoctoral fellow with the U.S. Department of Agriculture’s Agricultural Research Service who led a recent study on practices for deploying Osmia lignaria bees for pollination in orchards. Here, McCabe pauses next to a O. lignaria nest box during the season after bloom and bee foraging. In the next box, tunnels with “mud caps” are nests filled with immature O. lignaria bees.

Lindsie McCabe, Ph.D., a postdoctoral fellow with the U.S. Department of Agriculture’s Agricultural Research Service, says, “Honey bees are very, very methodical in how they collect pollen, and blue orchard bees are just like, ‘I’m going to get all into this flower and rub it everywhere.'” What that behavior means is that several hundred female BOBs can pollinate an acre of early fruit as effectively as thousands of honey bees.

Part of the reason honey bees continue to dominate is that how to use them is well-established, while how to use BOBs is still a work in progress. A study published last week in the Journal of Economic Entomology focuses on a way to streamline one aspect of blue orchard bee management—waking them up from their winter’s sleep.

BOBs spend the winter as adults in cocoons in a hibernation-like state called diapause. Managed BOB cocoons are kept in cold storage and need to be warmed up before the bees will emerge. An easy, standardized way to do that hasn’t been developed. For example, one grower warmed the bees in her house. Two days usually worked, but when they wouldn’t rouse one year she stuck them in the bathroom with a space heater set to 85 degrees Fahrenheit. It worked, but bees in the house seems like an unlikely method to promote widespread BOB use.

Plus, any method of warming bees inside means they are then thrust out into the cold. “This can cause a problem sometimes,” says McCabe, “especially when you get cold snaps in the orchard or in the western U.S. when it gets really cold at night. … It seems to take them longer to emerge when they don’t have heat below them.”

Since the flowering season for these trees is short, having the bees ready when bloom begins is critical. McCabe and colleagues tested a device to make waking BOBs up easier and more predictable: an incubator that sits on top of a honey bee hive.

The device is called the Hivetop Incubator (HTI). Wonderful Orchards, which previously experimented with using BOBs for almond pollination,⁠ owns the patent for the HTI. Honey bees keep the core temperature of their hive in the 90s Fahrenheit (mid 30s Celsius). Naturally, heat is lost. Since the two pollinators are often used together, that radiating heat can provide steady warmth for incubating BOBs.

The researchers conducted experiments in Utah and Washington State. The Utah experiment focused on potential adverse impacts to the honey bees from having the incubator (and BOBs) sitting on top of their hive. Half the hives had incubators on top, half didn’t. The researchers found that the internal temperatures of the two groups were “not significantly different.” They also tracked various parameters of colony health (bee health, brood quantity, percentage of empty cells, amount of stored food) and found no adverse impacts.

The researchers also looked at internal temperatures in the BOB incubators and emergence rates. Incubators on occupied hive boxes were approximately 7.8 degrees C warmer than those on empty boxes. Sixty percent of the BOBs in the warm incubators had emerged by the second day, a level not reached until the sixth day in the un-warmed incubators. Both sets of incubators had around 80 percent emergence after eight days.

At the Washington site, which was colder, 60 percent of the BOBs in the heated incubators had again emerged by the second day; however, it took eight days to reach that level in the unheated incubators. Also, after 10 days, 95 percent of the BOBs had emerged from the heated incubators, but only 65 percent from the unheated ones. At this site, the researchers also tracked whether bees spread throughout the orchard to nest or stayed near the incubation sites. Other than very close to the incubators (within 1 meter) the bees scattered randomly throughout the orchard. McCabe says that dispersed nesting leads to dispersed pollination, “so you don’t have these hotspots of pollination.”

The experiments showed the HTI to be highly successful, particularly in the colder Washington trial. McCabe says one thing they don’t know yet is whether the incubators might increase pathogen exchange between the two bee species. They’re looking into it. Nevertheless, the authors feel positive enough about the trial to write, “Our study supports the incorporation of the HTI into the best management practices for using O. lignaria in orchard pollination.”

Maybe someday soon HTIs will make BOB-waking easier (no bees in the bathroom), more reliable, and less jarring for the bees.

Paige Embry is a freelance science writer based in Seattle and author of Our Native Bees: North America’s Endangered Pollinators and the Fight to Save Them. Website:

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