CO-Horts Blog

Wednesday, January 4, 2023

Meant to Bee: Overwintering Strategies for Bees

Posted by: Lisa Mason, Arapahoe County Extension

Where do the bees go in the winter? With over 900 species of bees found in Colorado, it is an interesting question. How do they survive the snow and cold temperatures? Can we support bees during the winter? Bees and other insects have special adaptations so their species survives from year to year. Here is a look at bee adaptations and life cycles in the winter time.

Honey bees

Honey bees (Apis mellifera) are one of the 900+ species of bee in Colorado. They have a eusocial structure meaning the colony has: 1) overlapping generations, 2) a reproductive division of labor, and 3) cooperative care of the developing larvae. The female worker bees forage all summer and into fall bringing in food reserves to last them the winter. When temperatures start to drop, honey bees huddle together to make a cluster and shiver their wings. Shivering provides warmth for the hive. Their main goal is to keep the queen warm so the colony can survive. The core temperature in the hive can be as high as approximately 91 degrees Fahrenheit. A healthy hive with adequate food storage is more likely to survive, which reinforces the importance of best beekeeping practices by the beekeeper all year. Read how to prep a hive for winter here. When a beekeeper harvests honey, they have to be careful not to harvest too much. Beekeepers should leave 80-100 pounds of honey in the hive for the bees to feed on throughout the winter. Honey bees are often seen outside the hive warm winter days. Anytime temperatures rise above 55 degrees Fahrenheit, honey bees may take a cleansing flight which provides them the opportunity to relieve themselves. Honey bees may also forage for whatever they can find including tiny bits of protein at bird feeders.

The worker bees in the hive cluster around the queen during the winter to keep her warm and safe. Credit: Lisa Mason

Bumble bees

Colorado has 23 species of bumble bees (Bombus spp.). They live underground or in large cavities. They have a one-year life cycle, like solitary bees. They have a social structure within their colonies that include a reproductive queen and workers. Like other social insects, they have overlapping generations and cooperative brood care in the colony.

Newly mated queens are the only bumble bee survivors during the winter. The hardy queens find a place to hibernate in a protected place like leaf litter, a wood pile, or underground. When spring arrives, the queens will emerge, begin to forage, build a new nest, and lay eggs. The eggs will mostly be female worker bees at the beginning of the season. The queen will continue to lay eggs throughout the season. In late summer, new queens and male bumble bees will hatch and leave the colony to find mate. As temperatures drop, the colony from the current season will end except for the mated queens that will hibernate. Queen bumble bees are capable of living alone, unlike honey bee queens.

A queen bumble bee (Bombus nevadensis) foraging on Rocky Mountain bee plant. Photo: Lisa Mason 

Solitary bees

Solitary bees comprise of the the vast majority of bee species diversity. They live a one-year life cycle. During the life cycle, a female bee builds a nest underground or in a cavity. A cavity usually consists of a pre-existing tunnel. Some tunnels might be in a dead log, nooks and crannies in stone or brick, a human-made bee hotel, or hollow stems. The female bees collect pollen and nectar to bring back to the nest. All the collected pollen and nectar is made into a ball called “bee bread” which is all the food needed for one bee to grow from a larva to an adult. The female lays an egg on the bee bread and seals up the nest. After the egg hatches, the larva will go through full metamorphosis from a larva, to a pupa, and on to an adult before emerging from the nest the following season. The lives ended for the female and male solitary bees we saw flying around the previous summer, but their brood is warm for the winter deep underground or in a cavity. 

An underground, solitary bee nest (Agapostemon sp.). Photo: Rachelle Stoddard

Does “leaving the leaves” support the overwintering bees?

Leaf litter supports hibernating bumble bees by providing them a sheltered area for the winter. Solitary bees are well-protected in their underground or cavity nest but leaf litter may provide some insulation for their ground nest. Since honey bees live in hives, they don’t rely on leaf litter but they have been observed drinking water from damp leaf piles. Leaf litter also supports a wide diversity of arthropods by providing shelter during the winter. Wooly bear caterpillars are a great example of a moth that relies on leaf litter. They overwinter in the caterpillar stage by hibernating in leaf litter. They have an impressive adaptations to survive the winter. When summer arrives, they complete metamorphosis and transform to an adult Isabella tiger moth. Overall, leaf litter can provide a shelter for a diversity of overwinter arthropods. Leaf litter is also a great mulch for perennial and vegetable gardens. The leaves also return critical nutrients back to the soil. While leaf litter is a valuable component to the landscape, it is important to remove leaves from the lawn to prevent mold and disease growth, and to avoid smothering the turf.

When should spring garden clean-up begin to best support the overwintering bees?

Different bee species emerge at different times, so it is a challenge to find a perfect date for spring garden clean-up that supports bees. Here are some general considerations for supporting overwinter bees:

  • Bumble bees typically emerge from hibernation between mid-April and mid-May depending on weather and elevation. Bumble bees could be impacted if leaf litter is disturbed before they emerge.
  • Some solitary bees nest in hollow or pithy stems such as Rubus spp. (raspberries, blackberries, etc.), joe-pye weed, bee balm, etc. They will emerge in the spring. If hollow stems appear to be plugged, perhaps consider leaving them stems until they emerge, or stems could be carefully moved to a protected location where they won’t be disturbed.
  • Leafcutter bees are often some of the latest emerging bees. They nest in a wide variety of cavity locations from natural spaces to human infrastructure (e.g. patio stones, and occasionally even inside gardening gloves). Since they emerge later in the season, it is likely best not to postpone garden work for leafcutter bees unless specific locations of nests are known that could be protected.
  • Squash bees may nest in underground areas where squash and pumpkin plants are typically planted. Consider not tilling the vegetable garden to support these important pollinators.

  • All bees need early-season flowers that offer pollen and nectar. Consider adding plants that flower as early as April and May. Some options include golden currant, chokecherry, crocus, and pasque flowers.
A leafcutter bee (Megachile sp.) nest found inside a garden glove. Credit: Nicole Didero

More information

For more information on bee lifecycles, check out the Native Bee Watch Citizen Science Field Guide. Learn more about the 900+ native bee species in Colorado by checking out A Beginner’s Field Guide to Identifying Bees

For more information on what happens to other insects in the winter, refer to this CO-Horts Blog post written by Jessica Wong

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