New tri-coloured bumble bee (Bombus ternarius) queens (gynes) emerging from their underground nest. Photo: T. Harrison

Stumped: Why do bumble bees need forests?

Posted onApril 28, 2022|Bumble Bees, Native Pollinator Initiative, News and Events, Pollinators
When you think of bumble bees, where do you imagine them? Buzzing around flowers, of course! Most of us are used to seeing bumble bees in our gardens, at the park, or meadows, but these aren’t the only places you can find bumble bees.

Picture this. It’s early spring and you’re walking through a forest where the canopy only breaks enough to let a few rays of sunshine through. Leaf litter is still on the forest floor. There aren’t many flowers in bloom just yet, but you hear focused buzzing in the distance – your only cue that bumble bees are somewhere nearby. Following the sound, you’re able to spot a beautiful big queen off in the distance. What are they doing if not foraging?

Nest searching!

Flying in a distinctive zigzag like pattern while avoiding flowers, they release a loud and constant buzz as they go about their search for a safe nest space.

The bumble bee colony lifecycle. Forests fill many needs for bumble bees throughout their life cycle, including providing habitat for overwintering and nesting, and providing food resources in the spring. Illustration: Jeremy Hemberger.

Having somewhere safe to live is incredibly important, especially for eusocial insects who have a whole colony to protect, and forests give bumble bees the perfect place to nest.

Eusocial insects live together and meet the following requirements:

1) Cooperative brood care – working together to take care of their young

2) Reproductive division of labour – queen(s) will do all the reproduction (laying eggs), while the workers help with foraging and brood care, but don’t lay eggs. These roles are constant throughout their lifetimes.

3) Overlapping generations – since the queen(s) lay eggs all season, there are overlapping generations in the colony, where one bee might be twice as old as some of her sisters!

While we’re used to seeing bee colonies depicted in trees, or domesticated honey bees living in wooden box hives, bumble bees usually build their nests close to, or under the ground. This can include empty burrows (e.g. old mouse nests), hollow logs, or holes around tree roots. Unlike grasslands and gardens, forests provide lots of nooks and crannies for bumble bee queens to start their new colonies. Not only that, but the trees and shrubs in forests help slow down harsh winds, protect bees from the rain, and keep temperatures down during the heat of the summer by providing shade from the sun.

Left: Yellow-banded bumble bee (Bombus terricola) on Dutchman’s breeches flowers. Right: Half black bumble bee (Bombus vagans) on Solomon’s seal. Both of these spring ephemerals bloom early in the season, providing bumble bee queens with much needed nectar and pollen. Photo: T.Harrison

Bumble bees are also drawn to forested areas while nest searching because forests are home to the earliest-blooming flowers: spring ephemerals, such as the Dutchman’s breeches. When bumble bee queens first emerge from overwintering in April and May, they are extremely hungry (I mean, wouldn’t you be after sleeping the whole winter?), but there are very few flowers in bloom. Finding a nice patch of forest allows them to fill two needs with one deed: they get some delicious early spring nectar and pollen and find a new home to settle down into to raise their colony in.

Spring ephemerals are woodland wildflowers that begin to grow and bloom very early in the season, taking advantage of the bare trees that allow sunlight to reach the forest floor, and include:

  • Dutchman’s breeches
  • Solomon’s seal
  • Trilliums
  • Rue anemone
  • Trout lilies and blue bead lilies
  • Hepatica
  • Snowdrops

Overwintering: New bumble bee queens produced at the end of the colony lifecycle dig into soil and live underground in a state of slowed metabolism, becoming dormant over the winter until they emerge the next spring. Refer to the winter portion of the colony lifecycle diagram above.

The importance of multiple different habitats and plants to even just this one small group of insects really goes to show how important each and every piece of the natural world is. While you’re out looking for bumble bees this spring season, don’t forget to explore forests too. Maybe you’ll be lucky enough to see queens searching for their new home – keep an eye out for that zigzag pattern!

Watch the video for an example of this behaviour. You can witness the characteristic zigzag pattern that bumble bee queens use when scouting out a good place to live and raise a colony. Notice how she lands on the ground to check out any holes that might be good spots. Video: T. Harrison

Parker Smale

Native Pollinator Initiative

Parker’s passion for insects can be traced back to one fateful field entomology course, which also inspired his own entomology blog. As the lead technician for WPC’s 2022 bumble bee breeding program, he specializes in data wrangling, pollen chefery, and entomological match-making.

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