Where do wild bees go in winter?

In mid-October, gardens are abuzz with wild bees—eastern carpenter bees, bicolored striped-sweat bees, and common eastern bumble bee queens, to name a few. After mating, those bees have one goal: fatten up before winter. They are getting ready to hibernate for the next six months, and every flower counts.

fall bees fueling up on new england aster before winter
Fall active bees love fueling up on New England aster (Symphyotrichum novae-angliae) before winter. From left to right: Agapostemon sp., Halictus ligatus, Bombus impatiens.

Cueing into dropping temperatures and shorter days, the bees tuck away in quiet, secluded spots. Think: under leaves, rotting logs, long forgotten debris piles, or even back in their natal nests. Once in their resting place, they enter diapause—a state of lower metabolic activity—and cross their antennae. They will need enough energy to survive the next six months before spring.

Importantly, unlike honey bees, wild bees—of which there are more than 350 kinds in New England—are not active during winter. They cannot exit and re-enter diapause during a warm spell in February. Breaking hibernation before spring is a death sentence, and bees have many checks in place to ensure that this does not happen.

Although some species (like those mentioned above) actively fuel up before winter, most take a different strategy: they remain in their natal nest, surviving entirely on the nutrient-rich provisions left by mom. They have never seen the bright beautiful world. These bees will remain in the dark confines of their nest until emergence the following year.

Most wild bees overwinter in brood cells in their natal nest.

Bees that emerge in the spring overwinter as adults. In the fall, larvae metamorphose into adults before winter (yes, just like butterflies, bees undergo metamorphosis). They overwinter as adults in the nest, emerging when the temperatures rise in spring. For example, unequal cellophane bees Colletes inaequalis are harbingers of spring to eastern North America, emerging when there’s still snow on the ground because they overwintered as ready adults.

In contrast, bees that emerge in summer or fall hibernate as post-feeding larvae, also known as prepupae. Prepupae resume development after winter and pupating into adults sometime the following summer. Adults emerge when their favorite flowers are in bloom, but it is still a grand mystery how late-season solitary bees time their emergence so perfectly. Each year, when goldenrod blooms in our gardens in early August, hairy-banded mining bees Andrena hirticincta are there without fail.

Hairy-banded mining bees (Andrena hirticincta) female nectaring on goldenrod. This fall-active species overwinters as prepupae in the soil and develops into adults after winter.

Hibernating bees may be out of sight, but they can’t be out of mind. Think of the bee life cycle like a series circuit: if a single connection goes faulty, if a single life stage goes unsupported, all the little lights on our summer flowers go out. In this way, ensuring bees survive the winter is as important as ensuring adults find flowers in your garden in summer. Knowing how bees survive the winter, then taking intentional actions informed by this knowledge is the way forward:

  1. Plant fall blooming native plants: Asters and goldenrods are fountains of energy-rich nectar. Bees that fuel up before winter require enough fall-blooming flowers to build sufficient fat reserves. The very best pollinator gardens bloom until frost.
  2. #Leavetheleaves: Provide undisturbed nesting sites like patches of soil insulated with fallen leaves or intact hollow stems the right way. Resist the urge to neaten up your yard. Leave it messy until May and the bees will thank you. (Posting signage can help others know your good intentions!)
  3. Pay attention: And last, take time to notice wild bees in your gardens in fall. Are females drinking nectar, or are they still collecting pollen and provisioning nests? What flowers do they like best? Or maybe you spotted bumble bee queens digging their winter hibernacula (if so, get in touch with Queen Quest)? Often the most important life stages are the ones hiding in plain sight.
Leaving stems standing in your garden over winter helps pollinators like bees and butterflies survive until spring.

Agapostemon virescens: this green metallic sweat bee loves your garden

by Chloé Markovits and Nicholas Dorian

Ask a kid to draw a bee, and they will make it big and fuzzy, with black-and-yellow stripes. But did you know that not all bees look like that? One of the most common bees in eastern North America is slender, shiny, and green!

The bicolored striped-sweat bee (Agapostemon virescens) is a stunning green bee that you can find in your garden all year long. It is so beloved that it’s the official bee of Toronto. It lives mostly in the Midwestern and Northeastern United States (but with sightings from coast to coast). To help you spot one, let’s review its life cycle, flower preferences, and appearance. For a quick guide, reference our A. virescens field ID tips.

A. virescens are active for most of the growing season—typically from mid-May through late October—consisting of two generations of females. In mid-May, mated females emerge from hibernation and build nests underground. They use their natal nest or dig a new hole in sparsely vegetated soils. Mulched garden beds, sandy paths, patchy lawns are suitable nesting habitat.

Agapostemon virescens live in underground communal nests.
Agapostemon virescens live in underground communal nests. Females take turns guarding the nests from unwanted intruders like parasitic flies and cuckoo bees. PC: Nicholas Dorian

Unlike the social societies of honey bees or bumble bees, Agapostemon nest in communal societies of egalitarian females. Like apartment complexes in the human world, many females (up to 30!) will live in a single nest, but each will take care of her own offspring independently. A nest has one entrance (a hole at the surface of the ground) which is used by multiple females.

Ground-nesting bees are at risk from predators that aim to steal the nest contents. But Agapostemon has figured out a defense strategy: while some females are out foraging, at least one always stays back to guard the nest. Females take turns guarding the nest, and the presence of a guard reduces the chance of a visit from an unwanted intruder.

While out foraging, A. virescens females gather pollen and nectar. Females can be found on a wide variety of plants—from roses, to strawberries, to sunflowers—but in general have a penchant for plants in the Asteraceae family. In particular, summer-active females love collecting pollen from purple coneflower (Echinacea purpurea), cup-plant (Silphium perfoliatum), false sunflower (Heliopsis helianthoides), and lance-leaf coreopsis (Coreopsis lanceolata).

Agapostemon virescens loves asters like cup-plant.
Agapostemon virescens loves asters! Cup-plant (Silphium perfoliatum) is one of their favorite plants. PC: Nicholas Dorian

In late-July to early-August, a second generation of A. virescens emerges. This generation consists of both males and females. Males cloud around flowers waiting for the chance to mate. They are not welcome back in their natal nests, so males find places to sleep outside. Sometimes, males will sleep together in aggregate in sheltered nooks and crannies– bee slumber parties!

Fall-active females sip nectar from flowers in order to survive winter, but do not build nests or lay eggs. Look for fall-active A. virescens on asters like New England aster (Symphyotrichum novae-angliae), goldenrods (Solidago spp.), and sneezeweed (Helenium autumnale). When temperatures drop, mated females enter hibernation in their natal nests until spring, and males die.

A. virescens females can be easily identified on flowers or with photographs taken with a phone camera. Look for a metallic green thorax and black-and-white striped abdomen.  Males are trickier to distinguish from males of other Agapostemon species but can be identified by their metallic green thorax, black-and-yellow abdomen, and the lack of dark streak on their hind femur.

Agapostemon virescens females can be easily identified via the combination of a metallic green thorax and a black-and-white abdomen.
Female Agapostemon virescens can be easily identified via the combination of a metallic green thorax and a black-and-white abdomen. PC: Nicholas Dorian

If you’re lucky enough to spot one, take a moment to appreciate its fascinating life cycle. Is a male darting around in search of a mate? Or a female working hard to collect pollen for her offspring? Is she hovering close to the ground? That might mean you’ve found her nest! Despite its abundance, there is still much to be learned about this bee. What soils do they like for nesting? How far do they forage? How long do females live after provisioning nests in early summer? Why do females prefer aster pollen over all others? TPI scientists are hard at work addressing some of these questions.

Cellophane bees are the real first sign of spring

Step aside robins, unequal cellophane bees (Colletes inaequalis) are the true harbingers of spring in the eastern US. Bees emerge on the first warm day, often when snow is still on the ground, and they refuel on flowers after a long winter. And, they are already out in Boston: in 2022, cellophane bees emerged on March 16th!

Unequal cellophane bees are incredibly abundant, especially in suburban areas, so they are easily spotted around town. Before you head out to find them, read this blog, check out this video, and explore our field identification guide. After you enjoy your time with them, be sure to report photos to iNaturalist!

Colletes inaequalis female sits in her nest entrance, ready for spring. Females dig long tunnels underground 1-2 feet in length!

Flight Season

Unequal cellophane bees are active in very early spring, usually before trees have leafed out, and are active for four to five weeks. In Massachusetts, they are typically active from mid-March through mid-April.

Appearance

Unequal cellophane bees are small—like macaroni, about 0.75x the size of a honey bee—with “cute” heart-shaped faces, sand-colored hair, and bold bands on their abdomen. Males have long antennae and thick mustaches. Females are bigger than males, with cleaner faces, shorter antennae, and long hairs on their legs for carrying pollen.

Colletes inaequalis are have bold stripes on their abdomens and have sand-colored hairs. Males (right) are smaller than females (left), with longer antennae and fuzzy mustaches.

Nesting habits

Unequal cellophane bees are easily spotted while nesting. Females build underground nests, excavating sand into a small mound at the surface called a tumulus (like an ant hill but with an entrance about the width of a pencil). They live in bee neighborhoods—termed aggregations—each of which can contain thousands of nests. In the first few weeks of the year, males can often be seen zooming low through the aggregation looking for mates.

Nesting aggregations typically appear on trampled, sandy south-facing slopes. Lakeshores, cemeteries, trampled picnic areas, sandy paths on the edge of the woods, and even backyards are all good places to look!

Cellophane bee nests occur in large aggregations, often in bare sandy soil. Each nest looks like a mini-volcano of sand, and females often sun themselves in the nest entrance.

Similar Species

Cellophane bees Colletes can appear similar to many mining bees Andrena that also nest below ground. But, Andrena have obvious facial foveae and parallel eye margins, which Colletes do not.

Comparison of Colletes vs. Andrena in the field.

Favorite Flowers

Unequal cellophane bees are not picky eaters. To attract them to your yard, plant spring-blooming trees like red maples Acer rubrum, apples Malus, eastern redbud Cercis canadensis, and serviceberry Amelanchier. They can also be spotted on garden bulb flowers like snowdrops, blue squill and crocuses.

Colletes inaequalis has a penchant for red maple (Acer rubrum) flowers.

Safety

You do not need to worry about being stung–cellophane bees are incredibly gentle! Even the biggest aggregations consisting of thousands of nests pose absolutely no threat to people or pets.