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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.

Pipevine Swallowtails and the case for caterpillar gardening

When you think of gardening for pollinators, you most likely think of beautiful flowering plants which attract bees, butterflies, and other insects by offering copious amounts of nectar or pollen. But flowers cannot give all pollinators the food they need. Butterflies in particular can be very picky when deciding what to eat when first starting out their lives as caterpillars. Most butterfly species have evolved to specialize feeding on one or a handful host plants. In a sense, when it comes to supporting butterfly communities, providing ample host plants for these soon-to-be-butterflies could be just as beneficial as planting nectar flowers for adults!

Pipevine swallowtail eggs are laid in small clusters so that when they emerge, these caterpillars work together eating leaves in small groups

The Pipevine swallowtail Battus philenor is one butterfly that benefits tremendously from the gardening of its favorite plant, Pipevine Aristolochia, after which it is named. Both Pipevine and the butterfly with which it shares its name is native across eastern North America, including Massachusetts. The plant itself is highly toxic and could cause severe kidney problems or even cancer if ingested by humans, but Pipevine swallowtail caterpillars could not care less.

These caterpillars have evolved not only to cope with the toxins in the plants, but to actively seek them out. Pipevine swallowtails, like many other butterflies, including Monarchs and their beloved milkweed, acquire the toxic chemicals present in their host plant and sequester them. As caterpillars they build of reserves of this chemical making them unpalatable to potential predators for the rest of their life. This is part of the reason many butterflies display such bright and captivating scales – it can be a signal to predators that nothing good will come from eating them.

In the right conditions Pipevine can scale great heights clinging to trees or other supports

Though these caterpillars cannot get enough of Pipevine, the adult swallowtails themselves do not nectar on the flowers it produces. Pipevine, like the name suggests, produces curved flowers that resemble a smoking pipe which is not conducive for a butterfly proboscis. Dutchman’s pipe Aristolochia macrophylla, is the most common species in eastern North America, and produces small tubular flowers with a small circular entryway. Into that entryway go flies which are actually attracted to the flower’s fragrance. Upon entering the narrow pipe, the flies get a little stuck, catching some of the flower’s pollen and carrying it with them until they end up in another flower which they help fertilize. After fertilization, the plant produces great seed pods which, when rooted and grown, will provide more food for those hungry caterpillars.

Before you start thinking this toxic, fly-attracting plant might not be the right fit for your garden, consider that Pipevine has been an immensely popular ornamental plant since Victorian times. It is an ideal option for trellises, fences or pergolas, providing plenty of shade when mature! If you are interested in attracting these beautiful butterflies to your home or garden in the future be sure to water your Pipevine as it needs lots of water and give it a place to climb. Be sure, too, to get Aristolochia macrophylla specifically, as some species will not support these caterpillars or might even be invasive – moreover macrophylla will provide the best shade (it is given the Greek name meaning ‘large leaf’) and will attract some marvelous butterflies looking for the right place to lay their eggs.

Adult Pipevine swallowtails have a remarkable coloration, with black and iridescent blue wings that have orange and white spots on the underside (Photo credit: John Flannery)