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By Hillary Sardiñas

Washington Park Arboretum Bulletin, Summer 2016

This article is reprinted from our quarterly botanical magazine. To read more articles like it, become an Arboretum member today! For questions about bees, please contact the Xerces Society at pollinators@xerces.org.

With summer here, gardens are buzzing with pollinators—and not only the honey bees. The European honey bee is just one of more than 20,000 bee species worldwide. Honey bees were brought to America by settlers, only reaching the West Coast in the 1860s. Before that wild, unmanaged bees provided most of the pollination of wildflowers and crops. Native bees are still the most important pollinators of wild plants, helping to maintain ecosystem diversity. In addition, bees help feed the world: One out of every three bites of food we eat needs a pollinator to reproduce. Native bees play a large role in crop pollination, and they are often better pollinators than honey bees, spending more time on each flower and therefore helping to transfer more pollen.

Most bee diversity is concentrated in Mediterranean environments, however, the Pacific Northwest also hosts numerous species. The vast majority of bees are solitary, meaning they don’t form complex colonies with divisions of labor, like honey bees. Although social bumble bees nest in cavities, solitary wild bees either excavate nests in the ground, or find hollow stems to occupy. The rainy environment in the Pacific Northwest makes the latter nesting strategy much more attractive, as plant stems are less likely than soil to get inundated. Because most native bees are solitary, they don’t have a hive to protect and are therefore much less likely to sting than honey bees. In fact, you really have to get a native bee mad to provoke it into stinging you.

Causes of Bee Decline

Unfortunately, both native bees and honey bees are currently declining due to a number of threats. Habitat loss is one of the major drivers of biodiversity loss worldwide. Even remnant habitat fragments are losing value due to the spread of non-native invasive plants. The invasives compete with the native species most wild bees rely on for pollen and nectar. Disease spread is another major issue facing bee populations. Managed bees, like bumble bees used for tomato pollination, spread disease to native bumble bees. Many bumble bee species that were once widespread have disappeared throughout most of their range. The western bumble bee Bombus occidentalis used to be the most common species west of the Mississippi but is now rarely seen. Another species with a more limited range, Franklin’s bumble bee Bombus franklinii, hasn’t been recorded since 2006, despite extensive searches in its former Southern Oregon range.

How You Can Help

You can help with bee conservation by planting flowering plants in your yard for year-round, continuous bloom. Focus on native species because ornamental plants, while showy, often no longer produce the pollen and nectar bees rely on. Bees also need nest sites, either undisturbed bare soil or pithy-stemmed plants (like raspberries). See the “Native Plants for Native Bees” for some recommendations. More extensive plant lists for regional pollinators, as well as recommended nurseries and other resources, can be found at: www.xerces.org/pollinators-pacific-northwest-region.

Native Plants for Native Bees

Here are some handsome Northwest native plants—listed in order of bloom time—that provide good food and habitat for native bees.

Bigleaf lupine (Lupinus polyphyllus)
Slender clarkia (Clarkia gracilis)
Selfheal (Prunella vulgaris ssp. lanceolata)
Canada goldenrod (Solidago canadensis)
Hall’s aster (Symphytotrichum hallii)
Oregon grape (Mahonia aquifolium)
Blueblossom (Ceanothus thyrsiflorus)
Cascara (Rhamnus purshiana)
Nootka rose (Rosa nutkana)
Salal (Gaultheria shallon)
Oceanspray (Holodiscus discolor)
Coyotebrush (Baccharis pilularis)

Make sure that the plants you purchase weren’t treated with systemic pesticides like neonicotinoids. Systemic pesticides can be expressed in pollen and nectar, making them toxic to bees. In addition, it is important to avoid spraying any blooming plants with pesticides or herbicides, as these can be harmful to bees and other pollinating insects like syrphid flies.

Learn more by exploring www.xerces.org/pollinator-conservation.

Major Bee Groups in the Pacific Northwest

Of course, when you’re trying to contribute to a conservation effort, it can help to know a little bit about what it is you’re trying to conserve. Here are profiles of some of the major bee groups in our region. Next time you’re out in your garden, see how many of these groups you can identify.

yellow-faced bumble bee on yellow flower

Yellow-faced bumble bee, Bombus vosnesenskii. (Photo by Kevin Cole/Wikimedia Commons)


Life History: Bumble bees are some of the most important pollinators. They are active from early spring, when queens first emerge and search for food to start their colony, until late fall. Bumble bees have the ability to buzz pollinate, or vibrate a flower at a frequency that causes pollen otherwise locked within a flower to explode onto the bee. Bees that can’t buzz pollinate are ineffective pollinators of plants that require it, such as tomatoes. Bumble bees are also generalists, meaning that they visit a number of different plant species instead of focusing on one or a few. They also have large ranges, enabling them to pollinate plants over great distances. Bumble bees are keystone species in many ecosystems—meaning that a lot of other species in these systems depend on them.

Description: Bumble bees are easy to identify, with their large, fuzzy bodies and iconic black and yellow banding. Some species of bumble bee have white or orange markings as well. Like honey bees, bumble bees carry moistened balls of pollen on their hind legs.

Common species in the Pacific Northwest: Black tail bumble bee, Bombus melanopygus; fuzzy-horned bumble bee, Bombus mixtus; yellow-faced bumble bee, Bombus vosnesenskii; California bumble bee, Bombus californica.

mason bee on brown leaf

Blue orchard bee, Osmia lignaria. (Photo by Robert Engelhardt)


Life History: Mason bees are named for the way they use mud as a building material to create cell divisions in their nests. Providing access to mud can help support populations of wild mason bees. Mason bees are solitary, and they make their nests in naturally occurring, above-ground cavities. They mate in spring, and then the female bees collects nectar and pollen provisions for their nests. Each female then finds a hole or tunnel in which to create a compartmentalized nest for her eggs. With the eggs fully provisioned, the female then plugs up the entrance. The larvae hatch, feed, hibernate through winter in cocoons, and emerge in spring to start the cycle afresh. Mason bees are very efficient pollinators and are active from spring through late summer. Commercial mason bees are sold for pollination purposes, but these bees can potentially harm wild, locally native mason bees (see “Downside to the Mason Bee Trade).”

Description: Mason bees often have exoskeletons with metallic coloration ranging from green to teal. They have a narrower body than leafcutter bees, though they also carry pollen under their abdomen.

Common species in the Pacific Northwest: blue orchard mason, Osmia lignaria; Osmia brevis; Osmia occidentalis; Osmia pusilla.

Downside to Mason Bee Trade

Keeping mason bees for pollination in gardens and farms is increasingly popular as they are readily available from suppliers. The trade in mason bees, however, is leading to the spread of species outside their native ranges, which can create competition with native species. Although some mason bees, such as the blue orchard mason bee (Osmia lignaria), have large ranges, populations of individual species have local adaptations that help them survive in specific areas. The Xerces Society for Invertebrate Conservation recommends first creating habitat that supports locally populations of wild bees by supplying lots of floral bloom throughout the year. If you do choose to buy mason bees, consider purchasing them from your area and never using species that are not locally native, including Osmia rufa which is native to Europe.

While more research into the possible negative effects of non-native mason bees is required, taking precautions will help minimize potential future problems that could arise. If creating artificial nests for mason bees, such as bee hotels (straws of holes drilled in wood), be aware that these types of bee houses can build up parasites over time. It is important to periodically sanitize or burn nests every three to five years to avoid disease and parasite build-up that might harm wild mason bee populations.

Sweat bee on orange flower.

Sweat bee, Halictus tripartitus ((c) John Ascher 2006/www.discoverlife.org)


Life History: Sweat bees got their name because some species collect salt from animal sweat, including from humans. Sweat bees are small and dark; they don’t have the typically black and yellow coloring we associate with bees. For these reasons, sweat bees are often confused with flies. Because they are so small, they pollinate by climbing deep into flowers and spending a lot of time drinking nectar and collecting pollen. They are quasi-social, with mothers and daughters living together but not splitting labor or suppressing one another’s reproduction (in short, there’s no queen). They are often one of the most common bee species in urban and agricultural environments.

Description: Sweat bees are usually black or dark metallic grey. Some species have pale hairy strips on their abdomen. They carry dry pollen on their hind legs.

Common species in the Pacific Northwest: small sweat bee, Halictus tripartitus; medium sweat bee, Halictus ligatus; tiny sweat bee, Lasioglossum species.

mining bee on green flowers

Snowy adrena, Andrena nivalis. ((c) Michael Veit 2010/www.discoverlife.org)


Life History: Mining bees excavate nests underground, hence the common name. They are one of the main blueberry pollinators besides bumble bees, often nesting at the base of blueberry plants. (One native species, Andrena astragali, is a specialist pollinator of the death camas—Zigadenus species—and has the unfortunate name death camas bee.) They also have been known to nest in lawns. Mining bees are solitary, with one female creating a few nests during her lifetime, which often lasts just four to eight weeks. Each nest contains approximately five cells, or chambers, in the dirt provisioned with enough pollen for a single bee larvae. The female bee lays one egg per cell, then caps the cell with mud; she never sees her offspring. After an egg hatches, the larva consumes all the pollen, then metamorphoses into an adult bee. It spends the rest of the year and winter in diapause (a period of suspended development) underground, waiting for the right seasonal cues (temperature and moisture), which let it know that it’s time to emerge to pollinate and reproduce.

Description: Mining bees often have fuzzy orange or blond thoraxes (mid sections) and dark abdomens with light stripes. They carry their pollen dry on their hind legs, which makes them look like they are wearing chaps.

Common species in the Pacific Northwest: Death camas bee, Andrena astragali; snowy adrena, Andrena nivalis; Andrena evoluta.

leaf cutter bee on orange flower

Male western leafcutter bee, Megachile perihirta ((c) Celeste Ets-Hokin 2013/www.discoverlife.org)


Life History: Leafcutter bees are aptly named: They cut sections of leaves or petals to create cell divisions within their nests. Leafcutter bees have large mandibles (oral appendages used for cutting) and wide heads to help with their task of snipping pieces of plant. Leafcutter bees nest above-ground in holes in wood. Because of the tight nesting space, instead of carrying pollen on their legs, they carry it under their abdomen. Another name for leafcutter bees is “hairy-belly bees,” which refers to the specialized hairs, or “scopa,” on their underside. When they forage on flowers, leafcutter bees often lift their abdomen up, preventing it from wiping the pollen away on the flower. This posture distinguishes them from other bee species. Though leafcutters can cause superficial damage to some plants in the garden, they more than make up for it by being productive summer pollinators. (If they are targeting a prized rose or other plant, you can protect it temporarily with row cover cloth until the bees find something new to take pieces from.)

Description: Leafcutter bees are often more squat-looking than other bee species. They carry their pollen under their abdomens, which are usually black, with pale bands of hair.

Common species in the Pacific Northwest: western leafcutter bee, Megachile perihirta; silver-tailed petal-cutter beer, Megachile montivaga.

cuckoo bee on orange flower

Concave cuckoo bee, Triepeolus concavus. (Photo by Kathy Keatley Garvey/UC Davis Bug Squad Blog)


Life History: Cuckoo bees get their name from cuckoo birds, because they also are parasitic: Instead of creating their own nests, cuckoo bees lay eggs in the nests of other bees! When cuckoo bee larvae hatch from their eggs, they are able to move, and then search out and destroy the larvae of their host. Because cuckoo bees don’t need to collect pollen to provision nests for their young, they are often less hairy in appearance. Cuckoo bees are intimately connected to their host species, so that if host populations decline, cuckoo bees will likely also suffer.

Description: Cuckoo bees are often red or yellow, and sometimes have white markings. They are often nearly hairless, and so can have a wasp-like appearance.

Common species in the Pacific Northwest: concave cuckoo bee, Triepeolus concavus.

Hillary Sardiñas is the former Pacific Coast Pollinator Specialist for the Xerces Society for Invertebrate Conservation. She now works as a biologist for the Alameda County Resource Conservation District. For questions about this article or the Xerces Pollinator Program, email pollinators@xerces.org.

Header photo: The once-common western bumble bee, Bombus occidentalis. (USDA Agricultural Research Service)

The Xerces Society for Invertebrate Conservation

Robert Michael Pyle started the Xerces Society in 1971 to raise awareness about invertebrate conservation. Named after the extinct Xerces Blue butterfly, which used to live on the sand dunes of San Francisco, the Xerces Society continues to educate and advocate for critically important but often unseen species.

On-Farm Conservation: The Xerces Society’s Pollinator Program has worked with farmers across the US to create on-farm, flower-rich habitats. It has supported the creation of more than 250,000 acres of pollinator meadows and hedgerows. These areas not only support crop pollinators, but also provide habitat to natural enemies of crop pests. The Pollinator Program has offered trainings, field days and short courses to farmers in all 50 states. On-farm habitat benefits both pollinators and farmers, who see increased crop pollination and yields.

Endangered Species Conservation: The Xerces Society also works to monitor populations of imperiled pollinators. Project Bumble Bee combines science and advocacy to engage citizens, landowners and government agencies in the protection of declining bumble bee species. You too can get involved in efforts to track bumble bee species by joining Bumble Bee Watch, a citizen science initiative that helps researchers track species remotely. You can upload photos of bumble bees that will be identified by an expert at www.bumblebeewatch.org.