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Thursday, June 19, 2014

Yellow-faced Bumble Bee (Bombus vosnesenskii)

Yellow-faced Bumble Bee (Bombus vosnesenskii) approaches
Purple Sage (Salvia leucophylla)

The western United States is home to many unique and useful insects. Regional gardeners are starting to appreciate the importance of native pollinators, in part due to the activities of National Pollinator Week (the 3rd week in June).  To learn more about pollinators in general see our June 2013 posting (http://mother-natures-backyard.blogspot.com/2013/06/life-friendly-gardening-planning-for.html).

The larger insect pollinators are among the more interesting visitors to S. California gardens.  One of our favorites is the Yellow-faced Bumble Bee (Bombus vosnesenskii).   This large native bee can be seen from spring to fall in west coast gardens from British Columbia, Canada to Baja California, Mexico.   In our garden, we most commonly see them from June through August.

Bumble Bees are true bees belonging to the genus Bombus. There are around 250 species worldwide, with ~40 native to the Western United States.  Bombus species are notable for their large size, hairy bodies and ability to fly in cold, damp weather.  They are generalist pollinators (visit a number of plant species) and live in colonies with one or a few breeding Queens.   They collect both pollen and nectar and are equipped with pollen baskets on their hind legs.

Female Yellow-faced Bumble Bee (Bombus vosnesenskii) - front view 
on Yarrow (Achillea millefolia)

Female Yellow-faced Bumble Bee (Bombus vosnesenskii) - rear view 
on Yarrow (Achillea millefolia)


Yellow-faced Bumble Bees are large – ½ to ¾ inch (1.5 to 2 cm.) long - and their bodies and wings are mostly black.  They have a fuzzy yellow head, including a yellow ‘head pile’ on the thorax just behind the actual head (see above) and a single yellow stripe on lower abdomen (males have a partial second stripe).  You can see more pictures at: http://www.discoverlife.org/20/q?search=bombus+vosnesenskii.  

While Bombus vosnesenskii is the most common bumble bee in local gardens, it can be confused with other native Bombus species including B. caliginosus, B. californicus,  B. occidentalis and B. vandykei.  The California Bumblebee (B. californicus) is far less common and has a black face.  The others are more likely to be found in the local mountains (B. vandykei) or on the central or northern coast.   For a complete guide to western U.S. Bumble Bees see: http://www.xerces.org/wp-content/uploads/2008/09/Western_BB_guide.pdf
Female Yellow-faced Bumble Bee in early spring
 Like all bumble bees, Bombus vosnesenskii colonies contain a breeding female (the queen), non-breeding female workers and some breeding males.  The queen is the largest; she may be up to ¾ inch long while workers and males are usually about ½ inch.  Males and females look slightly different. Males have an additional yellow stripe on their abdomen.  They have longer antennae and no pollen baskets.

More importantly, female queens and workers have a stinger, while males do not.  In general, bumble bees are not very aggressive; they aren’t pesky like wasps and they do not swarm.  However, they can sting multiple times if annoyed, most commonly when their nest is disturbed.  Bumble Bees have smooth stingers which do not detach; this allows them to survive after stinging. 

If a Bumble Bee is circling you, it most likely is just curious.  Stand still or move slowly and don’t wave your arms wildly; it will usually soon lose interest. If you are allergic to bee stings (about 0.5% of children; 3% of adults), it’s always prudent to be prepared when around bees (have an EpiPen available).    Learn more about avoiding bee stings at: https://pollinator.org/PDFs/NAPPC.NoFear.brochFINAL.pdf   

The Yellow-faced Bumble Bee follows a predictable life cycle.  Fertilized queens are the only individuals that overwinter.  They emerge in early spring in coastal S. California (as early as February in a warm year) or after the snow melts in colder climates.  Newly emerged queens can sometimes be seen visiting early-blooming flowers and flying near the ground in search of nest sites.

Yellow-faced Bumble Bees nest in the ground, but they are not excavators.  In natural areas, they usually choose old rodent burrows; in fact, the number of bumble bees correlates with the availability of such burrows.  In gardens, they may also nest under brush or compost piles, in wood piles or in loose leaf litter. If provided, they may even nest in a constructed nest box: see http://www.bumblebee.org/nestbox_plans.htm and http://www.xerces.org/wp-content/uploads/2008/11/nests_for_native_bees_fact_sheet_xerces_society.pdf

After choosing a nest site, the queen collects nectar and pollen to create a food lump (pollen ball) in the nest.  The pollen ball, which is commonly about 1 inch in diameter, provides food for the developing offspring.  She lays her eggs on the food lump, then incubates them for three to four weeks until the first workers emerge.

Warm temperatures are critical for proper development.  The queen lays her abdomen over the eggs/larvae to keep them warm.  She vibrates her thoracic muscles to generate heat. During this period, the queen only emerges if she herself needs food.  The eggs develop into larvae which feed on the food lump.    The larvae transform into pupa and finally emerge as adult female workers.  These workers help to provision and care for subsequent summer offspring.

The queen continues to lay eggs through mid-summer; she is the only individual who can lay eggs.  Her workers can be seen visiting flowers up to 2800 m (about 1.7 miles) from the nest, collecting nectar and pollen for the colony.   Some workers remain in the nest to tend the young, clean the nest and maintain nest temperature.  If the nest gets too hot, they cool it by fanning their wings near the entrance hole. 

The last eggs of summer give rise to new queens (from fertilized eggs) and male drones (from unfertilized eggs).  These adults emerge in late summer and don’t re-enter the nest. You can sometimes see drones ‘sleeping’ on flowers or foliage at this time of year. The males fertilize the new queens; then they and the workers die in late summer/fall.   A fertilized queen forages for a few weeks before digging a small cavity (the ‘hibernaculatum’) in which she remains dormant over the winter.   She emerges in spring to begin the cycle again.   To learn more about Bumble Bees we recommend: http://www.bumblebee.org/
Yellow-faced Bumble Bee (Bombus vosnesenskii) foraging on White Sage
 (Salvia apiana) in Mother Nature's Backyard

Bumble Bees like the Yellow-faced Bumble Bee are very efficient pollinators.   First, they have the ability to warm their flight muscles at cool ambient temperatures.  They transfer heat from the abdomen to the thorax (mid-section) which contains the legs.  This allows them to fly in cool, wet conditions that ground other pollinators.

Bumble Bees are generalist pollinators, visiting a wide range of flowers in their search for nectar and pollen.  Many interesting facts are just emerging about Bumble Bee pollination.  For example, the foraging pattern of Bumble Bees (crawling around in the flowers) promotes self-pollination within a single plant.  In addition, Yellow-faced Bumble Bees tend to visit a limited number of species on a single foraging run.  Why certain flowers are selected is not yet clear; it may have to do with characteristics of the nectar or pollen itself. But this pattern tends to maximize pollination between plants of the same species.
Yellow-faced Bumble Bee (Bombus vosnesenskii) foraging on Dune Lupine (Lupinus chamissonis)
Madrona Marsh Preserve, Torrance CA
 Bumble Bee’s large size allows them fly greater distances and to pollinate flowers  inaccessible to smaller pollinators.  They can, for example, land on the ‘keel’ of a lupine flower, causing it to pop open for access to nectar and pollen (see above). This is really fun to watch!  The optimal strategy for accessing the ‘goods’ varies with flower type.  You can occasionally watch a Bomus improve her technique as she tries out different strategies – a learned behavior that is not passed from one bee to another.

The fact that Bumble Bees visit many flower species makes them useful alternate pollinators in the wilds, on farms and in home gardens.  Yellow-Faced and other Bumble Bees visit both wild and domesticated plants.  They are known to be particularly good pollinators of tomatoes and the squash family (Cucurbitaceae).   Among the crop plants pollinated by bumblebees are: Prunus species (plums, cherries), citrus, apples, currants/gooseberries, blackberries/raspberries, peaches, sunflowers, beans, peppers, tomatoes, squash, cucumbers, pumpkins and watermelon. 

There is currently great interest in the agricultural use of Bumble Bees as alternate pollinators.  In fact, their commercial use is already raising ecological dilemmas.  In Europe, North and South America, where native pollinators are experiencing serious declines, Bumble Bees are increasingly employed as pollinators.  In general, commercially raised Bumble Bees are limited to a few species raised in a small number of hatcheries.  Scientists in Europe and the U.S. are calling for increased regulation of commercial Bumble Bees - and for good reasons (see http://ecowatch.com/2013/10/29/demand-protection-wild-bumblebees/).

Among the serious problems associated with commercially-raised Bumble Bees are 1) the spread of bee diseases and parasites; 2) the decline of native populations and species due to competition with introduced species.     Examples of both problems have already been documented in Europe and the Americas.   A more prudent strategy is to promote the survival of native species. 

Bumble Bees are declining world-wide.  Overall, there has been a decline in both  numbers and species since the early 1990’s; some species are even thought to now be extinct. This has prompted several organizations to take an active role in promoting these important pollinators.  To learn more see the following:


Yellow-faced Bumble Bees are among the species whose numbers and range are stable or possibly even increasing – at least for now.  The reasons for this are unclear and likely to be complex.  One thing that is certain is that Bombus vosnesenskii can live in urbanized areas, even in S. California.   This makes our gardens even more important as pollinator habitat havens.

There are a few concrete things you can do to promote native Bumble Bees.
1.       Plant their favored plants.  While Bumble Bees are generalists, they do favor some plants over others.  Studies have shown that Yellow-faced Bumble Bees occur most often in areas with a preponderance of native plants.  But they are nearly as common in places that contain a combination of native and certain non-native plants. When choosing non-native plants, choose old-fashioned, ‘open pollinated’ or ‘heirloom’ varieties if possible.    And aim to have flowers blooming throughout the Bumble Bee season, from spring through early fall.

Here are some plants to consider planting:

Trees/large shrubs: Ceanothus species, Cercis orbiculatus (Western redbud), Chilopsis linearis (Desert willow), Heteromeles arbutifolia (Toyon), Prunus species, and native/non-native Senna/Cassia

Smaller shrubs/perennials: Achillea millefolia (Yarrow), Agastache species (Giant hyssop), Asclepias (native Milkweeds), Symphyotrichum chilense var. chilense/Aster chilensis (Coastal aster), Ericameria species, native Lonicera (Honeysuckles), native Lupines (Lupinus, including annual and bush forms), Monardella, Penstemon heterophyllus, Mountain Mints (Pycnanthemum), Rosa californica and R. woodsii, Salvias (native sages).  Among the non-native plants that attract Bumble Bees are: Gentians (genus Gentiana), Borage, Lavender, Bergamont/Lemon Mint/Bee Balm (Monarda species), Rubus species (Blackberry, Boysenberry, Raspberry)

Annual wildflowers: Cirsium occidentale (Cobwebby thistle), Clarkias & Collinsia species, Eschscholtzia californica (California poppy), Gilia capitata (Globe gilia), Mentzelia species, Phacelia tanacetifolia (Tansy phacelia) & other Phacelias.


2.  Use pesticides sparingly – or not at all.  Bees – including Bumble Bees – are very sensitive to certain pesticides. A relatively new class of insecticides, the neonicotinoids, have been implicated in several large Bumble Bee die-off events in the U.S.  The neonicotinoids, which are chemically similar to nicotine, act on an insect’s nervous system.  They are effective insecticides, but don’t discriminate between harmful and beneficial insects. Pesticides that contain neonicotinoids include Bayer Advanced Garden Insecticides, Dinotefuran (Safari) and any insecticide that contains the following: Acetamiprid, Clothianidin, Dinotefuran, Imidacloprid, Nitenpyram, Thiocloprid, Thiamethoxam.  For more see: http://www.xerces.org/wings-magazine/neonicotinoids-in-your-garden/  

Practicing Integrated Pest Management (IPM) is a safer, greener approach to managing garden pests.  Keep plants healthy, use simple preventive measures and use chemical pesticides only as a last resort.   To protect pollinators, never apply pesticides to blooming plants.  For more see: http://www.xerces.org/pesticides/


3.   Leave space for ground-dweller’s nests.  Ground-dwelling pollinators need bare ground, brush piles and areas covered by loose leaf litter.  They cannot nest under a thick layer of mulch.  Consider leaving the ground around mature shrubs un-mulched.  Tuck garden trimmings beneath/behind shrubs where they can decompose naturally while providing nest spots for beneficial insects.


4.    Provide nesting materials.  Bumble Bees need loose, dry material for their nest.  An easy way to provide is this to plant native bunch grasses or ornamental grasses. Leave some of the old, dry leaves each year for birds and insects to use as nesting materials.  


Learn more about how you can help promote native Bumble Bees at:


More extensive resources on Bumble Bees can be found at:



We encourage your comments below.   If you have questions about Yellow-faced Bumble Bees or other gardening topics you can e-mail us at :  mothernaturesbackyard10@gmail.com


  1. Really nice post - lots of info and again, thanks for your support of California native plants!

  2. Great information and well-presented. Please do stress in your article the importance of boycotting all pesticides that are currently responsible for the devastating loss of bee populations. Without these humble warriors, our human food cycle itself will be threatened.