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| 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.
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| Female Yellow-faced Bumble Bee (Bombus vosnesenskii) - front view on Yarrow (Achillea millefolia) |
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| 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
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| Female Yellow-faced Bumble Bee in early spring |
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/
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| 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.
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| Yellow-faced Bumble Bee (Bombus vosnesenskii) foraging on Dune Lupine (Lupinus chamissonis) Madrona Marsh Preserve, Torrance CA |
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:
Conserving Bumble Bees: Guidelines for Creating and
Managing Habitat for America’s Declining Pollinators.
More extensive resources on Bumble
Bees can be found at:
Moving a
hive - http://homeguides.sfgate.com/rid-bumblebees-grass-87348.html
_________________________________________
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
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