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U.S. Forest Service

Bombus sonorus, the Sonoran Bumblebee

By Stephen Buchmann, Pollinator Partnership

Distribution of Bombus sonorus. Distribution of Bombus sonorus. Courtesy Discover Life.

Sonoran bumblebee, Bombus sonorus. Sonoran bumblebee Bombus sonorus. Photo copyright John Ascher, 2006-2010, Discover Life.

Sonoran bumblebee, Bombus sonorus. Sonoran bumblebee Bombus sonorus. Photo copyright John Ascher, 2006-2010, Discover Life.

Sonoran bumblebee being eaten by a green lynx spider. Sonoran bumblebee being eaten by a green lynx spider (Peucetia viridans). Photo copyright John Ascher, 2006-2010, Discover Life.

The Sonoran bumblebee, Bombus sonorus, is a large and colorful native bee of the Sonoran desert and much of the western United States. Workers are usually 14 to 18 millimeters long and queens are even larger. Queen bumblebees and carpenter bees are the largest native bees in the United States. This species is placed in the subgenus Thoracobombus and was named in 1837 by Thomas Say. It is one of our most common and widespread bumblebees. Some scientists believe that B. sonorus is a subspecies of a wider-ranging bee, Bombus pennsylvanicus. For this essay, I consider B. sonorus a distinct species, but one confined to the western states. We need molecular taxonomic research such as bee barcoding to resolve the species identity matter. From the photos, you will notice that this bumblebee has an all black head with the front and back of the thorax yellow, but with a broad black transverse band between the wings. The abdomen is all yellow except for the hind three segments, which are black. In young individuals, the yellow body hairs can be a brilliant yellow. This bumblebee visits a very broad range of flowering plants including sunflowers, thistles, clover, Black-eyed Susans, and many others. Bombus sonorus was also featured visiting and buzzing a deadly nightshade blossom on the beautiful United States postage stamps developed and distributed during 2007.

On the accompanying map from Discover Life, small yellow dots mark localities from which specimens in various museums were collected in past years. Scientists who study bee taxonomy and ecology call such maps dot distribution maps. They quickly and easily give an excellent impression of the total geographic range occupied by a species of animal or plant.

In the fall months, dozens of virgin queens and are produced within healthy and successful B. sonorus colonies. The males are easily distinguished from workers and queens by their slender non pollen-carrying hind legs, or their gracile long antennae. The males patrol territories and mate with virgin females, then die in a few weeks. The mated queens store spermatozoa inside a special sac, the spematheca, inside their bodies. These cells fertilize all of the female-destined eggs in the colony that will be founded in the coming year. Did you know that in all ant, bee and wasp species, comprising the insect order Hymenoptera, nesting females choose the sex of their offspring? They control the release of sperm cells to eggs. Hymenopteran eggs that are fertilized become females with a normal 2n, or diploid, condition. This means each cell has two sets of chromosomes. Unfertilized eggs become males that are haploid, with only one set of chromosomes.

Newly mated queens disperse and locate in underground mouse nests or other suitable chambers to spend the winter months in a kind of insect hibernation known as diapause. In March or April, depending upon the location and its elevation, queens forager for nectar and make honey. They store the honey inside a wax pot inside the young nest. They begin to lay eggs in a cluster and may even “brood” the eggs and young larvae, heating them exactly as a mother bird would do for her chicks. Worker bees in the first brood are tiny but soon take over all the tasks within the nest and outside as foragers. As the population grows larger, more brood cells are added along with pollen and honey storage pots. Pollen and honey are stored separately and nurse bees bring these foods to the larvae until they are ready to pupate. Bumblebees spin a silk cocoon inside their waxen brood cells. At first glance, bumblebee nests appear chaotic and unorganized. Instead of back-to-back neat hexagonal cells like those fashioned of wax by honey bees, bumble bees create lumpy cells for pollen and nectar storage and egg-shaped ones in which larvae develop.

One amazing behavior that bumblebees perform is to “buzz” or sonicate certain flowers. About 8% of the world’s flowering plants have anthers that look like a saltshaker with two holes. The salt grains are the pollen grains. A bumblebee female bites into an anther and curls her body around, and then buzzes like mad. She is using her flight muscles exactly like a tuning fork. The pollen blasts out the pores and the female collects it for use as larval food. Bumblebees are excellent buzz pollinators of familiar crop plants including blueberry, cranberry, eggplant and tomato. The modern hydroponic tomato industry depends upon these bees to produce a crop. Honeybees do not perform this specialized floral buzzing.

Over the last 10 - 20 years, scientists and naturalists have noticed dramatic population declines and range constrictions among five of the 47 species of bumblebees found in the US (B. affinis, B. franklini, B. occidentalis, B. pensylvanicus and B. terricola). Some of these declines may be due to introduced microbial pathogens from Europe. We encourage you to join one of the citizen science efforts and become a bumblebee watcher in your area. Several of these monitoring efforts are listed at the end of this article.

Although bumblebees are large and colorful insects that fly slowly and noisily between flowers, they can be confusing to identify whether you are a backyard gardener or an entomologist. Fortunately, there are some excellent guides now available with keys and photographs to help you identify just which bumble bee species you saw on that wild rose bush. A printed and online version of a guide co-produced by the USDA Forest Service and the Pollinator Partnership is available on the Forest Service and Pollinator Partnership websites as a PDF booklet. Excellent multi-entry illustrated keys to bumble bees are on the Discover Life website, along with excellent photos our Bombus fauna.

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