Most Bumble bees are true natives of our soils and have coevolved with our native plants.
By Fotolia/Tomas Sereda
Bees are one of the most important insects to us. Not only are they great garden pollinators, they maintain biological balance and recycle soil nutrients. Learn all about bees — from bumble bee buzz pollination to what is threatening their population in Bees, Wasps, and Ants (Timber Press, 2010) by Eric Grissell. The following excerpt was taken from chapter 8, "The Garden’s Pollinators: Bees."
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We now move into the world of the truly social (that is, eusocial) bees. Worldwide there are more than 200 species of bumble bees (Bombus; Figure 128), with about 50 species in North America. These most often appear as colorful combinations of flying black-and-yellow fur coats, but there are a few black-and-orange or just plain black species as well. Bumble bees certainly receive much less press than honey bees, but unlike their cousins, most are true natives of our soils and have coevolved with our native plants. Bumble bees differ from honey bees in several respects. For one thing, neither bumble bee colonies nor the old queen survive the winter. In late summer or autumn a number of reproductives are produced, including males and new queens. Mating takes place outside the nest, either on the ground or in the air. Males die and new queens either return to the old abandoned nest or seek out an underground hibernation site in which to overwinter. The following spring each surviving female will become the founder of her own colony.
Bumble bees nest in a variety of situations, either above or below ground depending on the species. They are opportunists to a degree, seeking out hollow areas in which to build their relatively small nests. Abandoned rodent burrows are a preferred site, but I have seen nests in unused bird houses and have been told they use old mattresses and upholstered chairs lying in the dump. The queen begins her nest in the spring by constructing a hollow wax basin in which she places pollen and another in which she places nectar she has collected. When enough pollen has accumulated, she lays one to two dozen fertilized eggs and caps the basin with more wax. The collected nectar is used by the queen as fuel during this process and eventually for workers on their outbound flights. Then she sits astride the egg chamber, warming it with contractions from her wing muscles. Heating speeds egg hatch, which takes place in a few days. After the eggs hatch the queen takes off and gathers more pollen and nectar supplies, during which the larvae take one to two weeks to develop into adults. These then become the initial group of workers that create more cells, collect pollen and nectar, feed new offspring, and guard the nest while the queen continues to lay eggs. Workers perform functions as needed and may live from a couple of weeks to several months. Bumble bee colonies in temperate climates may reach a hundred or so individuals per nest. Heading into autumn the queen begins to lay unfertilized eggs, which develop into males, and workers supply more food to some of the larvae, which develop into queens. Reproductives leave the nest, mate, the males die, and the newly mated queens overwinter to begin a new colony the following spring. The old queen and her offspring perish.
Bumble bees are great pollinators of native plants, providing pollination to different varieties of plants than do honey bees, which are European natives. Because bumble bees are generally more active in cooler weather and lower light levels than honey bees, they are also adapted to service different sorts of flowers. Among edible plants, they are much better at pollinating tomatoes, peppers, squash, cucumbers, eggplants, and blueberries because these plants require buzz pollination (sonication) in which the bee dislodges pollen by vibrating the flower. Bumble bees are commercially available and managed as are honey bees, but they are more adapted for use in greenhouses to pollinate crops such as strawberries and tomatoes. Bombus impatiens is the only commercially significant native North American species, with more than 50,000 colonies sold per year (North American Pollinator Protection Campaign 2006). Attempts to introduce a European bumble bee, Bombus terrestris, into other regions of the world is meeting with controversy and opposition. This species has been introduced into Japan but now is declared an “invasive alien species” because of its negative effect on native bumble bees. In Australia, a continent with no native bumble bees, its introduction has been outlawed due to the fear that it may outcompete native bees (Aussie Bee, website).
About half a dozen bumble bee species resort to social (or brood) parasitism of other bumble bees. These are called cuckoo bumble bees and are often placed in their own genus (Psithyrus). Females of these species have no apparatus for collecting pollen, they cannot secrete wax, and they cannot create worker females. As a result they are unable to build and provision their own nests. To surmount these obstacles, they are well armored with thick skins and an excellent stinger, and they are sneaky. A cuckoo female invades the nest of an unsuspecting bumble bee species, and, if she is not killed outright, eventually assumes the odor of her new nest mates. Once accepted into the chemical fraternity that is a bee nest, her duty now is either to kill the rightful queen and control the workers or to slowly supplant the old queen until she fades out of the picture. The new queen will lay eggs of her own that will then be cared for by the workers of the former queen, and a new crop of social parasites will be born to strike yet another colony. This system works, in the end, because there are roughly eight times more social species to be usurped than cuckoo species to usurp them.
Now for some more bad news about bees. Bumble bees, like honey bees, are suffering declining numbers. Beginning in the 1990s researchers in the United States began to notice a decline in three common species of bumble bees. Their decline is likely due to a strain of fungus (Nosema bombi) introduced into the United States from Europe. The decline is occurring elsewhere as well. In Great Britain there are between 15 and 20 species of bumble bee. According to a report by BBC News (February 27, 2008), at least three of these have become extinct and another eight are in decline. To understand this decline, some individual bees are being outfitted with tiny radio frequency identification tags, which can be read by an electronic device placed at the colony entrance. By studying the comings and goings of individual bumble bees, scientists hope to learn more about their lives, not the least of which is why they are disappearing.
For those interested in bumble bee identification there is an interactive pictorial guide to species at the Discover Life. And for those without computers, there is an inexpensive little book entitled The Natural History of Bumblebees by Kearns and Thomson (2001) that features a photographic field guide to North American species. British species may be identified using the guides by Prys-Jones and Corbet (1991) and Edwards and Jenner (2009).
What's the Buzz? Learn more about bee families in All About Bees: The Great Garden Pollinators.
Reprinted with permission from Bees, Wasps, and Ants by Eric Grissell and published by Timber Press, Inc, 2010. Buy this book from our store: Bees, Wasps, and Ants.