Summer of the bumblebee

Posted: Sunday, August 23, 2009

It was a good summer for bumblebees, so hikers often saw them visiting alpine and trailside flowers. The frequent sightings stirred some conversation among hikers who also garden, and therefore want bees to visit their peas and beans.

This led to a wish to know more about bees. Whole books have been written about bees, so what can be told here is therefore a tiny part of the available information - it serves only as an introduction to a big topic.

Over much of North America, the most common bee is probably the European honeybee, Apis mellifera. This species was introduced to North America by European settlers early in U. S. history. It is the common hive bee that beekeepers tend, sometimes moving the hives around the countryside to pollinate certain agricultural crops.

Honeybees have changed the pollination ecology of many native flowers because their foraging patterns are different from those of native bees, and in some cases the introduced species may out-compete the native species for nectar supplies.

Honeybees are highly social. In contrast to the many species of solitary bees, in which females do not tend to their young after laying eggs and provisioning a tiny nest, queen honeybees produce numerous worker bees (sterile females) that raise thousands of other workers throughout the summer.

Workers go out foraging for nectar (carried back to the nest internally) or pollen (carried in pollen baskets on the hind legs), and they learn the locations of the richest food sources.

They can communicate this information to each other by performing certain "dance" movements on the hive when they return from a foraging trip. Workers feed the larvae on pollen and "bee milk" (a combination of nectar and glandular secretions from the workers). They also maintain a warm hive temperature by vibrating their flight muscles. Adult honeybees often feed and groom each other.

New colonies form, usually late in the season, when worker bees have raised new queens, who lead a swarm of workers away from the old hive and start a new one. Honeybees have become naturalized in much of the country, establishing colonies in hollow trees, stumps and sometimes attics.

Whole colonies of queen and workers overwinter communally, clustering together and warming the cluster by muscle vibration. The colder the outside temperature, the tighter the cluster. They can keep the temperature in the cluster as much as 100 degrees above ambient temperature.

Bumblebees are larger than honeybees, and are typically covered with dense "fur." Most species are black and yellow, but some have rusty red bands on the abdomen, and a few species have white patches. (My favorite, the giant bumblebee of temperate zones in South America, is a spectacular rusty red all over.)

Bumblebees are also social bees, but their colonies are much smaller than those of honeybees, usually only a few dozen or perhaps a few hundred bees.

Queen bumblebees hibernate solitarily in winter, having mated in the fall. Each queen makes a small nest, just a couple of inches in diameter, using shredded plant fibers, commonly in old mouse nests or abandoned bird nests.

Nests are often underground, but sometimes they are on the ground surface under closely overhanging vegetation, or in logs or cabin walls. The queen initially raises a small brood of workers who forage to feed the next and larger broods. Late in the season, a number of new queens and male bees (drones) are raised.

These mate when mature, and the fertilized queens search for hibernation sites. The drones and workers die, and thus each colony usually lasts only one season.

Adult bumblebees feed on nectar, which can be stored in tiny honey pots inside the nest, but the larvae are fed mostly on pollen. Worker bees all forage independently of each other. They do not communicate with each other about good food sources. Adults seldom feed each other. They may or may not return to the colony at night, and sometimes they sleep in flowers on their foraging routes.

Bumblebees are the principal pollinators of many native flowers including lupine, monkshood, blueberries, beach peas and some gentians, but they visit many other flowers, too. If you watch closely when a bumblebee is crawling around on an open flower, you can easily see clumps of pollen attached to the pollen baskets on the hind leg, ready to be taken back to the nest.

There are several hundred species of bumblebees in the world, widespread over the northern hemisphere but not native to Australia or most of Africa. Roughly 50 species may be found in North America, according to one report, and about 30 are known from Alaska. Bees are poorly studied in Southeast Alaska, and a species count for our area is not available.

•Mary F. Willson is a retired professor of ecology.

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