Fight of the bumblebee: insects, like people, are constantly threatened by disease. Bumblebees' simple but effective immune systems shed light on the evolution of immune defenses and the costs of maintaining them.

Publication: Natural History

Publication Date: 01-NOV-03

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COPYRIGHT 2003 Natural History Magazine, Inc.

Humming from flower to flower, a bumblebee worker busily collecting nectar and pollen for its colony is, for many, the epitome of nature's peace and tranquillity. Yet nothing could be further from the truth. Not only is the foraging bumblebee always on the verge of an energy crisis; it is also entangled in a lifelong battle with microscopic enemies that try to capitalize on its efforts.

All complex organisms, people included, face essentially the same predicament. Coping with actual disease, of course, makes prodigious demands on one's energy: taking to bed is often the only possible solution. Yet even as we go about the business of ordinary living--working, crowding together in close quarters, caring for children, shopping at a local market--keeping disease at bay takes a constant toll on the body's resources. Ironically, the insects that carry some of the disease organisms against which people must be most on guard, including malaria, dengue, West Nile virus, and leishmaniasis, are themselves locked in equally desperate battles with similar, if not identical, parasites.

Because of their importance as pollinators of fruit crops and flowers, bees have been a focus in the study of disease and disease resistance in "lower" organisms. The most prevalent disease in bumblebees is caused by the trypanosome Crithidia bombi--a mobile protozoan closely related to the microorganism that causes human sleeping sickness. C. bombi cells are left behind when an infected bee visits a flower, and those cells can survive for a day or two at the bottom of flower tubes. When the next bee visits the flower, the infectious cells are picked up and carried back to the nest, where a few dozen other workers and a queen are put at risk. The disease often spreads rapidly through the colony and then, via more flower visits, to other colonies in the population. By June almost all bumblebee colonies in a population have become infected by C. bombi, though a large fraction of workers within each colony do survive the infection.

Another health hazard of collecting nectar and pollen in flowering meadows is that workers are forced to fly slowly when they maneuver around flower stalks. Slower flying speeds invite attacks by female parasitic flies of the family Conopidae. The conopids inject their eggs into the abdomens of foraging worker bees. There the eggs hatch, and the parasite larvae develop reside each bee, rapidly consuming their host from the inside out. Between ten and twelve days later the...

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