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Why do pillbugs turn into zombies?

By Chelsie Vandaveer

October 25, 2004

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The pillbug (Armadillidium vulgare) is an isopod native to Europe. It arrived in North America, an accidental hitchhiker in straw or hay and other animal feed. Pillbugs are common in gardens; they are innocuous creatures living in colonies under logs or rocks, hiding by day, and venturing forth to forage on detritus at night or during cloudy humid weather. (See Renfield's Garden, July 14, 2004)

The starling (Sturnus vulgaris) is also native to Europe. It arrived intentionally; an attempt to have each bird species mentioned in Shakespeare's plays present in North America. Starlings proliferated. During migration, the vast flocks overwhelm gardens, farms, and entire towns. Starlings are an ecological disaster to native birds seizing nesting sites and competing for food resources. (See Renfield's Garden, September 15, 2004)

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Pillbugs and starlings have little in common, except for an unseen player. In 1981, Dr. Janice Moore discovered that pillbugs and starlings have a common parasite, an acanthocephalan or "thorny-headed" worm.

The acanthocephalan (Plagiorhyncus cylindraceus) alternates its host during its lifetime. During its mature stage, the parasite lives in the intestinal tract of the starling and is avirulent, apparently doing little harm to the bird. The thorny-headed worm attaches to the intestinal wall and lays eggs which are shed in the starling's feces. Pillbugs, as recyclers, eat bird feces as well as other garbage.

Once inside the gut of a pillbug, the egg hatches. The parasite commandeers the pillbug's body—an infected pillbug becomes food, shelter, and transportation. Although the isopod may grow to a mature size, it never matures. This is most evident in female pillbugs; they never develop ovaries. Most of the pillbug's energy is drained for the growing parasite.

As soon as the acanthocephalan is ready for its alternate host, the behavior of the pillbug changes. The little isopod gets reckless. When other pillbugs are resting in their dark, humid colony, the infected pillbug is out wandering. Indifferent to moist dark shelter, the 'zombified' pillbug heads out into the heat and dryness of sunlight carelessly crossing over light-colored surfaces where it is easily spotted by hungry starlings.

"Parasites and Parasitological Resources" of the College of Biological Sciences of Ohio State University has great micrographs of an acanthocephalan. To learn more about these thorny-headed worms and to view the photographs, click on the link:

http://www.biosci.ohio-state.edu/~parasite/acanthocephala.html

(Compiled from: "Sowbugs Revisited", Louise Kulzer, Bug of the Month: February 1996; "Parasite manipulation of hosts", Dr. Ben Bolker, Ecology and Evolution of Parasites, Pathogens, and Disease, Department of Zoology, University of Florida, 2002; "Evolution of Virulence", Coevolution 5401 Lecture Notes, Dr. Timothy Craig, Biology, University of Minnesota, Duluth, 2003)

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