Farmers, gardeners and many people with allergies know and loathe the giant ragweed. The plant can be found in every U.S. state but Hawaii, Alaska and Nevada, commonly tops 15 feet of height, and is responsible for a great deal of the nation's hay fever each fall. The plant is a bane to farmers in many areas, frequenting crop fields.
Yet the supposedly stubborn weed bears the classic characteristics of a poor seeding success rate. The seeds are large, not produced in large numbers by the plant, and face high losses due to mice and other granivores. So how does the weed manage so well? Perhaps, say Ohio State University weed scientists, thanks to an unlikely ally: the common nightcrawler, an earthworm species brought from Europe by settlers.
During field experiments recording how long ragweed seeds took to become buried in the soil, thus escaping granivores, Emilie Regnier noticed the seeds moving overnight, ending up around - and inside of - nightcrawler burrows. By burying seeds, the earthworms in effect save them from granivores on the surface.
"This relationship may explain why giant ragweed is able to establish itself, especially in no-tillage crop fields and undisturbed areas where granivores are plentiful and there are few means by which a large seed can become buried on its own," says Regnier.
Nightcrawlers regularly pull organic matter into their burrows, letting it decompose into digestible matter, but Regnier and colleague Kent Harrison, scientists at the Ohio Agricultural Research and Development Center, don't know for sure why the worms choose ragweed seeds. The worms may recognize the seed's hull as a food source, or may use the large seeds to help strengthen the structure of their burrows. The question of a co-evolution, in which earthworms take nutrients from the outer layers of the ragweed seed while the seed sprouts, is a sticky one.
"The association in the midwestern United States is all the more intriguing because nightcrawlers are an exotic earthworm species brought by settlers from Europe, and giant ragweed is a native plant, so the association of the two species is relatively new on the evolutionary scale," Regnier says.