Chewing Leaves to Fight Fleas

Wild rats may have figured out a way to "flea-bomb" their homes. The dusky-footed wood rat, which builds stick houses sometimes taller than 2 meters, likes to place nibbled leaves from California bay trees around its nest. Biologists now think they understand why: In laboratory tests, torn bay leaves turned out to be very toxic to fleas, researchers reported last week at a meeting of the Animal Behavior Society in Carbondale, Illinois.

In the eastern United States, many wood rats will simply abandon a flea-infested nest. That means they typically have to move and rebuild every 2 weeks, says Arlene Alvarado, a biologist at the University of California, Davis. But wood rats in California have apparently learned to take advantage of the local flora to stay in their homes a lot longer, up to 2 years.

Their secret seems to be the California bay, a tree with pungent leaves whose oil contains chemicals called monoterpenes--the active ingredient in citronella, a mosquito repellent. Richard Hemmes of Vassar College, who collaborates with Alvarado, exposed cat flea eggs to both whole and fragmented bay leaves. While 80% of the eggs exposed to whole leaves survived, none of the eggs exposed to the broken leaves did. Because the West Coast rats leave nibbled bay leaves only near the nest and not in their food piles, Hemmes's group believes that the rats are chewing the leaves not for nutrients but to release monoterpenes. To make sure, they plan to fumigate a rat's nest and see if it continues using the bay leaves.

The finding "seems intriguing," says Doug Gill, an evolutionary ecologist at the University of Maryland, College Park, who has worked on host-parasite interactions. He notes that birds have been observed to place mustard leaves in their nests to ward off pests. But if the rats are indeed using the leaves for pest control, it would be the first known example of a mammal (other than humans) modifying a plant to ward off pests.