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Stanford Report, April 25, 2001

Don't bother trying to prevent ant infestations, scholar says


Using bug spray, bait and other household pesticides to prevent ant invasions is futile, according to a new study by Stanford researchers to be published in the journal American Midland Naturalist.

"People spend a lot of money on year-round pesticides," says Deborah M. Gordon, associate professor of biological sciences and lead author of the study, "but it's not the pesticide that keeps ants out of your home, it's the weather."

The Argentine ant routinely invades homes and apartments throughout California during periods of rainy weather and drought. But using bug sprays and other pesticides to prevent ant infestations is futile, according to Stanford scientists. CREDIT: Jack Kelly Clark/UC Statewide IPM Project
©2001 Regents, University of California

Gordon, who received a Guggenheim Fellowship earlier this month in recognition of her research on ant behavior, is author of the book Ants at Work: How Insect Society Is Organized.

She and her colleagues based their pesticide study on an 18-month survey of homes and apartments in the greater San Francisco Bay Area, a region plagued by the Argentine ant (Linepithema humile) -- an invasive South American species introduced into California nearly a century ago.

An ant motif -- seen here on some toy blocks -- is prevalent in biological sciences professor Deborah Gordon’s office. photo: L.A. Cicero

Lacking natural enemies, Argentine ants have taken over large areas of the state, wiping out native ant species and routinely invading human households. The aggressive insect also has become a major pest in other parts of the world with mild winters -- including Hawaii, South Africa, Australia and the French Riviera.

Rain and drought

The Stanford study is the first to examine a phenomenon Californians have long suspected: that the majority of Argentine ant invasions occur during winter rainstorms and summer droughts.

"Our goal was to determine if there really is an association between ant invasions and weather," says Gordon, "and if so, does pesticide use affect the intensity of infestation."

To find out, the Stanford team surveyed 69 households in the heart of California's Silicon Valley -- from Redwood City to Gilroy -- between January 1998 and July 1999. Each week, participants were asked to estimate how many ants invaded their home and whether pesticides were used to control the invaders. Gordon and her co-workers also collected weekly temperature and rainfall data from nearby weather stations for comparison.

The results of the survey demonstrated an "impressive" relationship between weather and infestation, according to the authors of the study.

"Ants are most likely to enter homes in cold, wet conditions, typically in the winter in Northern California," they write, noting that a smaller peak in the level of infestation occurs during hot, dry conditions -- typically in August and September.

Ant killers

To control infestations, participants in the study reported using a variety of ant killers:

  • Cleansers, such as bleach, ammonia, soap, Windex and Formula 409;

  • Herbal and natural products, including hot pepper, chili oil, lemon and vinegar;

  • Sprays, such as Raid, Black Flag and Hot Shot;

  • Baits and traps, including Combat, Grant's and Ortho Ant Kill.

It turned out that none of these products were effective in preventing ant invasions, although some did reduce insect abundance when infestation was high following a rainstorm or during periods of drought. Even then, sprays proved to be only slightly more potent than household cleansers and baits in getting rid of ants, while herbal and natural remedies were the least effective.

"Our study shows that Argentine ant behavior is clearly tied to the weather," says Gordon, noting that ants probably invade kitchens and dining rooms to escape searing heat or excessive dampness -- and there is little we can do to stop them.

"When you don't have ants in your house, putting out pesticides won't make any difference," she concludes. "The most reliable cause of a decline in infestation may be a change in the weather. They come in because of the weather, and they go out because of the weather."

Unusual biology

One reason why Argentine ants are so difficult to control is because of their unusual biology, observes Gordon.

"Unlike other species, Argentine ants have many queens, and the workers can go back to any nest, so it's impossible to kill off a colony by killing off one queen," she notes.

Unfortunately, adds Gordon, most pesticides are designed to eliminate single-queen species -- a strategy that not only is ineffective on Argentine ants but also damages the environment.

"Pesticides are toxic to people, to our drinking water and to the San Francisco Bay," she says. "By putting out ant killers when there's no infestation, we're only harming ourselves."

Gordon advocates using alternative methods to eliminate marauding ants during cycles of rain and drought.

"Try plugging up holes in walls where ants might enter or using Windex to wipe up ant trails once they arrive," she suggests. "I also recommend building moats around pet food. If you put your cat bowl on a plate with soapy water, the ants won't be able to get across."

Gordon is quick to point out that cleanliness has little to do with Argentine ant invasions. The insects might go after leftovers on your dining room table, she says, but it's the weather -- not the food -- that brings them into your home in the first place.

In addition to Gordon, the other co-authors of the American Midland Naturalist study are Lincoln Moses, professor emeritus of statistics; Meira Falkovitz-Halpern, social sciences research assistant in the Department of Pediatric Infectious Diseases; and Emilia H. Wong, who graduated last year with a B.S. degree from the Department of Biological Sciences.