Mark Shwartz, News Service
Stanford University News
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."
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.
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 andrainfall 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
ne 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. |
