The ants go marching in

Pesticides won't stop 'em

Stanford study shows it's the weather that brings them inside

Eric Brazil / SF Chronicle 25apr01

The fierce and tenacious Argentine ant has taken over California flatlands from native ant species. Photo by Jack Kelly Clark, courtesy of University of California Statewide IPM Project Habitat: Native to Argentina and Brazil, this species was probably introduced at New Orleans via coffee shipments from Brazil and Is found throughout the southern states and in California. Isolated infestations are found in Illinois, Maryland, Missouri, Oregon and Washington. Biology: Found in moist situations near a food source, preferring sweets but will feed on any kind of food. Colonies contain from a few hundred to thousands. They invade buildings when Conditions outside are ether too wet or too dry for their comfort. Control: Eliminating food sources will discourage the ants from entering a household. Plugging hales in walls, wiping ant trolls with Windex and building moats of soapy water on pets' food dishes will help control invasions. Source: National Pest Control Management Assoc.

It's the weather that sends ants streaming into homes, and using pesticides, sprays and baits to keep the critters out is hopeless, a new Stanford University study concludes.

The 18-month study conducted in 69 households between Redwood City and Gilroy found that the heaviest ant invasions occurred during winter rainstorms and summer droughts.

Folk wisdom has tended to link ant invasions with wet weather, but the study is the first to examine and test that suspicion.

Researchers also discovered a grim bit of news for anyone trying to drive ants out of their kitchen.

"When ants aren't in the building, it's the weather, not the pesticides, that are keeping them out," said Deborah M. Gordon, associate professor of biological sciences and lead author of the study, to be published soon in the journal American Midland Naturalist.

The seemingly invincible Argentine ant, an exotic invader that was first noticed in California in 1908 and has overpowered all other species in the flatlands as far north as Chico, is the study's subject.

The Argentine ants have invaded so successfully because "different colonies of the same species don't fight each other," said Phil Ward, a professor of entomology at University of California at Davis and a leading authority on ants. "They treat separate nests as if they are all part of one big happy colony. Most ants spend a lot of time fighting their own species."

Another reason for the hardiness and durability of Argentine ants is that their nests contain multiple queens and males, a fact of biological life that enhances a colony's chances for survival, the Stanford study found.

The study -- by Gordon, Lincoln Moses, Meira Falkovitz-Halpern and Emilia H. Wong -- involved 69 households, which provided weekly reports on ant populations and the control methods they used. Simultaneously, they gathered weekly temperature and rainfall data from six meteorological stations.

Conclusion: Ant "abundance is highest in winter when the weather is cold and wet, and there is a second, smaller peak in the hotter, drier part of summer."

The researchers found that applying various foreign substances to ant routes did have some effect in reducing ant populations during unusually heavy invasions. But all of the remedies had about equal effects.

"We found no indication that certain pesticides are more effective than others . . ." the study reported. "For example, household cleansers apparently diminish ant abundance about as much as baits or traps. The results of this study suggest that it may not be helpful to use pesticides to control Argentine ant infestation when weather conditions make infestation unlikely."

Gordon, an ecologist and member of the Stanford faculty for 10 years, is the author of "Ants at Work," published in 1999. The book, based on her field studies in remote southeastern Arizona, examines ant behavior and how colonies function and expand without any central control.

Gordon was awarded a Guggenheim Fellowship this month and intends to use it to look deeper into the way in which Argentine ants have achieved such a dominant position in California and other areas. She will also study similarities between the ant colonies in which she has specialized and other biological systems.

The Argentine ant is just one of more than 200 species of ants in California, but it has been so successful at finding a niche for itself that it has actually caused the extirpation of some species. No one is sure just why, Gordon said. "We want to find out why it's able to disrupt native communities so much and the effects it has on the ecosystem," she said.

The ant is not a public health problem to humans, Gordon said, but it can be a pest, particularly in backyard gardens, because it tends and encourages scale insects like aphids, which give off a "honeydew," a sweet excretion the ants like.

Ward said that the advance of the Argentine ant is having a definite impact on California's ecosystem.

For example, Ward said, one of his students has found that the native horned lizard -- also known as the horned toad -- is being eliminated from patches of coastal sage scrub in the San Diego area because the native ants on which it feeds have been all but eliminated by the Argentine ants. "It's a case where they (ants) are causing a vertebrate to go extinct," Ward said.

For more information about Argentine ants, visit the University of California at Davis Web site at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7411.html

Argentine ant facts

Habitat: Native to Argentina and Brazil, this species was probably introduced at New Orleans via coffee shipments from Brazil and is found throughout the southern states and in California. Isolated infestations are found in Illinois, Maryland, Missouri, Oregon and Washington.

Biology: Found in moist situations near a food source, preferring sweets but will feed on any Colonies contain from a few hundred to thousands. They invade buildings when conditions outside are either too wet or too dry for their comfort.

Control: Eliminating food sources will discourage the ants from entering a household. Plugging holes in walls, wiping ant trails with Windex and building moats of soapy water on pets' food dishes will help control invasions.

Source: National Pest Management Assoc.

Chronicle Graphic

E-mail Eric Brazil at ebrazil@sfchronicle.com

Stanford study says ants are here to stay, despite pesticides

Ron Harris / AP 25apr01

SAN FRANCISCO -- Keeping ants out of the home has more to do with the weather than with stocking up on pesticides, a new study by Stanford researchers reveals.

A survey of 69 Northern California households from January 1998 to July 1999 found that Argentine ants, a tenacious non-native breed, routinely are invading households, driving out native ant species and all but ignoring top-selling pesticides.

The weather is the real key to when the ants will come in your home, and when they'll decide to leave, Deborah Gordon, lead author of the study, said Wednesday.

``Putting out pesticides won't make any difference,'' Gordon said. ``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.''

The pesticides kill ants, Gordon said, but do not drive them from homes.

Common off-the-shelf pesticide sprays such as Raid, Hot Shot and Black Flag were not effective in driving the ants out of the homes studied. Neither were bait traps such as Combat, Grant's and Ortho Ant Kill.

Messages seeking comment from The Scotts Company, which makes Ortho Ant Kill, were not immediately returned.

Participants in the study tried almost everything to get rid of the ants, even wiping counters with bleach, ammonia, soap, hot pepper and chili oil. The only impact noted was a slight reduction in the ant populations following rainstorms and during summer drought periods.

Gordon's study revealed the Argentine ants likely invade kitchens and dining rooms to escape extreme heat or excessive dampness. She suggested wiping ant trails with glass cleaners such as Windex and plugging holes in walls.

The Argentine ants' unusual biology makes them difficult to control using traditional methods, Gordon noted. The ants have several queens and most pesticides are designed to eradicate single-queen species.

``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,'' Gordon said.

The Argentine ant, Iridomyrmex humilis, lacks natural enemies and has taken over large areas throughout the state. The insects have begun to flourish in other mild winter locations around the world, including Hawaii, South Africa, Australia and the French Riviera.

The ants are native to Argentina and Brazil and are believed to have come to the United States by way of coffee shipments, according to the National Pest Management Association, which recommends boric acid dust and perimeter treatments around the home to ward off the pests.

California also is becoming home to increasing populations of fire ants, which likely would fight with the Argentine ants for territory, Gordon said.

``The meeting of the fire ants and the Argentine ants will be interesting and gruesome,'' Gordon said. ``If the fire ants take over, they may deal with the Argentine ants for us.''

• National Pest Management Association, www.pestworld.org