When the Ants Go Marching, It’s Because of the Weather
They’re always hungry and thirsty. They’re sensitive to the weather. They travel in groups and overstay their welcome. And they’re, well, pests.
Teenagers on summer vacation? Relatives visiting from out of town? No. They’re Argentine ants. And you can spray them, bait them, curse them and vacuum them, but you won’t get rid of them.
Linepithema humile, as these persistent pests are called in academic circles, simply come and go with the weather, it turns out, regardless of how clean you keep your kitchen or whether or not you spray insecticides, according to a new Stanford University study.
“Pesticides aren’t keeping them out, the weather is,” said Deborah Gordon, a Stanford biological sciences professor and author of the study. “These ants come into everyone’s homes at the same time, and leave at the same time.”
In California, that infestation time is winter, when it rains, and summer, when the heat and drought arrive, Gordon said.
To conduct the study, the Stanford team asked participants in 69 Northern California households to estimate the number of ants invading their homes between January 1998 and July 1999, and whether the participants used pesticides to try to keep them out. Gordon and her co-workers, meanwhile, collected weekly temperature and rainfall data from weather stations for comparison.
The results of their survey demonstrated a strong relationship between weather and infestation, even when chemical barriers were used.
“People imagine that ants come because there are crumbs on the counter, but that’s not it,” Gordon said. “They come inside to escape the heat or rain.”
One reason for the Argentine ants’ resistance to eradication is their natural tendency to create multiple nests, experts say. It is unlikely, therefore, that eliminating one nest, either by spraying or removal, will get rid of the problem.
The insects thrive under steppingstones, at the base of trees and in log piles. Small satellite nests are scattered everywhere, up to a dozen around the yard, scientists say.
Since their first sighting in California about a century ago, Argentine ants have traveled in vast armies, moving into the nests of other ant species. They are known to kill the queens of native species, then take over the colonies, scientists say.
But they are hard to kill, say entomologists who have studied their behavior to help the agricultural industry eliminate the pests.
The Stanford study confirmed that typical household insecticides are basically useless, a conclusion most scientists and California urban dwellers came to years ago. Participants in the study claimed that cleansers, including bleach, ammonia, soap and window cleaners, were ineffective, as were herbal and natural products.
Over-the-counter sprays, baits and traps thinned ant numbers once they entered the house, but failed to stop the onslaught into kitchens and bathrooms.
“Baits aren’t attractive to Argentine ants,” said Michael Rust, a UC Riverside professor of entomology. “[These ants] are experts at foraging. No matter how clean and tidy you are, they’ll get in.”
So what does work? Not much, according to experts.
Gordon said that the resumption of temperate weather--not pesticides--inevitably sends ants packing. But some entomologists have seen results with strong chemical barriers.
Rust said that pesticides in the pyrethroid family of chemicals have been effective as an outdoor barrier to Argentine ants, but deltamethrin and bifenthrin, two of the strongest and most effective pyrethroids, must be applied by professionals and used at the maximum recommended levels to work.
Most pest-control companies that utilize these compounds do not use the maximum strength, however, because consumers are unwilling to pay the extra fees, and public sentiment runs against the use of strong pesticides, according to Frank Meek, national technical manager of Orkin Pest Control.
Once inside the house, ants seek protein-and sugar-based foods, so keeping sugary cereals, snacks and soda cans tightly sealed will help reduce their numbers, and occasionally, encourage them to look elsewhere for food.
Experts recommend that pet owners place their pets’ food bowl inside a larger bowl containing water. Ants, who are attracted to pet food, will not cross the “moat.”
Homeowners also are advised to caulk cracks and crevices around a structure’s foundation, as well as entrance holes for water pipes, telephone lines or cable TV wires.
Ultimately, though, one’s best efforts probably will fall short.
“I believe we will never completely eliminate Argentine ants,” Meek said. “It costs so much to put out the materials, and the public isn’t ready for this level of pest control.”
(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)
Where to Get More Information
For information about pest control, visit the University of California’s Integrated Pest Management Web site, at www.ipm.ucdavis.edu.
You may also call the Department of Consumer Affair’s State Structural Pest Control Board, at (916) 263-2540
