WOODSIDE -- Two volunteers trudge uphill through poison oak and past stretches of grassland and gnarly blue oak trees. A dot on the screen of their hand-held GPS unit shows they have hit their target.
Hunched over at Stanford University's Jasper Ridge Biological Preserve, Matthew Bahls and his partner scour the ground for tiny, six-legged creatures. They see one. Then another, and another: small and brown and moving en masse.
"Generally, if there's one Argentine ant, there's probably 500," said Bahls, a volunteer who helps coordinate the Jasper Ridge Argentine ant survey. Twice a year, a group of about 20 volunteers tracks the stampede of the invasive ant species into the preserve and gauges how native ant species are holding up to the assault.
Non-native Argentine ant populations have exploded around the state in the past century. Their huge colonies threaten native ants, plague citrus growers and madden homeowners as they flood into kitchens in winter months. If you have a household ant problem, it's probably Argentine ants.
When Stanford biologist Deborah Gordon began to survey ants at Jasper Ridge two decades ago, she thought the invading Argentine ants would wipe out most of the natives. It was a scary thought because native ants spread the seeds of native plants and control other insects. A delicate ecosystem was at risk.
"At that time we were absolutely convinced that the reserve was going to be completely overrun by Argentine ants," said Philippe Cohen, the preserve's executive director.
But Gordon's team discovered that not only is the Argentine ant invasion slowing down, some native ants might be fighting back.
The survey has evolved from a research project run entirely by Gordon's lab into one powered by community volunteers. This year marks the project's 20th anniversary.
Argentine ants have a long history with people. Steamships crawling with Argentine ants regularly landed at U.S. ports beginning in the late 1800s. Stowed away in bags of sugar, they were first sighted in New Orleans in 1891. From there, the ants may have hopped a train to California, according to Andrew Suarez, a biology professor at the University of Illinois.
The ants showed up in San Bernardino County in 1907 and landed in the state's busy port cities in the following decades. Now, Argentine ants inhabit most of California's coast. In some drier inland counties they can be found along rivers and sprinkled throughout urban areas.
When Gordon began her ant survey in 1993, Argentine ants posted themselves along the preserve's edge. They scuttled between homes and suburban buildings where they slurped up water during hot summers and waited out chilly South Bay winters beneath floorboards and behind kitchen cabinets.
Banishing these ants from California would be nearly impossible. "It's a trap to think about dealing with invasive species in terms of removal," Cohen said. "By the time they're a problem, it's too late."
Argentine ants do well in California's Mediterranean climate. They live in an underground network of nests connected by trails -- with multiple queens. Hordes of Argentine ants descend on food before native ants can even find it. Argentine ants also act like tiny bodyguards that protect crop-destroying insects, like aphids, from predators. For the gesture, these insects reward the ants with a tasty, sugary liquid.
But the reach of Argentine ants seems to have hit its limit, at least in the preserve. Gordon's lab has found that for these ants, which depend on water from landscaping and warm buildings to escape the chill, some parts of the preserve are simply too inhospitable.
Many ants across the state have fallen victim to the invader, but deep in the reserve, a native species, the winter ant, seems to be holding its ground. Gordon's team discovered that, when threatened, winter ants can squirt a toxic fluid that can kill Argentine ants in minutes. Whether the winter ant is holding back Argentine ants is something the team hopes to understand.
Other native ants that are threatened by Argentine ants include carpenter, thief and harvester ants.
Argentine ants flourish in wet years and California has experienced a string of mostly drier than normal years, Gordon said. "It's possible that if we get out of the drought they will start to spread again."
Those unanswered questions keep the volunteers coming back to whack through brush and jot down ant species they see.
"It's kind of like crowd-sourcing," said Bahls, who works as a fundraiser at Stanford.
Bahls helps teach new volunteers to identify the ants that might crawl their way. Argentine ants are small and brown, with a spade-shaped head. They almost always travel in unavoidable numbers, he said.
Other ants travel alone or dart and zip across the earth, changing directions rapidly as if in a constant state of panic -- like ants in the genus Formica, said Cohen, the preserve's executive director. "Boy, they look like they're on a real amphetamine high," he said.
Give a squeeze to an ant named Tapinoma sessile and the smell of banana cream pie wafts through the air. The ant tastes like the sweet dessert, as well, Bahls said. "There's a subgroup of us who can ID quite a few ant species by taste," he said, laughing.
After each survey, volunteer and retired firefighter Gary Smith collects the ant data from the teams. He plugs away at a computer to create maps of where the Argentine ants have traveled that season. Each time promises a surprise, but he's always rooting for the native ants.
Contact Ryder Diaz at 408-920-5064. Follow him at Twitter.com/RyderKDiaz.
The Stanford spring ant survey is May 4; a training class is set for April 20. Volunteers must be 18 years old or accompanied by a parent volunteer. Volunteers are required to be able to walk for long periods of time. "Visit http://jrbp.stanford.edu/ for more details." Also: The California Academy of Sciences runs the Bay Area Ant Survey; interested volunteers can pick up a free collecting kit from the Naturalist Center inside the California Academy of Sciences in San Francisco. For details, go to www.calacademy.org/science/citizen_science/ants.
Common name: Argentine ant
Scientific name: Linepithema humile
Origin: Argentina, but they're now found on every continent except Antarctica.
Appearance: Workers are dull brown, 1/8-inch long and travel in large numbers.
Home base: Nests teem with millions of workers and multiple queens. They live in a cluster of shallow nests in the ground that are connected by trails.
Behavior: Large numbers of ants move in long trails in search of sweets. They eat sugary fluids produced by such insects as aphids. They're known to invade homes.
Impact: The invaders have spurred declines in native ants and lizards throughout the state.
Ants in your house?
Block entryways into your home with caulk and petroleum jelly.
Sponge up ants with soapy water.
Clean surfaces of food and garbage.
Remove infested potted plants and check new landscaping materials for ants.
Use baits to control the colony.
Source: University of California