When the Ants Come Marching In ... and How to Keep Them Out

This is the time of year when the ants come marching in.

The cold, wet weather outside drives them to the warm, dry inside of our houses where there's plenty of food. Sweet food, like holiday cookies. Right on the counter. With a flimsy, come-hither plastic wrap draped casually over the top.

If you see a line of what looks like a thousand little ants marching in through a window or from some mysterious spot under the dishwasher, you shouldn't freak out.

That line of ants is nothing compared to the giant nests of ants, tens of millions of ants, from which your little ant visitors hail and to which they're hell bent on returning, tiny mandibles gripping cookie crumbs for their comrades.

In other words, it's the nest you should be really worried about.

Of the 18 species of ants found in Alameda County, Argentine ants are among the most ubiquitous. Just 3 millimeters in length, they can squeeze through cracks that are just 1 millimeter wide and often set up their colonies in cracks in concrete or shallow leaf litter. They're not native to the US – they probably reached New Orleans on coffee boats from Brazil in the 1890s – but they thrive.

Boy, do they thrive.

Entomologists have long known that Argentine ant colonies can stretch beneath several city blocks. And they've also known of the existence of three "global colonies," including one that stretches 560 miles along the California coast. The other two global colonies are along the Mediterranean coast and along the west coast of Japan.

A global colony, explains Phil Ward, PhD, a professor of entomology at UC Davis, simply refers to the fact that when the ants arrived in this country there weren't enough of them to create high levels of genetic diversity. Today, even ants in colonies across huge swaths of land are so genetically similar, Dr. Ward says, they don't respond aggressively to ants from other colonies — as they do in their native Argentina. Instead, they groom each other, rub antennae, and display other friendly ant behaviors. Why? Because they're basically one big happy ant family.

But the B-movie quality of these itty-bitty creatures gets worse: after testing the hydrocarbons on cuticles of ants from each of the colonies and observing the way the ants from different colonies interacted (i.e., they liked each other) the researchers decided the three colonies were actually just part of one global "super-colony."

So, yes, they're taking over the world.

But that doesn't mean they have to take over your house.

How to get rid of — and prevent — ant infestations

As interesting as these little buggers are, with their vast, friendly (to each other) colonies once they get in your home, they become pests — by the thousands.

The good news is that it's relatively easy to get rid of ants using a relatively safe product — ant baits. A type of insecticide, ant baits work well because worker ants go to them to get food, then trundle back to the colony to share the food with other ants. Then, because ants are very social, those ants share with other ants, who share with still more ants, and, well, eventually, all the ants in the colony die.

Ant baits are also recommended because they're self contained — i.e., you don't end up spraying toxic chemicals all over in an attempt to get rid of one problem nest. In fact, ant bait is the only insecticide recommended by the University of California Cooperative Extension. That's a pretty strong endorsement.

The best way to use ant baits is to put them outside, so they draw the ants out of your house. Recomended baits include Fipronil and hydramethylnon, both of which act so slowly that the ants can make it back to their nest before dying, says Daniel Wilson, community relations coordinator for the Alameda County Vector Control Services District.

Some people also swear by natural remedies like sprinkling chili powder, cayenne pepper, or cinnamon where ants are getting into your home and planting herbs that repel ants, including cedar, basil, chili peppers, geranium, lemon balm, peppermint, rosemary, and thyme, along the base of your home. But these methods just "repel the ants," says Phil Ward, PhD, a professor of entomology at UC Davis. "They don't target the colony. And I've never seen any proof that these work." 

Likewise, simply sponging or vaccuming up ants (or blasting them with Windex or ant spray) is not all that effective — these methods will kill the ants on your counter, but they won't touch the thousands more in the colony near your back door.

The best way to repel ants before they get into your home is by limiting their food sources. How to do that? Follow these tips:

1. Clear nesting material (including wood debris and plants) from around your home.
2. Keep plants and trees that produce sugary fruit, sap or nectar, away from your house.
3. Check for sources of water near your home, including leaking taps and air conditioner drains.
4. Put food away. Don't leave it on the counter, the kitchen table, or on top of the refrigerator. And if you do leave it out, put it in an airtight container, which blocks the smell of the food from getting out and the little anties from getting in.
5. Clean up spills – whether it's soda, sugar, or grease from meat —immediately.
6. Take out garbage every day and keep the bottom of your garbage cans free of dirt and gook.
7. Change the food in pet bowls often and clean them periodically.
8. Seal or caulk openings (including cracks) in your floors, walls, drains, and basement and repair all your screens.

Wilson of Vector Control also recommends preventing ants all year round — not just when you see them.

"Humans tend to be out of sight, out of mind when it comes to ants," he says. "They don't think about ants unless there's an invasion. But they should."

As such, he recommends checking the yard for evidence of ant nests all year round and putting bait near any nests you find. "If you do that, there's a good chance you won't have ants for years," he says.