Know Your Enemy: Fire Ants

No series on arthropod pests would be complete without fire ants. They are violent. They are deadly. And they’re seemingly out to conquer the world. I’m exaggerating on that last point, but not by much.

There are over 20 fire ant species around the world, including at least four species that are native to the United States. But when most people think of fire ants, they are thinking specifically of Solenopsis invicta, also known as the red imported fire ant.

Originally from northern Argentina, invicta was first observed in the U.S. near Mobile, Ala., in the late 1920s or early 1930s. The species has since spread as far west as central Texas and as far north as Virginia. It has also gotten a foothold in southern California over the past 15 years. But it hasn’t stopped there. In the past 10 years, invicta has spread to Australia, China and Taiwan from the U.S. (You’re welcome!)

One reason that invicta is so problematic is that the species forms large colonies – up to 300,000 workers in a colony with a single queen. In single-queen colonies, a colony will occupy a single mound. Because these colonies are fiercely territorial (they’ll go to war with each other), the mounds have to be spaced apart. Still, researchers have found densities as high as 100 single-queen mounds in a single acre.

But these fire ants can also form colonies with multiple queens, which spread across multiple mounds. When multiple-queen colonies are involved, there can be 230 mounds – or more – on a single acre. Since each mound is home to hundreds of thousands of workers, we’re talking about millions of fire ants. Millions.

These workers are omnivorous and aggressive. If you disturb their mound, they will come pouring out (pro tip: don’t kick a fire ant mound). But they are also active foragers. So if you are simply standing still in the same general area as a fire ant mound, they’ll find you. Researchers have found that if you place a dead cricket on the ground in the general vicinity of a mound, the ants will be on it in 15 seconds or less.

While fire ants primarily eat other insects and plant matter, they’ll also consume larger prey – including vertebrates that aren’t able to get away, such as nestlings.

When defending their mounds or attacking prey, fire ants use the stingers on their abdomens. Each sting injects a venom made primarily of alkaloids that are cytotoxins – meaning the venom actually kills the cells it comes in contact with. If you’re stung, it feels like you’re being burned. White blood cells then rush to the site of the sting, and pustules form. These pustules can itch for up to a week, and may even produce scar tissue.

And due to their aggressive nature, victims are rarely stung once. Unlike bees, fire ants can sting repeatedly. Ants also usually swarm intruders and prey. As a result, victims are typically stung multiple times by multiple ants.

Worse still, the venom also contains trace amounts of proteins. Some people are highly allergic to these proteins and go into anaphylactic shock when stung. Every year there are severe cases in which people die as a result of fire ant stings.

What can you do to fend off fire ants? First off, you can avoid so-called “home remedies.” They don’t work. For example, some people swear that running over a mound with a lawnmower will wipe it out. Wrong. When a mound is disturbed, a colony builds a new mound. Because they abandon the original mound site, it might look like you killed them. But all you really did with your lawnmower was: A) run the risk of getting a lot of fire ant stings; and B) force the colony to build a new mound a few yards away.

There are, however, effective pesticides that can be used against fire ants. You can also pour between three and five gallons of boiling water into a mound. This will likely kill off the bulk of the ants and their queen (as well as the surrounding grass).

Unfortunately, if you live in an area where fire ants colonized your yard in the first place, they will almost certainly come back. It’s an ongoing battle.

Check out our other posts on arthropod pests: Japanese beetles, mosquitoes, ticks, horse flies, black widows and carpenter bees.

Note: Many thanks to Dr. Ed Vargo, professor of entomology at NC State, for taking the time to talk to me about fire ants. Any errors in the above post are mine alone.