Cottage Q&A: Ant nests on the septic bed

An ant nest with a few ants moving in and out By AG-PHOTOS/Shutterstock

Small, black ants are nesting on my septic bed. There are several nests. Could their tunnelling pose a danger to the overlying sand, leading to a collapse? If so, what’s the best way to get rid of them?—Feeling Antsy

“In my 32 years, I’ve never been asked about ants,” admits Dale McLure, the past-president of the Alberta Onsite Wastewater Management Association. He— like our other pros—thought it was unlikely that ants, even a whole bunch of them, could cause the bed to collapse.

“They’re ants. They’re tiny little things,” says pest control specialist Glen Robertson. And don’t worry, ant nests on your septic isn’t a red flag for a malfunctioning system. “The attraction to the septic field may be due to the nature of the ant-friendly soil, being sandy and well drained,” says Elizabeth Lew of Gunnell Engineering, a company in Newmarket, Ont., that specializes in designing on-site sewage systems.

This doesn’t mean you should ignore the nests. While we’re all for letting nature be nature—don’t hate the ants for finding a sweet piece of real estate—“an overabundance of ant colonies may inhibit the natural growth of grass needed on a septic field,” warns Lew.

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Robertson suspects these ants are field ants. If you are going to evict them, he recommends using bait drops (they’re widely available) on the nests. The ants will spread the bait through the colonies, killing them. Don’t dump bleach, insecticide, gasoline, antifreeze, or any other liquid onto your septic bed. This will destroy important bacteria in the soil and flood the field.

For local advice, McLure suggests asking an area septic or pest expert, who may have experience with the same problem. If the ants are attracted to the soil, and soil conditions vary by locale, he says, “this could be a regional issue.”

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