Garden ants vs house ants in BC: when to act and when to leave them alone

Beneficial outdoor ant species in BC yards

Thatching ants (Formica spp.) build the large, domed mounds of soil and plant material that often alarm Metro Vancouver homeowners. They are outdoor specialists — aggressive defenders of their mound territory, but essentially no indoor pest risk. They are active predators of caterpillars, aphids, and other garden insects, and their tunneling aerates soil. Western thatching ant (Formica obscuripes) mounds in Surrey and Delta yards can reach 60 cm diameter and are a sign of a healthy insect ecosystem. Treatment is only warranted if the mound is in a high-traffic area posing sting risk, or directly against a foundation.

BC outdoor vs indoor ant species — treatment implications.

Species	Indoor risk	When to treat
Thatching ant (Formica spp.)	Very low	Only if mound directly against foundation or in high-traffic area
Field ant (Formica fusca group)	Low	Only if entering structure
Pavement ant (Tetramorium)	High — enters via foundation cracks	When trail enters home
Odorous house ant (Tapinoma sessile)	High — nests indoors readily	When trail enters home or indoor nest confirmed
Carpenter ant (Camponotus modoc)	High — structural pest	Always when in or adjacent to structure

The ants-and-aphids garden dynamic

Many outdoor ant species, particularly Formica and Lasius species, actively farm aphids — they herd aphid colonies on plants and protect them from predators in exchange for honeydew (a sugary excretion). If you see ants running up rose bushes, fruit trees, or other plants in your Metro Vancouver garden, they are almost certainly tending aphids. The ants themselves rarely damage the plants; the aphids they protect do. Managing the aphid infestation (not the ants) resolves the plant damage — horticultural soap or a sticky barrier on the trunk prevents ant access. Treatment of the ant colony for this scenario is almost never warranted.

When outdoor ants do require treatment

Any ant species establishing foraging trails into the home — bait the indoor trail regardless of species.
Carpenter ants in logs, stumps, or dead wood within 3 metres of the house foundation — these are establishment reservoirs.
Pavement ant mounds at the foundation wall — the colony is in soil adjacent to the sill plate and is a likely future indoor ant source.
Any mound in a children's play area where sting risk is unacceptable.

Frequently asked questions

Do thatching ants damage lawns?+

Their mounds displace grass in the immediate mound area but do not significantly damage the larger lawn. The mound's soil-aeration effect is locally beneficial for soil health.

I have a large ant mound in my back corner that I have had for 20 years. Should I remove it?+

Only if it is causing a specific problem. Long-established thatching ant mounds are genuine ecosystems — large Formica colonies support populations of other invertebrates and contribute significantly to the garden's pest control.

Ants are running up my fruit tree. Is that a problem?+

The ants themselves no; the aphids they are farming likely yes. A sticky tree band or horticultural barrier on the trunk prevents ant access, disrupts the aphid-farming, and allows natural aphid predators to recover the aphid population.

How do I tell a garden ant from a pavement ant?+

Thatching/field ants (garden ants) are typically 5-8 mm, red-and-black, and build mounds in open soil. Pavement ants are 2-4 mm, dark brown-black, and prefer the seams under hardscape. If you are seeing large red-and-black ants near a mound in your lawn, it is almost certainly a garden ant.