Ask The Entomologist

Thatch Ants

A question we received last month – our apologies for the delay in answering.

Thank you to Sandra Elster for prompting this piece.

Our mound-building ants in this part of the country are Western Thatching Ants, Formica obscuripes. 5′ by 5′ is quite an impressive mound! I suspect it had quite a few active queens in it at one time… These ants are rather special because they generally have multiple active queens in a single colony – the young queens often help out and reproduce at home, instead of founding their own new colonies.

While Thatching Ant queens can live about a decade, they will eventually die. And when they do so, if there aren’t other queens waiting in the wings, the whole colony will go down with them. I’m guessing this is what happened at your daughter’s place.

Workers at the entrance of a Western Thatching Ant nest near my home.
Busy despite the overcast day.

This species is most interesting to me when it sets about starting new colonies… You see, Western Thatching Ants often start out as social parasites! Queens of the red-colored wood ants, including our species of Thatch Ant, don’t start from scratch… They use others ants’ labor to get started.

The Queen is dead, long live the queen!”

A young queen of the Western Thatch Ant, instead of going to the backbreaking work of digging and founding a new colony all by herself, will tend to infiltrate nests of related ant species. Once inside, the young queen kills the old queen, acquires her smell, and steps into her role.

Over time, the new queen’s offspring will far outnumber those of the old queen, and the nest will eventually be a single species again. Fancy folks call this “temporary social parasitism“.

If the colony is very successful, it may divide into smaller colonies – a way new colonies sometimes form without using social parasitism. If a colony gets sufficiently large, daughter queens may take control of certain sections of it, forming a “satellite colony” instead of leaving to form an entirely separate one. Many linked colonies form a “supercolony”. The largest I’ve heard for this species is 210 linked colonies in eastern Oregon.

I wrote about wasp control not too long ago… Well, ants are your number one natural means of keeping wasps, and most other pest insects, in check. If a wasp colony is under stress, ants will often invade and carry away the baby wasps to be food for the colony. Controlling your Thatch Ants may lead to you having more wasp problems.

That said, if you want to exterminate your Western Thatch Ant colony, Washington State University Extension has some advice.

When they emerge, I’ll address Carpenter Ants & how to control them.

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