Moisture Ants/Cornfield Ant - Lasius alienus
This ant gets its name from the material they infest. They are commonly found in locations with high moisture content; including but not limited to damp rotting wood and damp soils beneath concrete slabs or foundation walls and are usually, but not always the sign of a moisture problem.
Common areas you will find them are under leaking toilets, behind tile shower enclosures, under leaking hot water tanks and under leaking dishwashers and washing machines.
They will not cause rot, but where it exists, they will bring more moisture in to accelerate rot and fungal growth. If after the water leak or moisture issue is resolved and the damaged wood material is not removed, they may continue to bring moisture in, furthering the damage. Even though the source of the water leak has been resolved, a chemical application may be required to kill the nest.
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Courtesy of Washington State University Extension