The large ant mounds in the garden belong to an omnivorous ant (Formica obscuripes Forel), that is the most common thatching ant in the western USA. They protect the azaleas and other plants from caterpillar infestations. The plants in this garden were routinely partly defoliated by green caterpillars in their prior homes. Here at the LWA the plants are not damaged because the ants carry the small caterpillars back to their mounds. Ants will bite if disturbed but if left alone they are not a problem.
The nests extend 4 feet or more below the surface. Above ground the nest is surmounted with a mound typically 1 foot high by up to 5 feet wide. The above ground portion is made up of thatch (fragments of plant debris) that helps to regulate temperature and humidity in the nest. A large nest can contain up to 20,000 worker ants. They aggressively defend their nest against intruders.