Western Thatching Ant   (Formica obscuripes)

Quick Look Pest Stats

Color: Reddish-orange and black, to all black

Legs: 6

Shape: Segmented; oval, with a notch on top of thorax 

Size: 3/16″ – 5/16″ 

Antennae: Yes

Region: Prevalent in the Pacific Northwest, also found in the south and east of the U.S.




Inhabits a wide range of habitats, including grasslands, prairies, sagebrush, shrub-steppe, mixed deciduous forest up to pinyon-juniper, coniferous forests, coastal and inland dunes, and alpine meadows. Nests are often found along the edges of meadows and other open areas. Their nest are distinct dome-shaped mounds composed of varying materials found in the nest’s surrounding environment. This composition is primarily vegetation and is commonly called “thatch”. The mounds are typically constructed in areas devoid of cover to expose the nest to sunlight. The size of these mounds is highly variable and is mostly determined by the age and health of the colony. The height ranges from 1 inch to 18 inches, although nests of larger heights are not uncommon. They can extend down to 4 feet in the ground, with chambers present in both the thatched mound and the soil. The condition of the thatched mound is in a constant state of change due to activities by the ants and changes in the environment. To prevent plants from shading the nest, the ants may chew off the bark at the base of plants growing on or nearby the mound. Formic acid is then sprayed into the open layer eventually killing the plants.


The Western thatching ants preys upon insects and other arthropods. Foraging takes place both on the ground and on vegetation, including high in trees. They also harvest honeydew from aphids and other homopterans and are known to occasionally eat plant tissue.


Their large thatched mounds and huge colonies with tens or hundreds of thousands of workers are distinctive and when encountered there is little doubt as to their identity. When at high density they seem to suppress other Formica diversity in the vicinity of their nests. Workers are variable in size (polymorphic), forage during the day in large numbers across the ground and on vegetation, and perform nest maintenance on their mounds and in the immediate vicinity. Colonies are polydomous (single colonies have multiple separate nests) and polygynous (single colonies have multiple queens). The number of workers and individual mounds in a colony can be exceptionally high. Near Lehman Hot Springs, Blue Mountains, Oregon, a supercolony was found that occupied an area around 2.4 acres and consisted of 210 active nests with an estimated population in excess of 56 million (McIver et al., 1997). Mating takes place during nuptial flights during June and July. Total time of development from egg to adult takes 61 to 122 days.


The landscape can be visually disrupted by the presence of their mounds. Physical contact with them is also displeasing, since they can bite quite hard and usually spray the area they have bitten with formic acid to produce a painful sensation or even blistering if the skin is not washed.


If you suspect a infestation of Western Thatching ants on your property, it is advised to involve a professional pest control company to put in place a pest management solution for you. 


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