Ant Mounds In Pastures

By Adam J. Varenhorst

Pastures and rangeland host numerous insect species. While the majority of these insects are benign, some are considered pests. The latter is especially true when insects inhabit areas where they were not previously observed, or areas where their presence is a nuisance. In some instances, these perceived pests are actually providing valuable ecosystem services.

Recently, a report came in of large ant mounds in a pasture. After investigation it was determined that the mounds are the creation of the Allegheny mound ants (Formica exsectoides), which are native to North America. The name is slightly misleading as the range of the Allegheny ants extends from Nova Scotia south to the mountains of Georgia, and west to the eastern edge of the Rocky Mountains. However, the habitat that promotes Allegheny mound ant colonies is sporadic in the known range.

Mound Profile

The mounds observed in South Dakota vary in size but are approximately 6-12 inches in height and vary in diameter from 6-24 inches (Figure 1). In other parts of their range these mounds can potentially reach sizes of 3.5 feet tall and 18 feet in diameter. The mounds have a characteristic bare patch on the top, which allows energy from the sun to heat the belowground colony. The top layers of the colony consist of thatch, which is used to protect the mound from extreme weather. The diameter of the mound can be used to estimate the population density of the ants within (Figure 2). A nest that is 6-18 inches in diameter would have approximately 500-3000 ants, 18-36 inches approximately 1,000-6,000 ants, and 36-60 inches approximately 3,000-10,000 ants.

Figure 1. (Above) Ant mound in a pasture in South Dakota. Note the characteristic bare top of the mound. Photo by Adam Varenhorst.

The worker ants remove plants from their mounds by biting them and excreting formic acid into the wound, and can easily be viewed crawling on the bare patch on the top of the mound (Figure 3). If a mound is severely disturbed the Allegheny ants will establish a new mound nearby.

Allegheny Mound Ant Identification

The worker caste of the Allegheny mound ants are the most abundant in the colony. The workers have a red head and thorax (middle region of the body) and a black abdomen (posterior region of the body) (Figure 2). They have black antennae and legs. The workers have large mandibles and should be handled with care as they can inflict a painful bite.

Figure 2. Allegheny mound ant worker. Photo by Adam Varenhorst.

Benefits & downsides associated with Allegheny Mound Ants

There are many positive aspects associated with the presence of Allegheny mound ants in pasture or rangeland. These ants feed on pest insects including aphids, grasshoppers, and other insects that can negatively affect grass. They also feed on weed seeds and can reduce weed populations.

One downside associated with Allegheny mound ants is that the mounds can complicate mowing. Increases in the number of mounds or colonies are associated with reduced ground cover and the subsequent reductions in shade. When mounds are shaded the chances for the ants within the mound to survive are greatly diminished. Another downside to having ant mounds present is that the amount of plant growth on ant mounds can be significantly reduced, causing small-scale patchiness throughout the affected area (Blomqvist et al. 2000).