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Are Ant Mounds in Pastures Bad?

By Adam 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. However, sometimes these perceived pests are actually providing valuable ecosystem services. Take thatching ants (Formica obscuripes) for example. We often receive reports of large ant mounds in a pastures and rangeland. These mounds are the creation of thatching ants, which are native to North America and common in South Dakota. Although these mounds are often considered a nuisance, the ants may play an important role as predators of potential pest insects.
Mound Description
The size of the mounds observed in South Dakota can vary dramatically from approximately 1 to 18 inches in height. These mounds also vary in diameter from 1 foot to as much as 11 feet (Figure 1). 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 mounds are built on top of below-ground chambers where much of the thatching ant activity occurs.
Figure 1. Thatching ant mound in a pasture in South Dakota. Note the characteristic bare top of the mound. 
The diameter of the mound can be used to estimate the population density of the ants within. Larger mounds indicate that the colonies are more successful. Worker ants remove plants from their mounds by biting them and excreting formic acid into the wound. The ants can easily be viewed crawling on the bare patch at the top of the mound (Figure 2). If a mound is severely disturbed, the thatching ants will establish a new mound nearby.
Figure 2. Thatching ants at the top of a mound.
About Thatching Ants
The worker caste of the thatching 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 3). 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 3. Thatching ant. 
The main positive aspect associated with the presence of thatching ants is that they prey on grasshoppers and other insects that can negatively affect grass forage quality. The downsides are that the mounds can complicate mowing and that the amount of plant growth on ant mounds can be significantly reduced, thus causing small-scale patchiness throughout the affected area. Note that disturbing or breaking apart a large mound may actually increase the number of mounds in an area, as the ants will disperse and form new colonies.