Common Name: Field Ant
Scientific Name: Formica spp.
The common name of field ant probably comes from their abundance in outdoor situations. This is the largest genus of ants in America north of Mexico, containing about one-sixth of our entire ant fauna. Some species are commonly called thatching ants because of their habit of constructing a mound or thatch of plant material, often grass. They are found throughout the United States.
Workers polymorphic, 1/8-3/8″ (3-9 mm) long. Color brown, black, reddish or a combination of these colors. Head with ocelli distinct, posterior border not indented. Antenna 12-segmented, without a club. Thorax lacks spines, profile not evenly rounded on upper side. Pedicel 1-segmented, node usually rounded dorsally. Gaster with anal opening round, surrounded by circlet of hairs. Stinger absent, but will bite and spray formic acid into wound if provoked.
(1) Allegheny mound ant (Formica exsectoides) has posterior border of head with a distinct emargination/indentation, node with sharp dorsal edge, and head and thorax red, abdomen and legs blackish brown. (2) Carpenter ants (Camponotus spp.) With profile of thorax evenly rounded. (3) Cornfield ant (Lssius alienus) with ocelli indistinct or absent.
Because of the size and diversity of this genus, few generalizations can be made. They exhibit such behavior as slave-making and temporary social parasitism of various kinds, and several different methods of nest constructions. Colony founding is by several means including by a single inseminated female. Some species, such as F. rufibarbis Wheeler, have gynecoid workers or workers which can and do lay eggs so they can also function as substitute queens. Colony size varies considerably, for example, colonies of F rufa McCook have nests of about 20,000-94,000 ants.
The habits are diverse within this genus. However, most species causing problems around structures are either one of those species called thatching ants or are associated with masonry walls, concrete sidewalks, etc. Thatching ant species construct their mound of plant materials, often grass but also twigs, leaves, and/or pine needles. Such nests are often located around small trees shrubs, or rocks. Other Formica typically construct their nests in the cracks of sidewalks, along foundations walls, at the base of trees, etc. Such ants include the California red-and-black field ant, F. occidua Wheeler, and the brown field ant, F. cinerea Mayr. Members of the genus Formica rarely nest in homes but occasionally enter in search of sweets. Field and thatching ants feed primarily on honeydew from aphids (plantlice), mealybugs, scale insects, etc. found on trees and shrubs. However, some such as the silky ant, F. fusca Linnaeus, tend aphids for honeydew but are also general scavengers predators and are not attracted to meats.
A perimeter treatment with a micro encapsulated or wettable powder formulation of pyrethroid will discourage structural entrance. If a particular problem develops, follow the ants back to their mound and treat it with an appropriately labelled pesticide. Mound injection using a high-pressure (160 psi) aerosol and heavy-duty 4-way probe is particularly effective, or treatment by digging in granules is very effective.