Formica workers.

In the middle of summer many of us as children have kicked at a pile of thatch that was alive with crawling ants and then run away before we would experience their wrath. One could detect the odor of the ants as they pursued us and sprayed their formic acid in defense. Later in the fall, encountering the same pile of lifeless debris on the ground one may have felt vindicated. Were the ants now dead?

Formica is the genus name for thatching ants that includes more than 75 species in North America. These ants seem to be everywhere as they inhabit open fields, forested areas, and in our gardens and yards. These ants cause problems in golf courses, shooting ranges, parks and recreational areas. They live in mountain habitats at high altitudes and in sand dunes near sea level. Some of these species may become nuisance pests in urban areas and around homes in natural areas. Are these ants harmful? Do these ants cause damage? Or are these ants beneficial and should they be protected? It all depends on how near human habitats and environments that these ants establish their colonies.

Large Formica mound.
Like all ants, Formica have winged males and females that swarm in a nuptial flight, mate and start new colonies. The male dies after mating and the inseminated queen finds a nesting site, produces eggs and nurtures her first brood to adulthood. The role of the first worker ants includes all the activities of the colony. In older colonies, budding occurs when a queen or a few queens leave with a group of workers to form a neighboring nest. They may divide to provide a larger foraging arena or to provide more favorable environmental conditions for the nest.

This large and varied Formica group includes ants referred to as thatching ants, field ants and wood ants. The ants are large and closely resemble carpenter ants both in size and color. It is important that correct identification be made as carpenter ants may cause serious damage to wood in structures and Formica ants are beneficial or at most may become nuisance ants around human dwellings. Formica, like carpenter ants, have a single node and a terminal, circular anal orifice. The difference occurs in the profile of the mesosoma (middle part of the ant). The dorsal surface is evenly convex in carpenter ants while the dorsal profile in Formica has a “notch” or dip in the middle. (See illustration.)

Sizes of the two groups of ants also will overlap. Sizes of carpenter ants vary from 6 to 13 mm in some species to 3.5 to 8 mm in other species. Formica vary from 3 to 9 mm in length. Colors are also similar with both groups; ant color varies from brown to black and red and black. Both group of ants also will be polymorphic or have different size workers within the colony.

Differences occur in that Formica are commonly polygyne (more than one queen) and also polydomous (more than one nest) whereas a carpenter ant colony may consist of a parent colony and satellite nests but only one queen. Formica nests may have multiple nests and share queens between nests.

The differences in the profile of a carpenter ant (top) and Formica ant (bottom).

In a recent survey of pest ants in the Pacific Northwest, more than 12 percent of the nearly 1,600 samples submitted by pest management professionals over a two-year period were identified as a species of Formica. These ants were problems to homeowners and were treated as nuisance pests for management.

With this large and diverse group, two major groups of Formica have become nuisance pests when located near structures: thatching ants and field ants. Both groups of ants also may be injurious to seedling trees and other plants and have been known to attack buds of fruit in the spring.

Similarities occur in the biology and identification but nest construction differs. Thatching ants are usually red and black and field ants are often all black or brown.

THATCHING ANTS. Thatching ant (or wood ant) colonies are recognized by the mounds they construct over their nests — it may be a single small mound or large multiple mounds more than 10 feet in diameter and up to 5 feet in height. Nests may extend 4 feet below ground and in winter will go below the frost-line until spring. Although the colony may have been initiated by a single queen, other inseminated queens may join this colony as it grows. Colonies may persist for up to 20 years and contain many thousands of individuals.

The role of the queen(s) is egg production and roles of workers include caring for the queen and brood, foraging for food, building and repairing the nest, cleaning the nest and defending the colony. As with all ants, development is by complete metamorphosis with eggs hatching to larvae that develop to pupae and eventually to adult workers. Development time will vary from 61 to 122 days. In northern climates, egg production commences in April and lasts through August. Mating flights occur in June and July.

Formica nest under attic insulation.

Some species of thatching ants also may nest in rotting wood, bringing in thatching materials to line galleries created in the wood. These ants also burrow through the wood, making pathways and moving brood through the rotted log to create conducive environments.

The beneficial qualities of these ants include their predaceous and scavenger foraging behavior. These ants will “farm” aphids in that the ants will collect honeydew, and guard and protect the aphids from predators such as flies and beetles. The ants prey upon a huge quantity of other insect pests, particularly in forested environments. They also feed on carrion of larger animals, helping to clean the environment and recycle nutrients.

Ants also will feed on extra-floral nectaries and seeds. Their nesting habits aerate the soil and promote water absorption and are important in soil building. The ants, in turn, are also part of the complex food web in natural areas in that the ants are prey to spiders, toads, a number of bird species and even bears, who tear apart nests to eat the brood held near the surface.

Thatching ants have the ability to control temperature and humidity by manipulating the thatch during the day to open “windows” or close “windows” by directing light rays. This activity ensures the proper rearing environments for the brood. “Windows” should not be mistaken for entrances. Ants may enter at these points; however, many entrances are employed by the ants. By moving brood within the nest, temperature and humidity requirements are also controlled. Thatching ants forage most actively in the morning and early evening and remain inside the nest during the warmest part of the day.

Birds may sit on a thatching ant nest, allowing ants to crawl into their feathers spraying formic acid in a defensive behavior. This activity has been observed with robins, jays, crows and other song birds. The acid penetrates the bird feathers and eliminates bird lice. This process has been dubbed “anting.”

Surface of a Formica nest showing workers.

FIELD ANTS. Field ant colonies live in the soil often under plant material or at the base of plants. These ants also may nest under rocks, under patio stones or bricks, under sidewalks, or under landscaping timbers. Their mounds, when present, are composed of soil particles and seldom reach more than a few inches in height. When these ants nest under a lawn, the mound tops may not reach the top of the grass, making their nesting sites difficult to locate and treat. The incidental bites from the ants inhibit activities of children, pets and adults on lawn-infested areas.

MANAGEMENT. The decision to manage these ants should take into account their beneficial qualities as well as the damage they might produce. Nests near structures where people may experience the bite and the formic acid spray that produces a painful sensation or even a blister if the skin is not washed should be controlled. Nests at a distance away from a home that is out of the foraging arena of the ants should be allowed to remain to preserve the ecological benefits these ants contribute.

Nests near foundations may allow ants to enter structures under the lower edge of siding and establish nests in wall voids. These ants do not excavate wood but may carry thatching material into the void. When thatching materials appear below an electrical outlet covering, remove the covering and inject pesticide dust into the void on the exterior side of the electrical box. This may occur in homes in wooded areas and is not a common occurrence in urban areas.

A nest can be treated by application to the surface. Do not merely apply to the nest entry holes as there are many entrances. The entire nest surface and subterranean portion should be thoroughly treated. One method is to penetrate the nest by digging deeply and stirring the nest contents with a study shovel before applying the chemical. Remember the ants will defend the nest! Always follow label directions. Some chemicals, particularly granular formulations, need water for the toxicant to be carried into the nest.

Be certain that these ants are indeed a threat by considering the beneficial qualities they provide and the distance the nests of these ants are from human habitation.

The author is an instructor in the biology department at Spokane Falls Community College, Spokane, Wash.