Solenopsis richteri

Black Imported Fire Ants (BIFA), Solenopsis richteri, are a native of South America. The range of these ants in South America extends from northern Argentina throughout Uruguay and into southern Brazil. The species was accidentally introduced into the southeastern United States around 1918 in the city of Mobile, Alabama. From there it spread outward until the it came into competition with Red Imported Fire Ants Solentopsis invicta upon the introduction of this latter species approximately twenty years later. The range of BIFA in the United States has now stabilized in Mississippi and Alabama, although the species is beginning to spread into western Tennessee. (Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

Solenopsis richteri prefers open area grasslands, particularly pastures and lawns. The pampas of Argentina was its original preferred habitat. Young BIFA colonies prefer moister areas in which to build their mounds, whereas more mature colonies tend to emigrate to drier soil as they grow larger. Most BIFA colonies are found at lower elevations, although these ants are quite adaptable. In South America, BIFA colonies are found in seasonally marshy areas as well as at elevations as high as 12,000 feet. (Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

Characteristics common to all fire ants include a two-segmented waist, an absence of spines on the back of the ant (specifically, the propodeum), and a long hair (or seta) in the middle of the clypeus (just above the jaws). Workers have ten-segmented antennae, the last two segments of which form a distinct club. A characteristic seen only among the two species of imported fire ant is the median tooth centered along the front edge of the clypeus, which is flanked by two lateral teeth. The final distinctions used to identify S. richteri are its black or dark brown coloration from which its name derives and a characteristic yellow spot on its gaster.

Caste differentiation among BIFA is relatively easy. Males are winged, stingless, and larger than the females (with exception of the queen). Before having mated, unfertilized (virgin) queens are winged, and their gaster (abdominal area) is much larger than males or workers. After mating, fertilized queens quickly lose their wings, but their size and swollen gaster continues to make them quite distinct. Sterile females are dimorphic, with majors being distinctly larger than minors (although as colonies grow older sizes vary more). (Arnett, Jr., 1985; Campbell, et al., 1999; Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

Eggs hatch after after five to eight days, with the variance in time due mainly to the caste and sex of the egg. There are four larval stages, or instars, followed by the pupal and then adult stages.

After hatching, workers apply small amounts of venom to the larvae to prevent infection. Between each instar the larvae, shed their skins with the help of workers. Each brood forms a ball, held together by a sticky coating and the hairs that develop progressively with each stage.

When a larva enters the pupal stage, it is moved into storage with other pupae. The pupae are moved about the nest regularly to the nest chambers with the optimal humidity for development. During the pupal stage, tissues reorganize themselves into the adult form. Finally, the ant ecloses (emerges from the pupal skin) with the help of workers. The shed skin is eaten by nestmates. Young adults are called callows and are soft and pale, becoming darker and harder within a couple of days. (Grzimek, 1972; Holldobler and Wilson, 1990; Taber, 2000)

As with many other ants, BIFA reproduction occurs during a "nuptial flight." Virgin queens emerge from the mound by the hundreds along with males and copulation takes place in the air. Favorable conditions to trigger the onset of a nuptial flight are air temperature of between 75 and 90 degrees, ground temperature above 65 degrees, and high humidity. New nest mounds tend to form downwind of the parent mound, indicating that wind may be an important factor as well. Nuptial flights usually occur in the afternoon. In the United States, they occur in every month except January. In South America, they usually occur between January and April.

Colonies mature in two years. In the United States, egg production is seasonal, beginning in March. Sexual broods are laid before worker broods. Egg production ends with the onset of winter. In South America, egg production peaks in the summer (January to March) and in the winter (July to September). Eggs laid in the summer only develop into workers, whereas all castes develop in the winter. (Grzimek, 1972; Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

Longevity in S. richteri is dependent upon caste. Males can be quite long-lived relative to workers, but will die within days of leaving the mound and mating. Workers live a little over half a year in the wild, and live anywhere from 10 to 70 weeks in laboritories. Successful queens live approximately five years in the wild, and 6 to 7 years in captivity. (Holldobler and Wilson, 1990; Taber, 2000)

Solenopsis richteri, like other fire ants, is well known for aggressiveness and mound- building behaviors. Mounds grow to three feet in height and three feet in width, and under the best conditions mounds lie approximately forty feet from each other. BIFA mounds are often covered with a "roof" of vegetation. In South America, giant mounds with multiple fertilized queens are common. These mounds, termed Mima mounds, can grow to ten feet in height and eighty feet in width. Rain stimulates mound building in BIFA, and colonies often build more than one mound in a day and then travel back and forth. The primary purpose of the mound is to aid in controlling the humidity at which larvae and pupae are kept, which contributes to the awesome breeding capacity of fire ants.

BIFA are highly territorial and respond to any threat to the mound with overwhelming force. Swarms of ants pour forth to defend their home, and the painfulness of their sting is second only to the sheer number of stings delivered by the attacking ants. (Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

BIFA share the complicated system of communication utilized by most ant species. They communicate primarily through pheromones, releasing them to communicate information and particularly to mark trails. The most common use of these trails is to lead the way back to a food source found by a foraging worker. BIFA scouts also commonly use trails to mark the path to competing mounds. Nearby mounds of other species will be overrun and the colony destroyed, while nearby colonies of other BIFA are often raided for larvae.

Much of BIFA communication is ritualized or instinctual. Ritualized tapping of the antennae sends signals through a substrate while antennae are also used to communicate a variety of signals. Touching certain areas (usually with forelimbs) is also used, primarily to initiate the regurgitation of food. This is probably instinctual, as human hair has been demonstrated to produce the same effect when it is used as a probe.

BIFA also have a very high pitched squeal, barely audible to the human ear, that is used as a distress call. (Grzimek, 1972; Holldobler and Wilson, 1990; Holldobler and Wilson, 1994; Taber, 2000)

The optimal foraging conditions for the BIFA are air temperature between 70 and 85 degrees and ground temperature of less than 95 degrees. BIFA also divide foraging duties. For instance, some foragers locate and disable food while others carry the booty back to the mound. BIFA eat primarily other insects, and they also feed upon oily seeds. Some of the prey insects of S. richteri are not killed but instead kept in the mound and "milked" for their fluids.

Foods eaten: long-horned grasshoppers, cucumber beetles, various ground beetles, spittle bugs various spiders, stink bugs, house flies, mealybugs, various insect larvae and oily seeds. (Taber, 2000)

The most effective anti-predator adaptation of S. richteri is its venomous sting. The poisonous compounds in its venom are very painful and have been demonstrated to be particularly deadly to termites. Also, the aggression and sheer numbers of a fire ant swarm have a large deterrant affect. (Taber, 2000)

Solenopsis richteri acts to control the insect populations throughout its range. In turn, it is also a source of food for many insects, accounting for up to 75% of the diet of some organisms. (Taber, 2000)

Solenopsis richteri has relatively little positive impact in the United States. What beneficial effects it did have in controlling crop, livestock, and other pests has largely been usurped by Red Imported Fire Ants. In South America, S. richteri is not considered to be beneficial to humans. (Taber, 2000)

Solenopsis richteri is considered a pest and a nuisance in the United States. In particular, it is responsible for significant amounts of damage done to farm machinery. It is also a nuisance because of its preference to build mounds on lawns and because this connection with humans often leads to painful stings. In Brazil, these ants are a pest to the potato crop, eating the tubers and branches. Interestingly, it is considered to be a benign resident in Argentina, neither a pest nor a benefit. (Taber, 2000)

There are no conservation efforts on behalf of S. richteri. The U.S. Department of Agriculture has also stopped all efforts towards the eradication of imported fire ants, deeming them virtually impossible.