The Argentine ant is considered to be both an invasive and tramp species, which thrives in disturbed habitats where there is abundant moisture and low ant diversity. As an invasive species, they become established in an area and then spread into the surrounding environment aggressively displacing other native ants. Their populations lack colony borders, and can extend over entire habitats.
Argentine ant (Linepithema humile) workers are 2.2 to 2.6 mm long with one-segmented petioles and 12-segmented antennae without clubs. Their body varies in color from light to dark brown, with somewhat lighter legs. Their mandibles are yellowish and dentate. Argentine ants cannot sting but emit a musty odor when crushed. Queens are brown, and 4 to 6 mm long. They perform other duties besides egg-laying, such as foraging, and feeding and grooming the colony’s young. Males are dark brown and 2.8 to 3.0 mm long.
Distribution. Argentine ants are found worldwide in mild temperate Mediterranean climates. Their native range is subtropical South America in lowland areas of the Paraná River basin. They were probably introduced into the United States at New Orleans in the late 1800s on ships transporting coffee from Brazil. They are now established throughout the southeastern United States, and also found in Maryland, Illinois, Indiana, Missouri, California, Oregon, Washington and Hawaii. Based on genetic studies, it appears that the Argentine ant population in California was introduced from the southeastern U.S., thereby undergoing a double genetic bottleneck. As a consequence, the California population has less genetic diversity and intercolony aggression than the southeastern population.
Argentine ants also have been transported to Europe, Australia, South Africa, Central America, the Mediterranean, and Caribbean and Atlantic Islands. There are two enormous supercolonies in southern Europe, the largest one stretching more than 6,000 kilometers from Portugal to northern Italy.
Biology and Habits. The Argentine ant is considered to be both an invasive and tramp species, which thrives in disturbed habitats where there is abundant moisture and low ant diversity. As an invasive species, they become established in an area and then spread into the surrounding environment aggressively displacing other native ants. Their populations lack colony borders, and can extend over entire habitats. Indeed, their unicoloniality is a key attribute of their ecological domination and success. In their native South America, however, they coexist with other ant species and maintain colony borders, which they defend against other neighboring Argentine ant colonies. The loss of intraspecific aggression in introduced populations provides them with a competitive edge over other less populous species.
In common with other tramp species, Argentine ants are tied closely to human activities and are readily transferred by commerce. In addition, they are polygynous, unicolonial and achieve colony multiplication by budding.
The colony structure is fluid with workers, brood, and food moving between nests depending on the distribution of resources. Colonies have tremendous capacity for growth and expansion due to numerous queens and budding. Argentine ant populations can reach astronomical proportions. For example, in a citrus grove in San Diego County, Calif., it was estimated that from 50,000 to 600,000 ants ascended each tree daily in order to tend homopterans; and in a residential area in Southern California, it was estimated 176,000 to 538,000 ant visits per home occurred over a 24-hour period.
The typical queen to worker ratio is 15⁄1000, which is reduced to 1⁄1000 after the workers execute queens at the beginning of the reproductive season in spring. Queen execution is believed to trigger the production of new reproductives and increase genetic relatedness, thereby promoting colony cohesion.
Virgin queens do not disperse on nuptial flights, but mate within the nest although without a significant level of inbreeding possibly due to the mating flights of males and the mobility of queens and sexual brood. New colonies are initiated by budding from the parent colony. Small propagules consisting of workers and brood either with or without queens can grow and reproduce quickly. Males either mate with virgin queens in their own nest or leave on mating flights to mate with females in other nests.
In the Argentine ant life cycle, the egg, larval and pupal stages are hidden within the underground nest and are only seen when nests are disturbed or when workers are carrying them to a different location. The microscopic eggs are white and approximately 0.3 mm long. Although queens lay eggs throughout the year, most of them are laid in spring and summer. The incubation period varies depending on the temperature, from 12 to 55 days, with an average of 28 days. The length of time spent in the larval and pupal stages also varies depending on temperature. Following the four larval instars (stages), the larvae molt into pupae, which look like adults except their legs and antennae are held tight against the body. These pupae are initially white, but begin to turn darker as they mature. In the final molt, an adult ant emerges from the pupal stage. The time needed to complete the cycle from egg to adult ranges from 33 to 141 days, with an average of 74 days
The colony cycle is seasonal. In southern California, for example, worker production starts in mid-March and ends in October. Colonies grow throughout the spring and summer until about mid-October to November, when a massive die-off occurs. Reproductives overwinter with workers and begin laying eggs in the late winter and early spring that will develop into new alates and workers.
In winter, Argentine ants often nest out in the open with small piles of excavated soil revealing their nest sites. During summer, they move into shade to avoid direct sunlight on the nest. In adverse conditions or the coming of winter, colonies sometimes merge and form supercolonies. With the coming of spring, these supercolonies break up into smaller colonies that disperse to find new nest sites.
Typically, nests are shallow (20 cm or 0.66 feet), but in dry soils, they can be as deep as 60 centimeters (1.97 feet). In the urban environment, outside nests are located beneath boards, stones, concrete, and within decaying plant matter and mulch. Nests are often found at the base of plants or trees that are infested with Homoptera. Indoor infestations are commonly found in kitchens and bathrooms, where water and food are available. Indoor nests are often located in voids below kitchen and bathroom cabinets, around and behind dishwashers, and under stairwells. In addition, potted house plants, bath traps, and sub areas sometimes harbor nests. Homeowners sometimes complain of massive numbers of dead ants that are found in unexpected places, such as in a bathtub or on the garage floor. These corpse piles probably arise from large colonies that are cleaning out their nest after the fall and winter die-off.
Argentine ants forage systematically. Unlike many ants that deposit odor trails only on the way back to the nest from a food source, Argentine ants deposit their trails continuously. This ensures that they cover new ground when searching for food, and are not revisiting the same locations. When they find food, the ants reinforce the trail in order to recruit additional ants to collect the resource. Once trails are established, individual ants show a high degree of site fidelity to a specific resource. Argentine ants are often seen trailing along sidewalks, driveways, and other structural edges that provide guidelines for foraging ants. Trails may consist of thousands of ants traveling to and from nests.
Approximately 99 percent of the food brought into an Argentine ant nest is in liquid form. Their main dietary staple is honeydew, which consists of sugars, amino acids, lipids, vitamins and minerals. Their diet changes over the year.
Control. The seasonal patterns of Argentine ant infestations in the urban environment vary by region. For example, in Berkeley, Calif., there is an initial invasion of buildings in August, which coincides with the decline of homopteran populations, and again in fall when the first substantial rains inundate colonies. In southern California, invasions are highly variable usually beginning in early June and ending by late summer with indoor infestations being associated with increasing temperature and aridity, reduction of homopterans in nearby plants and rainfall. Ideally, control measures for Argentine ants should be initiated early in the season when colonies are undergoing their most rapid growth and development. Stunting colony growth early will result in decreased numbers later in the season.
Perimeter treatments with residual insecticides or baits are currently the primary methods of controlling Argentine ants. Various pyrethroid spray and granular formulations, for example, are commonly used to create barriers around homes. There are other slow-acting, non-repellent insecticides that are readily transferred from one ant to another, acting more like bait than a barrier, available to treat for Argentine ants.
Depending on the insecticide and formulation, barriers have different degrees of contact activity and repellency. Some pyrethroid formulations are highly toxic to ants, often creating additional problems when ants indoors are cut off from their colony outdoors. The trapped ants may increase their activity inside searching for an escape route, thus increasing their visibility. Synthetic pyrethroids like bifenthrin suppress recruitment giving the appearance of being highly repellent. At the maximum label rate, an application of a pyrethroid can provide eight to ten weeks of control.
Creating an effective barrier against Argentine ants is difficult because any small gap will provide an opening. Other factors that may reduce efficacy include chemical degradation, irrigation, dense groundcover, mulch, high temperature, substrate alkalinity and direct sunlight. To be most effective, a residual insecticide should be applied thoroughly, not only as a band around the foundation, but also where the ants trail such as along the edges of sidewalks and stepping stones; at the base of trees, potted plants, and garbage cans; and, where nests are found. One drawback to a perimeter treatment is the broad-spectrum effects, which may kill beneficial arthropods, resulting in secondary pest outbreaks.
In contrast, baits are more target-specific and environmentally friendly particularly when delivered in bait stations and not broadcasted. Populations of Argentine ants are vulnerable to baits due to the continuous movement of ants between nests. It is estimated that more than 50 percent of the worker population was exchanged among neighboring nests in five days. Thus, bait consumed by ants from one nest will gradually spread into surrounding nests. Baits are particularly effective when applied early in the seasonal life cycle of Argentine ants when the colony’s demand for food is high but its availability is relatively low. Protein-based baits are most attractive at this time of the year, probably due to heightened egg production and increasing numbers of developing brood.
Their foraging behavior, however, can create problems for baiting programs, since the foraging range of Argentine ants is more than 60 meters. In a residential setting this may encompass several homes, requiring area-wide control. Due to their resource and site fidelity, another challenge is how to redirect foragers from an established trail to a bait. One possibility is a pheromone-enhanced guideline, such as a string treated with Z9-16:Ald (synthetic trail pheromone) in order to draw ants off a trail and into a bait station.