The Argentine ant, Linepithema humile, is a small, dark-brown insect native to the Paraná River drainage in South America. Accidentally transported globally through human commerce, this species has become established in many regions with Mediterranean and mild temperate climates, including parts of Europe, Australia, New Zealand, and the United States. Ranked among the world’s 100 worst invasive alien species, the Argentine ant is a major pest in both urban and agricultural environments. The species causes substantial harm, stemming from its unique social biology that allows it to outcompete local fauna and disrupt human systems.
Unique Biological Traits Driving Their Success
The success of the Argentine ant as an invader is directly linked to its unusual colony structure, known as polygyny, meaning colonies possess multiple queens. In their introduced range, a single nest can house hundreds of reproductive queens, which allows for exceptionally fast population growth. This contrasts sharply with most native ant species, which typically have only one queen per colony.
The ants also form vast “supercolonies,” which are large networks of spatially separated but socially connected nests. This cooperative structure is possible because the introduced populations exhibit low levels of aggression toward ants from neighboring nests of the same species. Genetic uniformity among the ants in these invasive regions prevents the workers from recognizing nestmates as different from non-nestmates, eliminating internal territorial conflict.
This lack of aggression enables a massive, cooperative workforce to spread across vast territories without the energy drain of constant warfare. Colonies expand rapidly through a process called “budding,” where one or more queens walk to a nearby location with workers to establish a new, fully cooperative nest. This social structure bypasses the dispersal limitations faced by most other ant species, allowing the Argentine ant to quickly dominate new environments.
Ecological Impact: Displacement of Native Species
The massive populations and cooperative nature of the Argentine ant lead to a devastating impact on local ecology. They are aggressive competitors and often displace most native ant species in invaded areas, with native ant populations sometimes declining by over 90%. This displacement is not limited to other ants, as the invasive species also competes for resources with native arthropods and small vertebrates.
The reduction or eradication of native ant species causes a severe disruption in natural food webs. For example, the decline in native ants, which are a primary food source, has been linked to the falling populations of other animals, such as the coast horned lizard in Southern California. The invasive ants also disrupt ecological processes mediated by native fauna.
Disruption of Seed Dispersal
One significant disruption is the effect on myrmecochory, the process of seed dispersal by ants. Many native plants rely on local ant species to bury their seeds, which is important for protection from fire or to aid germination. When Argentine ants displace these native species, the seeds are often left on the soil surface, leading to a failure of plant recruitment and a decline in native flora. This disruption can also indirectly promote the spread of other invasive plant species whose seeds the Argentine ant may prefer to disperse.
Economic Costs and Agricultural Harm
Beyond the natural environment, the Argentine ant imposes a heavy financial burden through its interactions with agriculture. The species is notorious for forming a mutualistic relationship with sap-feeding insects, such as aphids, scale insects, and mealybugs. These pests excrete honeydew, a sugary waste product, which the ants consume.
In exchange for this food source, the Argentine ants actively protect the sap-feeders from their natural predators, including parasitic wasps and ladybugs. This protection allows the pest populations to flourish, leading to higher densities and increased damage to commercial crops like citrus and avocados. The ants may even move the pests to healthier parts of the plant to maximize honeydew production, further accelerating crop damage.
The economic damage is compounded by the high cost of control and management efforts. Agricultural operations often see the Argentine ant as the primary factor driving the need for increased pesticide application. Municipalities and professional pest control companies face substantial annual expenses managing widespread infestations in urban and suburban areas. Furthermore, the ants have been known to chew through irrigation pipes to access water, adding to infrastructure repair costs.
The Direct Nuisance of Home Invasion
For the average homeowner, the primary issue with the Argentine ant is its persistent presence indoors. Unlike many other ant species, Argentine ants invade in massive numbers, particularly when weather conditions outside become unfavorable, such as during heavy rain or drought. They are constantly seeking food, water, and shelter, making homes an attractive target.
Their foraging behavior is characterized by the formation of wide, distinct trails that can be seen climbing walls, utility lines, and across kitchen counters. These trails are noticeable and can consist of hundreds or even thousands of individuals moving in continuous columns. The ants’ small size, approximately 1/8 inch long, allows them to easily gain entry through tiny cracks and crevices in foundations, windows, and doors.
Once inside, they contaminate food sources and surfaces as they forage for sweets, meats, and fats. The sheer volume of ants involved in an infestation makes them a severe nuisance pest. Their persistence means that if one nest is eliminated, ants from the interconnected supercolony nearby quickly move in to re-colonize the area.