Argentine ants (Linepithema humile) are a globally widespread invasive species. Originally from South America, they have established themselves across many parts of the world, particularly in regions with Mediterranean climates, including South Africa, New Zealand, Japan, Europe, and the United States. Understanding their biology and behavior is important due to their presence in agricultural, urban, and natural environments.
Identifying Argentine Ants
Identifying Argentine ants often begins with observing their physical characteristics and behaviors. Worker ants are small, measuring 2 to 3 millimeters (about 1/8 inch) in length. They exhibit a dull light to dark brown coloration and possess a single, scale-like node on their waist. These ants lack hairs on their head and thorax and do not sting.
They are known for distinct foraging patterns, moving quickly and erratically. When searching for food or water, they form wide, well-defined trails. A crushed Argentine ant may emit a faint, musty odor, sometimes described as resembling rotten coconuts or stale butter. Their ability to slip into small cracks and crevices and swarm in large numbers often leads to their presence in homes, especially in kitchens and pantries.
The Supercolony Phenomenon
The success of Argentine ants as an invasive species is due to their unique social structure, particularly their ability to form supercolonies. A supercolony is an expansive, interconnected network of nests functioning as a single, enormous colony. Unlike most ant species, individual nests within a supercolony exhibit low aggression towards each other, which allows for high population densities and cooperative behavior across vast areas. This low aggression is supported by high genetic similarity among individuals.
These ants originated in the Paraná River drainage in northern Argentina, Uruguay, Paraguay, and southern Brazil. Their global spread was primarily facilitated by human commerce, via shipping and the movement of plants. For example, they were first recorded in California in 1907, believed to have arrived from Brazil on coffee ships. This human-mediated dispersal has allowed them to establish populations on every continent except Antarctica.
The cooperative nature and rapid reproduction within supercolonies contribute to their dominance. Each supercolony can contain multiple queens, sometimes as many as eight queens for every 1,000 workers, ensuring continuous and rapid population growth. This structure allows for efficient resource exploitation, as they can quickly mobilize large numbers of workers to new food sources or nesting sites without internal conflict. The ability to form such massive, unified colonies provides Argentine ants with a competitive advantage, enabling them to displace many native ant species wherever they invade.
Impact on Ecosystems and Homes
The widespread presence of Argentine ants has significant impacts for both natural ecosystems and human dwellings. Ecologically, they are highly disruptive, often outcompeting and displacing native ant species. This displacement can cascade through the food web, affecting other native invertebrates and small vertebrates that rely on native ants for food. Native plants dependent on specific ant species for seed dispersal are also impacted, as Argentine ants may not fulfill this role or may consume the seeds.
Argentine ants engage in a symbiotic relationship with sap-feeding insects like aphids and scale insects. They actively protect these pests from predators and parasitoids in exchange for honeydew, a sugary excretion produced by these insects. This protection can lead to increased populations of these plant pests, resulting in greater damage to agricultural crops and natural vegetation. Their presence can also reduce the number of pollinators visiting natural flowering plants, potentially impacting plant reproduction.
Within homes, Argentine ants become a nuisance due to their large numbers and persistent foraging. They invade kitchens, pantries, and other areas in search of food and water, often contaminating food supplies. Their tendency to appear in large swarms, especially during hot or wet weather, can make them bothersome for residents. The aggressive and invasive nature of these ants makes their control a challenge for pest management.