The Red Imported Fire Ant (Solenopsis invicta) is an aggressive invasive species native to South America, known for its painful sting and large, conspicuous mounds. These ants are small, typically 2 to 6 millimeters long, and are reddish-brown with a darker abdomen. When disturbed, they swarm and sting repeatedly, delivering venom that causes a burning sensation and often results in white, fluid-filled pustules. This species presents a substantial public health threat, as stings can lead to severe allergic reactions, including anaphylaxis, and they can cause significant damage to agriculture and infrastructure.
Current Status of Fire Ants in Massachusetts
The Red Imported Fire Ant (S. invicta) is not currently established as a permanent, overwintering population in Massachusetts. While this species has spread throughout the Southern United States, the New England climate has historically prevented its successful establishment. Any transient colonies found in the state are typically isolated incidents resulting from accidental transport rather than a self-sustaining presence. State agencies maintain a surveillance program to monitor for accidental introductions, particularly in high-risk areas like shipping ports and nurseries.
Massachusetts does host a different, non-native stinging species known as the European Fire Ant (Myrmica rubra). This species was first documented in the state in the early 1900s and can be a significant pest, especially in coastal and urban neighborhoods. The European Fire Ant does not build the large, characteristic earthen mounds associated with S. invicta, and the two are biologically distinct species.
Identifying Native Ants That Resemble Fire Ants
Misidentification of S. invicta is common, as several native or established ant species in Massachusetts share a similar reddish-brown color or build mounds. The most reliable way to distinguish the Red Imported Fire Ant is by examining its physical characteristics, particularly its waist structure. True fire ants have two distinct nodes, or segments, between the thorax and the abdomen. They are also polymorphic, meaning worker ants within a single colony vary significantly in size, ranging from small to large.
Common local ants like the Pavement Ant (Tetramorium species) also have two waist nodes and are often found nesting under sidewalks, creating small soil mounds. Pavement Ant workers are generally smaller and more uniformly sized than S. invicta workers. Field Ants (Formica species) can be mistaken for fire ants because some build large earthen mounds, although they lack the two-node waist structure and the painful sting. Carpenter Ants (Camponotus species) are large and common but are typically black or dark brown, have only one node, and do not sting.
Factors Preventing Establishment
The primary factor preventing the permanent establishment of Red Imported Fire Ants in Massachusetts is the region’s cold winter climate. Solenopsis invicta colonies cannot survive prolonged, deep winter freezes, a condition common across New England. Research indicates that the northern expansion of the species is largely limited by the minimum average temperature of the coldest month.
For a colony to survive and reproduce, the nest must maintain a temperature that supports brood development, which is typically above 70°F. While the ants can retreat deep into their nests to avoid freezing temperatures, extended exposure to near-freezing conditions significantly increases mortality. Laboratory studies show that 100% mortality occurs after seven days at -4°C (24.8°F), and even at 0.5°C (32.9°F), mortality rates rise sharply. The consistently low temperatures during a Massachusetts winter prevent successful overwintering.
Pathways for Accidental Introduction
Despite the cold, fire ants can temporarily appear in Massachusetts through human-mediated transport from infested areas in the South. This accidental introduction is the most common way the ants reach non-endemic states. The ants often “hitchhike” in high-risk commodities that contain soil or are stored outdoors in infested areas.
The most frequent pathways include the movement of nursery stock, potted plants, sod, and other landscaping materials shipped north. Colonies or individual queens can also be transported in shipping containers, vehicles, or soil-moving equipment. Vigilance is required when inspecting goods imported from quarantined Southern states, as a single mated queen surviving the trip can initiate a temporary colony until the cold weather eliminates it.