The black fire ant is an invasive pest that stings. Fire ants belong to the genus Solenopsis, which contains over 200 species worldwide. The imported species are well-known for their aggressive behavior and painful sting. While the red imported fire ant (Solenopsis invicta) receives the most attention, the black imported fire ant is also a significant species. These ants are recognized as invasive pests that cause medical and economic harm across the southeastern states.
The Black Imported Fire Ant: Identification and Origin
The black imported fire ant, Solenopsis richteri, was the first invasive imported fire ant to arrive in the United States. Native to South America, its original range extended from northern Argentina through Uruguay and southern Brazil. It was likely introduced accidentally around 1918 to the port of Mobile, Alabama, possibly as a stowaway in ship ballast soil.
The most distinguishing physical characteristic is its color, which ranges from dark reddish-brown to black. Workers exhibit polymorphism, ranging from approximately 1.0 mm to over 4.0 mm in length. Identifying features common to all fire ants include a two-segmented “waist” (pedicel) and the absence of spines on the rear segment of the thorax (propodeum). Workers also possess ten-segmented antennae, with the final two segments forming a distinct club. A unique feature shared only by the two imported fire ant species is a median tooth on the front edge of the clypeus, flanked by two lateral teeth.
Behavioral Differences Between Black and Red Fire Ants
The black fire ant (S. richteri) displays several behavioral traits that differentiate it from its red counterpart (S. invicta). S. richteri is less aggressive and has been largely outcompeted by the red species across much of the southern range. Despite this, black fire ants are known for their mound-building behavior and aggressively defend their colonies when disturbed.
Black fire ants tend to build their earthen mounds larger and denser than the red species, often preferring shaded areas or locations near structures. In contrast, the red imported fire ant favors open, sunny locations for its nests. S. richteri is also better adapted to cold temperatures than S. invicta. This allows black fire ants to be more active at slightly cooler temperatures, enabling them to forage earlier or later in the day.
While both species deliver a painful sting, the venom reaction from the black fire ant is slightly milder than that of the red imported fire ant. However, the sting from any imported fire ant results in the formation of a sterile pustule, distinguishing it from other insect bites or stings. The black fire ant’s sting still poses a medical threat, and they are highly territorial.
Geographic Range and Hybridization
The pure black fire ant (S. richteri) is currently restricted to a smaller geographic area in the United States compared to the red species. Pure populations are largely concentrated in parts of northeastern Mississippi, northwestern Alabama, and southern Tennessee. This distribution is due to the black species’ greater tolerance for cold temperatures, which limits the northward spread of the red imported fire ant.
Hybridization
Identification based on color alone is complicated by extensive hybridization between the two species. The black and red fire ants have interbred, creating a hybrid form, Solenopsis invicta x richteri, which is widespread across the infested Southeast. This mixing has resulted in a broad “hybrid zone” stretching through Alabama, Georgia, Mississippi, and Tennessee.
The hybrid ants exhibit a mix of traits from both parent species, and their appearance can vary significantly, sometimes appearing reddish, dark, or a color in between. In areas like Tennessee, the hybrid form is often the most common type encountered. The most reliable method for accurately distinguishing pure species from the hybrid involves specialized laboratory techniques, such as analyzing cuticular hydrocarbons or venom alkaloids.