Red Imported Fire Ant

Red imported fire ant workers range in size from small to medium, making them polymorphic. Workers measure between 2.4 and 6.0 mm (0.094 and 0.236 in). The head measures 0.66 to 1.41 mm (0.026 to 0.056 in) and is 0.65 to 1.43 mm (0.026 to 0.056 in) wide. In the larger workers (as in the major workers), their heads measure 1.35 to 1.40 mm (0.053 to 0.055 in) and 1.39 to 1.42 mm (0.055 to 0.056 in) wide. The antenna scapes measure 0.96 to 1.02 mm (0.038 to 0.040 in) and the thoracic length is 1.70 to 1.73 mm (0.067 to 0.068 in). The head becomes wider behind the eyes with rounded occipital lobes present, and unlike the similar-looking S. richteri, the lobes peak further than the midline, but the occipital excision is not as crease-like. The scapes in major workers do not extend beyond occipital peak by one or two scape diameters; this feature is more noticeable in S. richteri. In medium-sized workers, the scapes reach the occipital peaks and exceed the rear border in the smallest workers. In small and medium workers, the head tends to have more elliptical sides. The head of small workers is wider out front than it is behind. In the major workers, the pronotum does not have any angular shoulders, nor does it have any sunken posteromedian area. The promesonotum is convex and the propodeum base is rounded and also convex. The base and declivity are of equal length. …

Red imported fire ants are native to the tropical areas of central South America, where they have an expansive geographical range that extends from southeastern Peru to central Argentina, and to the south of Brazil. In contrast to its geographical range in North America, its range in South America is significantly different. It has an extremely long north–south range, but a very narrow east–west distribution. The northernmost record of the red imported fire ant is Porto Velho in Brazil, and its southernmost record is Resistencia in Argentina; this is a distance of about 3,000 km (1,900 mi). In comparison, the width of its narrow range is about 350 km (220 mi), and this is most likely narrower into southern Argentina and Paraguay and into the northern areas of the Amazon River basin. Most known records of the red imported fire ant are around the Pantanal region of Brazil. However, the interior of this area has not been examined thoroughly, but it is certain that the species occurs in favourable locations around it. The Pantanal region is thought to be the original homeland of the red imported fire ant; hydrochore dispersal via floating ant rafts could easily account for the far south populations around the Paraguay and Guaporé Rivers. The western extent of its range is not known exactly, but its abundance there may be limited. …

Red imported fire ants are extremely resilient and have adapted to contend with both flooding and drought conditions. If the ants sense increased water levels in their nests, they link together and form a ball or raft that floats, with the workers on the outside and the queen inside. The brood is transported to the highest surface. They are also used as the founding structure of the raft, except for the eggs and smaller larvae. Before submerging, the ants will tip themselves into the water and sever connections with the dry land. In some cases, workers may deliberately remove all males from the raft, resulting in the males drowning. The longevity of a raft can be as long as 12 days. Ants that are trapped underwater escape by lifting themselves to the surface using bubbles which are collected from submerged substrate. Owing to their greater vulnerability to predators, red imported fire ants are significantly more aggressive when rafting. Workers tend to deliver higher doses of venom, which reduces the threat of other animals attacking. Due to this, and because a higher workforce of ants is available, rafts are potentially dangerous to those that encounter them. Necrophoric behaviour occurs in the red imported fire ant. Workers discard uneaten food and other such wastes away from the nest. …

The red imported fire ant is a significant agricultural pest in areas where it is not native. They are capable of damaging crops, and they threaten pastures and orchards. The mounds themselves can destroy agricultural equipment such as irrigation systems and damage machinery during harvest time. The ants are known to invade soybean crops causing lower yields, and could cause $156 million in losses for soybean crops in the southeastern United States. Reports from Georgia and North Carolina claim that 16.8 to 49.1 kg/ha of soybeans could not be harvested due to interference from fire ant mounds; combine harvesters skipped over the mounds preventing the crops from being harvested, and farmers raised the cutter bars on their combine harvesters to avoid impacting the mounds. The feeding behaviour of red imported fire ants can result in significant damage to many other crops, including: beans, cabbage, citrus, corn, cucumber, eggplant, okra, peanut, potato, sorghum, sunflower, and sweet potato. The ants also interfere with plants' root systems and feed on young growth. Sometimes, colonies build mounds around or near the base of citrus trees chew off new growth and feed on the flowers or developing fruit. Citrus trees are often girdled or killed. Despite its pest status and notoriety, the red imported fire ant can be beneficial. …

The species name of the red imported fire ant, invicta, derives from Latin, and means "invincible" or "unconquered". The epithet originates from the phrase Roma invicta ("unconquered Rome"), used as an inspirational quote until the fall of the Western Roman Empire in 476 AD. The generic name, Solenopsis, translates as "appearance of a pipe". It is a compound of two Ancient Greek words, solen, meaning "pipe" or "channel", and opsis, meaning "appearance" or "sight". The ant is commonly known as the "red imported fire ant" (abbreviated as RIFA) outside of South America; the "fire ant" part is because of the burning sensation caused by its sting. Alternative names include "fire ant", "red ant" or "tramp ant". In Brazil, locals call the ant toicinhera, which derives from the Portuguese word toicinho (pork fat).