Red Imported Fire Ant

The **red imported fire ant** (*Solenopsis invicta*), also known as the **fire ant** or **RIFA**, is a species of ant native to South America. A member of the genus *Solenopsis* in the subfamily Myrmicinae, it was described by Swiss entomologist Felix Santschi as a variant in 1916. Its current name *invicta* was given to the ant in 1972 as a separate species. However, the variant and species were the same ant, and the name was preserved due to its wide use. The red imported fire ant is native to South America but it has been accidentally introduced in Australia, New Zealand, several Asian and Caribbean countries and the United States. The red imported fire ant is polymorphic as workers appear in different shapes and sizes. The ant's colours are red and somewhat yellowish with a brown or black gaster. However, males are completely black. Red imported fire ants are dominant in altered areas and live in a wide variety of habitats. They can be found in rain forests, in disturbed areas, deserts, grasslands, alongside roads and buildings, and in electrical equipment. Colonies form large mounds constructed from dirt with no visible entrances because foraging tunnels are built and workers emerge far away from the nest.

These ants exhibit a wide variety of behaviours, such as building rafts when they sense that water levels are rising. They also show necrophoric behaviour, where nestmates discard scraps or dead ants on refuse piles outside the nest. Foraging takes place on warm or hot days, although they may remain outside at night. Workers communicate by a series of semiochemicals and pheromones which are used for recruitment, foraging, and defence. They are omnivores and eat dead mammals, arthropods, insects, seeds, and sweet substances such as honeydew from hemipteran insects with whom they have developed relationships. Predators include arachnids, birds and many insects including other ants, dragonflies, earwigs, and beetles. The ant is a host to parasites and to a number of pathogens, nematodes, and viruses which have been viewed as potential biological control agents. Nuptial flight occurs during the warm seasons, and the alates may mate for as long as 30 minutes. Colony founding can be done via a single queen or a group of queens, who will later contest for dominance once the first workers emerge. Workers can live for several months while queens can live for years; colony numbers can vary from 100,000 to 250,000 individuals. Two forms of society in the red imported fire ant exist: polygynous colonies (nests with multiple queens) and monogynous colonies (nests with one queen).

Venom plays an important role in the ant's life as it is used to capture prey or for defence. About 95% of the venom's components are water-insoluble piperidine alkaloids, and it is particularly potent on sensitive humans. More than 14 million people are stung by them in the United States annually, where many are expected to develop allergies to the venom. Most victims experience intense burning and swelling, followed by the formation of sterile pustules, which may remain for several days. However 0.6% to 6.0% of people may suffer from anaphylaxis, which can be fatal if left untreated. Common symptoms include dizziness, chest pain, nausea, severe sweating, low blood pressure, loss of breath, and slurred speech. More than 80 deaths have been recorded from red imported fire ant attacks. Treatment depends on the symptoms; those who only experience pain and pustule formation require no medical attention, but those who suffer from anaphylaxis are given epinephrines. Whole body extract immunotherapy is used to treat victims and is regarded as highly effective.

The ant is viewed as a notorious pest, causing billions of dollars in damages annually and impacting wildlife. The ants thrive in urban areas, so their presence may deter outdoor activities. Nests can be built under structures such as pavements and foundations, which may cause structural problems, or cause them to collapse. Not only can they damage or destroy structures, but red imported fire ants also can damage equipment and infrastructure and impact business, land, and property values. As workers are attracted to electricity, they can swarm electrical equipment and destroy it. In agriculture, they can damage crops, damage machinery, and threaten pastures. They are known to invade a wide variety of crops, and mounds built on farmland may prevent harvesting. They also pose a threat to animals and livestock, capable of inflicting serious injury or killing them, especially weak or sick animals. Despite this, they may be beneficial because they consume common pest insects on crops. Common methods of controlling these ants include baiting and fumigation; other methods may be ineffective or dangerous. Due to its notoriety and importance, the ant has become one of the most studied insects on the planet, even rivalling the western honey bee (*Apis mellifera*).

## Description

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 thoraciclength 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. The suture of the promesonotum is either strong or weak in larger workers. The petiole has a thick and blunt scale; if observed from behind, it is not as rounded above in contrast to *S. richteri*, and sometimes it may be subtruncate. The postpetiole is large and broad, and in the larger workers, it is broader than its length. The postpetiole tends to be less broad in front and broader behind. On the rear side of the dorsal surface, a transverse impression is present. In *S. richteri*, this feature is also present but much weaker.

The sculpture is very similar to *S. richteri*. The punctures are from where pilosity arises, and these are often elongated on the dorsal and ventral portions of the head. On the thorax, striae are present, but they are less engraved with fewer punctures than in *S. richteri*. On the petiole, the punctates are located on the sides. The postpetiole, when viewed above, has a strong shagreen with distinct transverse punctostriae. The sides are covered in deep punctures, where they appear smaller but deeper. In *S. richteri*, the punctures are larger and more shallow. This gives a more opaque appearance to the surface. In some cases, punctostriae may be present around the rear portion. The pilosity appears similar to that of *S. richteri*. These hairs are erect and vary in length, appearing long on each side of the pronotum and mesonotum; on the head, the long hairs are seen in longitudinal rows. Numerous appressed pubescent hairs are on the petiolar scale; this is the opposite in *S. richteri*, as these hairs are sparse. Workers appear red and somewhat yellowish with a brown or completely black gaster. Gastric spots are sometimes seen in larger workers, where they are not as brightly coloured as those in *S. richteri*. The gastric spot usually covers a small portion of the first gastric tergite. The thorax is concolorous, ranging from light reddish-brown to dark-brown. The legs and coxae are usually lightly shaded. The head has a consistent colour pattern in large workers, with the occiput and vertex appearing brown. Other parts of the head, including the front, genae, and the central region of the clypeus, are either yellowish or yellowish brown. The anterior borders of the genae and mandibles are dark-brown; they also both appear to share the same coloured shade with the occiput. The scapes and funiculi range from being the same colour as the head or shares the same shade with the occiput. Light-coloured areas of the head in small to medium-sized workers is restricted to only the frontal region, with a dark mark resembling an arrow or rocket being present. On occasion, nests may have a series of different colours. For example, workers may be much darker, and the gastric spot may be completely absent or appear dark-brown.

Queens have a head length of 1.27 to 1.29 mm (0.050 to 0.051 in) and a width of 1.32 to 1.33 mm (0.052 to 0.052 in). The scapes measure 0.95 to 0.98 mm (0.037 to 0.039 in) and the thorax is 2.60 to 2.63 mm (0.102 to 0.104 in). The head is almost indistinguishable from *S. richteri*, but the occipital excision is less crease-like and the scapes are considerably shorter. Its petiolar scale is convex and resembles that of *S. richteri*. The postpetiole has straight sides that never concave, unlike in *S. richteri* where they concave. The thorax is almost identical, but the clear space between the metapleural striate area and propodeal spiracles is either a narrow crease or not present. The side portions of the petiole are punctate. The sides of the postpetiole are opaque with punctures present, but no irregular roughening is seen. The anterior of the dorsum is shagreen, and the middle and rear regions bear transverse puncto-striae. All these regions have erect hairs. The anterior portions of both the petiole and postpetiole have appressed pubescence that is also seen on the propodeum. The colour of the queen is similar to that of a worker: the gaster is dark brown and the legs, scapes, and thorax are light brown with dark streaks on the mesoscutum. The head is yellowish or yellowish-brown around the central regions, the occiput and mandibles are a similar colour to the thorax, and the wing veins range from colourless to pale brown. Males appear similar to *S. richteri*, but the upper borders of the petiolar scales are more concave. In both species, the postpetiole's and petiole's spiracles strongly project. The whole body of the male is concolorous black, but the antennae are whitish. Like the queen, the wing veins are colourless or pale brown.

The red imported ant can be misidentified as the similar-looking *S. richteri*. The two species can be distinguished from each other through morphological examinations of the head, thorax, and postpetiole. In *S. richteri*, the sides of the head are broadly elliptical and the cordate shape seen in the red imported fire ant is absent. The region of the occipital lobes that are situated nearby the midline and occipital excision appear more crease-like in *S. richteri* than it does in the red imported fire ant. The scapes of *S. richteri* are longer than they are in the red imported fire ant, and the pronotum has strong angulate shoulders. Such character is almost absent in the red imported fire ant. A shallow but sunken area is only known in the larger workers of *S. richteri*, which is located in the posterior region of the dorsum of the pronotum. This feature is completely absent in larger red imported fire ant workers. The red imported fire ant's promesonotum is strongly convex, whereas this feature is weakly convex in *S. richteri*. Upon examination, the base of the propodeum is elongated and straight in *S. richteri*, while convex and shorter in the red imported fire ant. It also has a wide postpetiole with either straight or diverging sides. The postpetiole in *S. richteri* is narrower with converging sides. In *S. richteri*, the transverse impression on the posterodorsal portion of the postpetiole is strong, but weak or absent in the red imported fire ant. As well as that, *S. richteri* workers are 15% larger than red imported fire ant workers, are blackish-brown, and have a yellow stripe on the dorsal side of the gaster.

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