Solenopsis invicta

Solenopsis invicta

The red imported fire ant or fire ant, RIFA (Solenopsis invicta Buren, 1972) is an insect belonging to the Formicidae family.

Systematic –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Phylum Arthropoda,
Class Insecta,
Order Hymenoptera,
Suborder Apocrite,
Superfamily Vespoidea,
Formicidae family,
Subfamily Myrmicinae,
Solenopsidini Tribe,
Genus Solenopsis,
Species S. invicta.
The terms are synonymous:
– Solenopsis saevissima var. wagneri Santschi, 1916;
– Solenopsis wagneri Santschi, 1916.

Geographic Distribution and Habitat –
Solenopsis invicta is an ant native to a vast range that includes: Argentina, Brazil, Uruguay and Paraguay, up to 33° south latitude, 64° west longitude and 1100 m above sea level. However, it was not found in Bolivia.
This ant, in its native countries, is controlled in its expansion by natural enemies.
However, this species has been introduced in various places in the world, such as New Zealand, Australia, Taiwan, the Philippines and China, but above all in the southern United States, where, in the absence of natural enemies, it has spread rapidly causing extensive damage, and is classified as one of the most harmful invasive species in the world.
Finally, in 2023, it also landed in Italy, after having already conquered a large part of the globe: The first sighting was of 88 nests in Sicily, near Syracuse, and it is the first official sighting for Europe.
S. invicta is an ant that successfully competes against other local ants, expanding its range, especially in the United States, where it has gradually spread throughout the North and West, despite intense efforts to stop or eliminate it. They can currently be found in most Southern states: Alabama, Arkansas, Florida, Georgia, Louisiana, Maryland, Mississippi, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, and Virginia. The U.S. Department of Agriculture also lists a county in New Mexico, another in California and the entire island of Puerto Rico as infested. Additionally, there are reports of S. invicta ants in the San Francisco area.

Morphology –
Solenopsis invicta is an ant that can be identified following the above descriptions.
Workers range in size from small to medium, making them polymorphic. They measure between 2.4 and 6.0 mm.
They have a poisonous stinger that inflicts very painful stings, comparable to a lit match stuck under the skin.
The head measures 0.66 to 1.41 mm and is 0.65 to 1.43 mm wide. In the largest workers their heads measure 1.35 to 1.40 mm and 1.39 to 1.42 mm wide.
The antennal scapes measure 0.96 to 1.02 mm and the thoracic length is 1.70 to 1.73 mm.
The head widens behind the eyes with the presence of rounded occipital lobes, and unlike similar-looking S. richteri, the lobes peak beyond the midline, but the occipital excision is not as crease-like. The scapes in the major workers do not extend beyond the occipital peak by one or two scape diameters; this feature is more evident in S. richteri. In medium-sized workers the scapes reach the occipital tips and exceed the posterior edge in smaller workers. In small and medium-sized workers the head tends to have more elliptical sides. The head of the little workers is wider at the front than at the back. In the larger workers the pronotum does not have angular shoulders or a sunken posteromedian area. The promesonotum is convex and the base of the propodeum is rounded and also convex. The base and the slope are the same length. The promesonotal suture is strong or weak in larger workers. The petiole has a thick and obtuse scale; when viewed from behind, it is not as rounded superiorly unlike S. richteri, and can sometimes be subtruncated. The postpetiole is large and broad and, in larger workers, is wider than its length. The postpetiole tends to be narrower in front and wider in back. On the posterior side of the dorsal surface there is a transverse impression. This characteristic is also present in S. richteri but much weaker.
There are striae on the thorax, but they are less incised and with fewer punctures than S. richteri. On the petiole dotted dots are located on the sides. The postpetiole, when viewed from above, has a strong shagreen with distinct transverse punctostriae. The sides are covered with deep punctures, where they appear smaller but deeper.
The hairiness appears similar to that of S. richteri. These hairs are erect and of variable length, appearing long on each side of the pronotum and mesonotum; on the head the long hairs are seen in longitudinal rows. Numerous learned pubescent hairs are found on the petiole 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 colorful as those of S. richteri. The gastric stain usually covers a small portion of the first gastric tergite. The thorax is co-colored, from light reddish-brown to dark brown. The legs and coxa are generally lightly shaded. The head has a consistent color pattern in large workers, with the occiput and vertex appearing brown. Other parts of the head, including the anterior part, the gene and the central region of the clypeus, are yellowish or yellowish brown. The anterior edges of the gene and mandibles are dark brown; they also appear to share the same color tone with the occiput. The scapes and funiculi vary from being the same color as the head or sharing the same shade with the occiput. The light areas of the head in small and medium-sized workers are limited to the frontal region only, with the presence of a dark mark resembling an arrow or rocket.
Queens have a head length of 1.27 to 1.29 mm and a width of 1.32 to 1.33 mm.
The scapes measure from 0.95 to 0.98 mm and the thorax from 2.60 to 2.63 mm.
The head is almost indistinguishable from S. richteri, but the occipital excision is less folded and the scapes are noticeably shorter. Its petiole scale is convex and resembles that of S. richteri. The postpetiole has straight sides and never concave, unlike S. richteri where they are concave. The thorax is almost identical, but the free space between the metapleural striated area and the propodeal spiracles is a narrow fold or is not present. The lateral portions of the petiole are dotted. The sides of the postpetiole are dull with punctures present but no irregular roughening is seen. The anterior part of the back is shagreen green and the central and posterior regions bear transverse punctual striae. All these regions have erect hairs. The anterior portions of both the petiole and the postpetiole have an appreciated pubescence which is also seen on the propodeum.
The color 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 similar in color to the thorax, and the wing veins are colorless to light brown. Males appear similar to S. richteri, but the upper edges of the petiole scales are more concave. In both species, the spiracles of the postpetiole and petiole are strongly protruding. The entire body of the male is black with color, but the antennae are whitish. As in the queen, the wing veins are colorless or light brown.

Aptitude and biological cycle –
Solenopsis invicta has a reproductive cycle that begins with the nuptial flight that begins during the warmest seasons of the year (spring and summer), usually two days after the rain. The time when the alates emerge is between noon and 3pm.
Males are the first to leave the nest and both sexes readily undertake flight with little or no preflight activity. However, the workers swarm onto the mound stimulated with excitement by the mandibular glands inside the head of the alates. Since the mounds have no holes, the workers form holes during the nuptial flight to let the alates emerge. This behavior in workers, elicited by pheromones, includes rapid running, back-and-forth movements and increased aggression.
Workers also cluster around the alates as they climb vegetation and, in some cases, attempt to pull them down before they take flight. Chemical signals from males and females during the nuptial flight attract workers, but chemical signals released by workers do not attract other nestmates. It also induces alarm recruitment behaviors in workers which results in a higher rate of late recovery.
Males fly at higher altitudes than females: captured males are usually 100 to 300 m above the surface, while females are only 60 to 120 m above the surface.
A nuptial flight lasts about half an hour, and females generally fly less than a mile before landing.
About 95% of queens mate successfully and mate only once; some males may be sterile due to failure of the testicular lobes to develop. In polygynous colonies males do not play a significant role and most are therefore sterile; one reason is to avoid mating with other ant species. This also makes male mortality selective, which can affect the breeding system, mating success and gene flow.
Ideal conditions for initiating a nuptial flight are when humidity levels are above 80% and when ground temperatures are above 18°C (64°F).
Nuptial flights occur only when the ambient temperature is between 24 and 32 °C (75–89 °F).
Queens are often found 1-2.3 miles from the nest from which they flew. The founding of colonies can be carried out by an individual or by groups, a phenomenon known as pleometrosis. This joint effort by the co-founders contributes to the growth and survival of the nascent colony; Nests founded by multiple queens begin the growth period with three times as many workers as colonies founded by a single queen.
Despite this, such associations are not always stable. The emergence of the first workers instigates the struggle between queen-queen and queen-worker. Under pleometrotic conditions, only one queen wins, while the losing queens are subsequently killed by the workers. The two factors that could influence the survival of individual queens are their relative fighting abilities and their relative contribution to worker production. Size, an indicator of fighting capacity, is positively correlated with survival rates. However, manipulating the queen’s relative contribution to worker production had no correlation with survival rate.
A single queen lays approximately 10-15 eggs 24 hours after mating. In established nests, a queen applies poison to each egg, which perhaps contains a signal telling workers to move it. These eggs remain the same size for a week until they hatch and become larvae. At this point, the queen will have laid 75 to 125 more eggs. The larvae that emerge from the eggs usually remain covered by the shell membranes for several days. The larvae can free their mouthparts from the shell using body movements, but still require assistance from workers during hatching. The larval stage is divided into four stages, as observed during the moulting stages. At the end of each moult, a piece of unknown material is seen connected to the exuviae if isolated from the workers. The larval stage lasts six to 12 days before their bodies expand significantly and become pupae; the pupal phase lasts from nine to 16 days.
As soon as the first individuals reach the pupal stage, the queen stops producing eggs until the first workers mature. This process takes from two weeks to a month. The young larvae are fed with oils regurgitated from its crop, as well as with eggs or trophic secretions. It also nourishes the young with its wing muscles, providing them with the necessary nutrients. The first generation of workers is always small due to the limit of nutrients needed for development. These workers are known as minims or nanites, who burrow out of the queen’s chamber and begin searching for food needed by the colony. The construction of mounds also takes place in this period.
Within a month of the birth of the first generation, larger workers (major workers) begin to develop, and within six months the mound will be visible, if observed, and home to several thousand residents. A mature queen is capable of laying 1,500 eggs a day; remember that all workers are sterile, therefore they cannot reproduce.
A colony can grow exceptionally fast. Colonies housing 15-20 workers in May grew to over 7,000 in September. Field tests found that these colonies began producing reproductive ants when they were one year old and by the time they were two years old they had over 25,000 workers. The population doubled to 50,000 by the time these colonies were three years old. At maturity, a colony can host 100,000 to 250,000 individuals, but other reports suggest that colonies can contain more than 400,000. colonies.
The factors that contribute to the growth of the colony are different. Temperature plays an important role in the growth and development of colonies; Colony growth ceases below 24°C and development time decreases from 55 days at 24°C to 23 days at 35°C. Growth in established colonies occurs only at temperatures between 24 and 36 °C. The dwarf brood also develops much more quickly than the lesser worker brood (about 35% faster), which is advantageous for establishing colonies. Colonies that have access to an unlimited supply of insect prey are known to grow substantially, but this growth is further accelerated if they are able to access plant resources colonized by hemipteran insects. In incipient monogynous colonies where diploid males are produced, colony mortality rates are significantly high and colony growth is slow. In some cases, monogynous colonies have 100% mortality rates in the early stages of development.
The life expectancy of a worker ant depends on its size, although the overall average is around 62 days. Minor workers are expected to live 30 to 60 days, while larger workers live much longer. Larger workers, who have a life expectancy of between 60 and 180 days, live 50-140% longer than their smaller colleagues, but this depends on temperature. However, workers kept in laboratory conditions are known to live 10 to 70 weeks (70 days to 490 days); the maximum recorded longevity of a worker is 97 weeks (or 679 days). Queens live much longer than workers, with a lifespan ranging from two to nearly seven years.
In colonies, queens are the only ants capable of altering sex ratios that can be predicted. For example, queens from male-producing colonies tend to produce predominantly males, while queens from female-favored colonies tend to produce females. Queens also exert control over the production of sexual organs through pheromones that influence the workers’ behavior towards both male and female larvae.

Ecological Role –
Solenopsis invicta is a more aggressive ant than most ant species native to North America and has, as mentioned, a painful sting. Generally, a person encounters them by carelessly stepping on one of their mounds, which causes the ants to climb up the person’s legs, attacking en masse. Ants respond to pheromones released by the first attacking ant. They then bite at the same time, often causing the death of small animals by overloading their immune systems.
This ant is always on the move, often traveling from one area to another among grasses, nursery roots, and other agricultural products. They are harmful to humans, not only because of the physical pain they can inflict, but because the construction of their anthills can damage plant roots, resulting in crop loss, and interfere with mechanized cultivation. It is quite common for several mounds of S. invicta to suddenly appear in a suburban garden or farmer’s field, seemingly overnight.
Their stings rarely pose a threat to the lives of people or large animals, although they have caused the death of at least eighty people from anaphylactic shock due to allergy to their sting. However, they often kill small animals such as birds or even small newborn calves if they don’t get up fast enough. The bite of S. invicta contains a venom containing a natural alkaloid that displays potent necrotoxic activity and causes both pain and the formation of white pustules that appear one day after the bite. The rest of the venom contains an aqueous solution of proteins, peptides, and other small molecules that produce the allergic reaction in hypersensitive individuals.
This species is very hardy and has adapted to survive both flood and drought conditions. If the ants sense a rise in the water level in their colonies, they will band together to form a huge ball or raft that can float on the water, with the workers on the outside and the queen on the inside. Once the ball hits a tree or other stationary object, the ants climb onto it and wait for the water level to recede. To survive in drought conditions, their colony structure contains a network of underground collection tunnels that extend below the water table. Additionally, although they do not hibernate during the winter, colonies can survive in cold conditions at temperatures as low as 9°C.
The preferential diet of S. invicta consists of the protein substances it obtains from the insects it preys on. In any case, the diet can include various components of animal (invertebrates and vertebrates) or vegetal origin, including oily and sugary substances, although it does not often consume extrafloral nectar and rarely carries out collection activities.
Solenopsis invicta have negative effects on seed germination. The extent of damage, however, depends on the duration of seed vulnerability (dryness and germination) and the abundance of ants. One study has shown that while these ants are attracted to and remove seeds that have adapted to ant dispersal, this species damages these seeds or moves them to locations unfavorable for germination. In seeds donated to colonies, 80% of Sanguinaria canadensis seeds were scarified and 86% of Viola rotundifolia seeds were destroyed. Small percentages of Pinus palustris seeds deposited by workers successfully germinate, thus providing evidence that imported fire ants aid seed movement in the longleaf pine ecosystem. Seeds containing elaiosomes are harvested at a higher rate than seeds not containing elaiosomes and are not stored in their nests, but rather in surface litter piles in the vicinity of the mound.
The fire ant is an important agricultural pest in areas where it is not native. They are capable of damaging crops and threatening pastures and orchards. The mounds themselves can destroy agricultural equipment such as irrigation systems and damage machinery during harvest time. In some states, in addition to large crop losses, combines jumped over mounds, preventing crops from being harvested, and farmers raised the cutter bars of their combines to avoid hitting the mounds, losing some of the threshing. The feeding behavior of imported fire ants can cause significant damage to many other crops, including: beans, cabbage, citrus, corn, cucumbers, eggplant, okra, peanuts, potatoes, sorghum, sunflower, and sweet potatoes. Ants also interfere with the root system of plants and feed on young shoots. Sometimes, colonies build mounds around or near the base of citrus trees as they chew on new growth and feed on the developing flowers or fruit. Citrus trees are often girdled or killed.
However, despite these aspects, the red ant can be useful. The ant is an effective insect predator, so it can serve as a biological agent against other pest species, especially in sugarcane fields. Pest insects that the ant kills include: weevils (Anthonomus grandis) in cotton crops, sugarcane borers (Diatraea saccharalis) in sugarcane fields, flies of the genus Haematobia irritans in manure, velvet bean caterpillars (Anticarsia gemmatalis ) in soybeans and whiteflies found in greenhouses. Numerous studies indicate that fire ants do not interfere with or attack economically important insects in cotton fields, which has led many farmers in the southeastern regions of the United States to consider these ants beneficial.
However, some scientists have suggested that the beneficial status of the fire ant is difficult to predict when geography, plant size, season, soil moisture and insecticide use are not taken into account. These factors can reduce the effectiveness of fire ants as pest control agents. Another factor is that workers are indiscriminate and kill beneficial insects and other pests in pastures and predators of aphids and scale insects. They also reduce the effectiveness of parasitic wasps against pest species by eating the larvae and pupae.
To contain this ant, both conventional and biological control methods have been developed.
Phoridae, parasitoid dipterans belonging to the Pseudacteon tricuspis and Pseudacteon curvatus species, or anteaters are often used in biological control.
Compared to other ant species such as Anoplolepis gracilipes, which quickly take over areas where they have been contained, fire ants are quite easy to control. The first proposals for ant control date back to 1957, when the U.S. Congress authorized an eradication program using federal and state funding. Research on the ant and its biology continued after an eradication program was established, and many chemicals were used to eliminate it. However, scientists found that these insecticides were killing native wildlife, and the Environmental Protection Agency subsequently outlawed them. Some scientists have even questioned whether ants are parasites or not. Today, the fire ant is unlikely to be eradicated in areas such as the United States. Populations can be managed adequately if an integrated approach is used. Some scientists have considered using the ants’ natural enemies against it; this includes Kneallhazia solenopsae and B. bassiana. Phorid flies have also been viewed as potential biological agents, as they can reduce foraging activity in imported fire ants and affect population levels. However, they are unable to influence the growth rate of colonies. Additionally, parasitic ants, parasitic wasps, mites, other pathogens, nematodes, and fungi have been considered potential biological agents. Others suggest that populations can be maintained or reduced by manipulating various ecological factors.
Among the various control systems, different baits have been used to control populations. The mounds are destroyed within a few weeks if bait is used on them. Baits are considered effective and simple to use against fire ants, compared to drenching, dusting or fumigation. They are sprinkled on the mound then the ants take them and consume them. Some baits, such as growth regulator baits and boric acid and sucrose water baits, benefit native wildlife and low concentrations are usually needed to kill a colony. Other baits used against red imported fire ants include Amdro, Ascend, hydramethylnon and Maxforce.
In addition to chemical control (the negative counter-effects of which are increasingly known), other methods that can be employed against these ants include mechanical and electrical devices. However, it is not known whether these devices are effective or not. Ant protection can be effective against colonies nesting inside buildings by sealing cracks, successfully suppressing the population outside the walls. Homeowners have used their own methods to remove the piles by pouring boiling water on them or lighting them with flammable liquids. While these methods can be effective, they are not recommended because they can be harmful to humans and the environment.
Solenopsis invicta has been included by IUCN specialists in the list of the 100 most harmful invasive species in the world.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– Russo G., 1976. Agricultural Entomology. Special Part. Liguori Editore, Naples.
– Pollini A., 2002. Manual of applied entomology. Edagricole, Bologna.
– Tremblay E., 1997. Applied entomology. Liguori Editore, Naples.

Photo source:
– https://inaturalist-open-data.s3.amazonaws.com/photos/265526419/original.jpg