Red imported fire ant
The red imported fire ant (Solenopsis invicta), or simply RIFA, is one of over 280 species in the widespread genus Solenopsis. Although the red imported fire ant is native to South America, it has become a pest in the southern United States, Australia, the Caribbean, Thailand, Taiwan, the Philippines, Hong Kong, the southern Chinese provinces of Guangdong, Guangxi and Fujian, and Macau (the former Portuguese enclave that borders the province of Guangdong). RIFA are known to have a painful, persistently irritating sting that often leaves a pustule on the skin.
- 1 Overview
- 2 Morphology
- 3 Physiology
- 4 Behavior
- 4.1 Queen and worker behavior
- 4.2 Green Beard Altruism
- 4.3 Necrophoric Behavior
- 4.4 Behavioral Theory on Invasive Species
- 5 Economic impact
- 6 Countermeasures
- 7 Genomics
- 8 See also
- 9 References
- 10 External links
RIFAs are more aggressive than most ant species, and have a painful sting. Animals, including humans, often encounter them by inadvertently stepping on one of their mounds, which causes the ants to swarm up the legs, attacking en masse. The ants respond to pheromones released by the first ant that attacks, thereafter stinging in concert. They can even kill smaller animals by overloading their immune systems.
RIFAs successfully compete against other ants, and have been expanding their range. Recently, colonies of Rasberry crazy ant (also known as old world crazy ants ) have been introduced in the same ranges as RIFAs. These Crazy Ants are ecologically dominant over fire ants, which has been limiting their range slightly.
They are considered to be a pest, not only because of the physical pain they can inflict, but also because their mound-building activity can damage plant roots, lead to loss of crops, and interfere with mechanical cultivation. It is not uncommon for several fire ant mounds to appear suddenly in a suburban yard or a farmer's field, seemingly overnight. Their stings are rarely life-threatening to humans and other large animals, causing only 80 documented deaths as of 2006. However, they often kill smaller animals, such as birds. They sometimes kill newborn calves if they do not get on their feet quickly enough. The sting of the RIFA has venom composed of a necrotizing alkaloid, which causes both pain and the formation of white pustules that appear one day after the sting.
Fire ants are excellent natural predators and can be used as biological controls for pests such as the sugarcane borer, the rice stink bug, the striped earwig, aphids, the boll weevil, the soybean looper, the cotton leafworm, the hornfly, and many other pests harmful to crops. However, they also kill beneficial pollinators, such as ground-nesting bee species. Seeds, fruits, leaves, roots, bark, nectar, sap, fungi, and carrion are all fire ant prey. They are proficient enough at overwhelming intruders that they can virtually clear an area of invertebrates, lizards, and ground-dwelling birds.
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 will come together and form a ball or raft that floats, with the workers on the outside and the queen inside. Once the ball hits a tree or other stationary object, the ants swarm onto it and wait for the water levels to recede. To contend with drought conditions, their nest structure includes a network of underground foraging tunnels that extends down to the water table. Also, although they do not hibernate during the winter, colonies can survive temperatures as low as 16 °F (−9 °C).
Red imported fire ants have both a pedicel and postpediole. In other words, they belong to a group of ants that have two humps between the thorax and abdomen. The workers have ten antennal segments terminating in a two-segmented club. It is often difficult to distinguish between the red imported fire ant, Solenopsis invicta, and some other species in the genus. Characteristic differences are not always consistent between the black imported fire ant (Solenopsis richteri) or hybrids between the two species. In fact, positive identifications can often only be made using high performance liquid chromatography (HPLC) to show differences in cuticular hydrocarbons.
Like other insects, S. invicta breathes through a system of gas filled tubes called tracheae connected to the external environment through spiracles. The terminal tracheal branches (tracheoles) make direct contact with internal organs and tissue. The transport of oxygen (O2) to cells (and carbon dioxide (CO2) out of cells) occurs through diffusion of gasses between the tracheoles and the surrounding tissue and is assisted by discontinuous gas exchange (DGC). As with other insects, the direct communication between the tracheal system and tissues eliminates the need for a circulating fluid network to transport O2. Thus, S. invicta and other arthropods can have a modest circulatory system even though they have highly expensive metabolic demands.
S. invicta faces many respiratory challenges due to its highly variable environment, which can cause increased dessication, hypoxia, and hypercapnia. Hot, humid climates cause an increase in heart rate and respiration which increases energy and water loss. Hypoxia and hypercapnia can result from S. invicta colonies living in poorly ventilated thermoregulatory mounds and underground nests. DGC may allow ants to survive the hypercapnic and hypoxic conditions frequently found in S. invicta burrows. DGC is ideal for adapting with these conditions. It allows the ants to increase the period of O2 intake and CO2 expulsion independently through spiracle manipulation.
Metabolic rate, which indirectly affects respiration, is also influenced by environmental temperature. Peak metabolism occurs at approximately 32°C. Metabolism, and therefore respiration rate, increases consistently as temperature increases. DGC stops above 25°C, although the reason for this is currently unknown.
Respiration rate also appears to be significantly influenced by caste. Male S. invicta show a considerably higher rate of respiration than females and workers. This is due, in part, to their capability for flight and higher muscle mass. In general, male S. invicta have more muscle and less fat, resulting in a higher metabolic O2 demand. While metabolic rate is highest at 32°C, colonies often thrive at slightly cooler temperatures (around 25°C). The high rate of metabolic activity associated with warmer temperatures is a limiting factor on colony growth because the need for food consumption is also increased. As a result, larger colonies tend to be found in cooler conditions because the metabolic demands required to sustain a colony are also decreased.
Queen and worker behavior
Multiple studies have been conducted on the sex ratios exhibited within colonies of S. invicta. More specifically, it was observed that the queen actually controls the sex ratios. In an experiment, 24 field colonies were selected with highly biased sex ratios in a monogyne population. Eleven of these colonies were male specialists (numerical proportion of males, range: .77 to 1.0), and 13 were female specialists (numerical proportion of males, range: 0.0 to .09). After exchanging queens, twenty-two of the 24 colonies accepted the foreign queen, and 21 of these colonies produced a new batch of offspring 5 weeks later.
Luc Passera and colleagues found that based on the colony that the queen was originally from, they could predict the sex ratios of the of new colony after the switch. For example, post-switching, a colony produced predominantly males if the queen came from a male-producing colony, even if the host colony originally produced mainly females. It is not surprising, then, that they found that queens that came from a male-favored sex ratio colony produced no significant change in the sex-ratio of another male-favored colony post-switch. The same was true for a queen that came from a female-favored sex ratio colony and switched into another female-favored colony. Therefore, it is the queen that determines the sex ratio, not the workers.
Another study compared the inhibition of the number of sexuals (male and female) produced in a single queen colony and a queenless colony. Freshly killed corpses of functional (egg-laying) queens were added daily to queenless colonies. These effectively inhibited the production of sexuals through the excretion of pheromones, although not as effectively as living queens. Conversely, corpses of non-egg-laying queens did not inhibit the production of sexuals when added to queenless colonies. Also, when queens were introduced into queenless colonies that already had developed sexual larvae, workers in the colony executed these larvae. This indicates that the queen’s control over the production of sexuals can be enforced retroactively, even after the larvae are sexualized. These results provide evidence that functional queens exert control over the production of sexuals in S. invicta through pheromones that influence the behaviors of workers toward both male and female larvae.
S. invicta also presents a paradox for kin selection theory. In multiple-queen (polygyne) colonies, the egg-laying queens are, on average, unrelated to one another, so the workers appear to raise new sexuals that are no more closely related to them than are random individuals in a population. This was tested by removing worker/queen pairs engaged in trophallaxis with forceps, and then sampling the allele frequency to estimate for the reference population. Frequencies of the most common allele at each locus in workers from the study colonies are .89, .92, .78, .71, and .81 respectively—the frequencies of these markers have been found to conform to Hardy-Weinberg expectations in past studies. Genotypic data were used to estimate relatedness between the workers and the winged-queens they tended, and it was virtually zero. The results indicate that S. invicta workers tending queens in polygyne nests do so without respect to the relatedness of those queens.
Unrelated queens commonly found a colony cooperatively. This joint effort of the co-foundresses contributes to the growth and survival of the incipient colony. However, such associations are not always stable. The emergence of the first workers instigates queen-queen and queen-worker fighting. The two factors that could affect the survival of individual queens are their relative fighting capabilities and their relative contribution to worker production. Experimentation indicates that size, an indicator of fighting capacity, positively correlates with survival rates. However, manipulation of the queen’s relative contribution to worker production had no correlation with survival rate. It can be assumed that the worker brood cannot favor its mother based on these results.
It has been observed that S. invicta workers not only tend to queens indiscriminately, but they also indiscriminately attack them. Queens producing diploid males reared fewer offspring but were as likely to survive as queens producing only workers. It would have been assumed that if workers controlled queen mortality, they would be expected to discriminate in favor of their mother, therefore increasing their inclusive fitness. This however should favor the queen with the greatest number of daughters during the period of queen execution. The data actually shows that the fights among queens themselves have a strong role in determining which queen survives—the heavier co-foundress was more likely to win. Thus, queen survival is enhanced by high fighting ability relative to co-foundresses, rather than by the number of offspring she has. Workers respond to these queen differences by attacking the previously injured queen to reinforce the effects of competition among the queens.
Green Beard Altruism
The Concept of Green Beard Altruism
W.D. Hamilton (1964) proposed the hypothetical idea that organisms may have certain 'recognition alleles' that are phenotypically observable. Richard Dawkins (1976) later coined this concept the ‘green beard effect.’ Amongst certain species, such a gene imparts three elements upon the bearer: (i) it causes the bearer to present an observable and unique trait; (ii) it enables the bearer to distinguish between individuals that do and do not display this characteristic; and (iii) it leads the bearer to act altruistically toward those that exhibit the trait. Today, there is evidence that a greenbeard gene exists in the fire ant.
Discovery of a Green Beard in S. Invicta
Keller and Ross (1998) discovered the green beard gene in S.invicta at the Gp-9 locus. In their study, Keller and Ross provided an explanation of the deaths of queen that were homozygous dominant at this locus (BB). They found that in polygyne (multiple-queen) colonies, the heterozygous Bb workers execute the homozygous BB queens, initiating reproduction. A red fire ant’s genomic identity at the Gp-9 locus causes Bb workers to murder the queens that are not Bb. These executions are correlated with an odor cue that workers used to differentiate between BB and Bb queens. While this does not ideally represent the classical green-beard principles,where green-beard wearers kill non-wearers, this mechanism of selection is comparable. In correspondence with the Keller and Ross study, Grafen (1998) showed that the Gp-9 locus may be linked to maintaining monogyny and polygyny and genetic separation of the subpopulation. He also pointed out that in their experiment, each BB queen remained alive when it lived in a small colony fragment of a few hundred workers; she was only executed once she returned to the main polygynous colony that already had queens. Workers thereby will only tolerate a BB queen in a colony in which she is the only queen.
Recent Findings of a "Social Chromosome" in S. invicta 
In a recent study, Wang et al. (2013) expanded upon the earlier findings of Keller and Ross, characterizing the genomic region responsible for the fire ants’ social polymorphism; they found that this region is “part of a pair of heteromorphic chromosomes that have many of the key properties of sex chromosomes" (664). Utilizing restriction-site-associated DNA (RAD) tag sequencing, the researchers found that the Sb chromosome only occurs in one type of social organization, similar to the “selection regime” in Y and W chromosomes. The Sb chromosome includes “a large non-recombining region, inversions, an increased amount of repetitive elements and deleterious mutations, resulting in Sb/Sb individuals being non-viable" (664). The various genes within the larger supergene provide integrated control to maintain the phenotypes that Keller and Ross originally described above. In a commentary upon this recent study, Bourke (2013) categorized the Gp-9 system as a subsidiary arrangement that emerged after “the evolution of polygyny, and long after eusociality” (612). Thus, he gave credit to Wang et al. for showing that supergenes buttress both the social behavior and social structure of S. invicta.
Necrophoric Behavior in S. invicta
Necrophoric behavior refers to the disposal of corpses. In many species of ants, workers discard uneaten food and other such wastes in a refuse pile. Pioneering work was undertaken by Wilson (I958), who studied the impetus behind corpse disposal in worker ants, Pogonomyrmex badius Latreille. Filter paper squares treated with acetone extracts of P. badius corpses were carried to the refuse pile in the same manner as corpses. The active component was not identified, but the fatty acids accumulating as a result of decomposition were implicated and bits of paper coated with synthetic oleic acid typically elicited a necrophoric response. The process behind this behavior in imported red fire ants was confirmed by Blum (1970): unsaturated fats, such as oleic acid, elicit corpse removal behavior. Freshly frozen workers of the red imported fire ant were not treated like corpses by their nest mates and were not carried away since there was no decomposition and therefore no accumulation of fatty acids. Furthermore, after healthy workers received a light application of oleic acid and were returned to their nest, workers encountering the treated ants quickly seized them and transported them to the refuse pile.
Social Factors Affecting Necrophoric Behavior
It has been empirically proven that social factors can affect an animal’s response to a chemical cue. Gordon continued the work of Wilson (1958), by examining the effect of social context on the response of ant colonies to oleic acid. He found that colonies responded differently to oleic acid in different social contexts, carrying objects to the midden only in certain situations. Gordon’s results indicate that when a large majority of ants (greater than 15%) are doing midden work or nest maintenance, treated objects were taken to the midden. However, if the majority of the ants were foraging or convening, treated objects were taken to the nest. The colonies respond to oleic acid by quickly relocating the treated object to destinations that are appropriate for their current activities. It is assumed that if a plurality of the colony’s work force is participating in an activity, it is likely that a worker encountering the treated paper is engaging in that activity. Thus, if most ants are participating in midden work, it is likely that the paper will be encountered by a midden worker and will be carried to the midden. On the other hand, if a large percentage of the ants outside the nest are feeding or are part of a group being recruited to a food source, a forager will probably discover the treated object and carry it into the nest as food. The response to oleic acid depends on colony activities at the time treated objects are encountered.
Behavioral Theory on Invasive Species
This area of study distinguishes between behavioral factors influencing different facets of invasion success, such as colonization, establishment, and spread, in order to determine which behaviors are responsible for the effectiveness of some invasive species. For example, high dispersal ability, omnivory, gregariousness, and asexuality enhance the probability of colonization and establishment, but these behaviors do not influence a species competitive ability or its subsequent spread.
Invasive Tendencies in monogyne and polygyne colonies
In its introduced habitat, red imported fire ants exist in two forms: single-queened colonies (monogyne) and multi-queened colonies (polygyne). Introduced populations of fire ants maintain higher nest densities than native populations due to a lack of host specific pathogens and parasites. The higher population densities decrease opportunities for independent colony founding and increase the prevalence of polygynous colonies as dependent colony founding. In such scenarios, mated queens are frequently adopted into established colonies. Nestmate discrimination in polygyne colonies of red imported fire ants is greatly reduced relative to monogyne colonies as a consequence of increased queen number. The reduction in intraspecific aggression leads to a breakdown in territoriality, thus further increasing population density. Due to this phenomenon, polygyne populations are on average twice as dense as monogyne populations. The higher nest densities in turn influence the competitive asymmetry between red imported fire ants and native ants. Although both forms of S. invicta can effectively displace native ants, polygyne populations displace more species than do monogyne populations. The greater abundance of the polygyne form is indicative of its stronger competitive ability, as numerical advantages generally determine the outcome of competitive interactions in ants. The apparent disparity in competitive ability between the two social forms of S. invicta illustrates both how variation in behavioral characteristics can influence invasion success and how shifts in behavioral characteristics following introduction can enhance competitive ability.
An outbreak of the RIFA in Queensland, Australia, was discovered on 22 February 2001. The ants were believed to be present in shipping containers arriving at the Port of Brisbane from the United States. Anecdotal evidence suggests fire ants may have been present in Australia for six to eight years prior to formal identification. While the outbreak is restricted to a small (800 km2) region of southeast Queensland in and around Brisbane, the potential social, economic, and ecological damage prompted the Australian government to respond rapidly. The initial emergency response was followed by the formation of the Fire Ant Control Centre in September 2001. Joint state and federal funding of A$175 million was granted for a six-year eradication program involving the employment of more than 600 staff and the broad-scale baiting of approximately 678.9 km2 between 8 and 12 times, followed by two years of surveillance. Following the completion of the fourth year of the eradication program, the Fire Ant Control Centre estimated eradication rates of greater than 99% from previously infested properties. The latest (May 8) Federal budget confirmed the Program will receive extended Commonwealth funding of approximately A$10 million for at least another two years, until June 2009, to treat the residual infestations found most recently, and to fund validation of the overall treatment and surveillance program. (see:) As in previous years, the States have agreed in principle to match the Federal funding. That decision is set to be ratified in June 2007.[dated info]
According to a press briefing of the Agriculture, Fisheries and Conservation Department of Hong Kong, the territory's authorities have also located several ant-hills of Solenopsis invicta in an artificial wetland in Hong Kong's northwestern section.
People's Republic of China
In the People's Republic of China in January 2005, a controversy arose when it became known that Guangdong's provincial government had suppressed all information about the spread of fire ants in the province since the middle of 2004. Newspapers in neighbouring Hong Kong, including Apple Daily, Ming Pao, Hong Kong Economic Times, Sing Tao Daily and Takungpao (the latter funded by the Chinese government), have also reported the ants have been found in both Shenzhen and Wuchuan in Guangdong province.
There have also been reports of colonies in metro Manila and the Province of Cavite in the Philippines since July 2005; however, since early 2007, they have spread now as far as the Bicol Region.
Since September 2004, Taiwan has been seriously affected by the red fire ant. A few people are reported to have succumbed to venom from the ant stings. A large campaign to kill the ants has been partially effective, but it has not been able to eliminate all of them.
The Food and Drug Administration (FDA) estimates more than US$5 billion is spent annually on medical treatment, damage, and control in RIFA-infested areas. Further, the ants cause approximately US$750 million in damage to agricultural assets, including veterinary bills and livestock loss, as well as crop loss.
Many scientists and agencies are attempting to develop methods to stop the spread of the RIFA. Typically, control has been achieved through pesticide use. From the 1950s into the 1970s, Mirex was extensively used in an attempt to eradicate the species. However, the pesticide inadvertently aided the fire ants' spread by killing numerous native ant species that compete successfully with them. Mirex also caused even broader ecological harm that was often attributed to the fire ants. For example, it was first thought that the ants were linked to the decline of overwintering birds (e.g. the Loggerhead Shrike), but a later study showed that the pesticides were largely to blame. RIFAs have virtually no natural biological control agents native to, or naturalized in, the United States, China, Philippines, or Australia. Current research is focused on introducing biological control agents from the RIFA's native range.
The microsporidian protozoan Thelohania solenopsae and the fungus Beauveria bassiana are promising pathogens. Solenopsis daguerrei, a parasitic ant, invades RIFA colonies to replace the queen in hopes of gaining control of the colony. For this reason, its use as a biological control agent is also being explored.
Pseudacteon tricuspis and Pseudacteon curvatus are parasitoid phorid flies from South America which parasitize the ants. The female flies each lay an egg at the junction of head and thorax of their victims, prompting a jerky dance manoeuvre by the ants. The larva then slowly consumes the contents of the head, decapitating the ant in the process, and uses the exoskeleton as a pupal case.
Phorid flies have been introduced in many places in southeastern United States, and are slowly reproducing and spreading to cover the entire RIFA range. The amount of actual damage done to the ants by phorid flies is minimal, but the ants appear to be aware of the hovering flies, losing their social organization and ceasing foraging. In addition, phorid flies are very species-specific, and should in theory leave native ant species (the fire ants' prime competitor) unmolested.
Scientists at the US Agricultural Research Service also have been able to infect phorid flies with Kneallhazia solenopsae, a spore-producing insect pathogen, to control the population of red imported fire ants. The flies are unharmed by the pathogen and serve as vectors in transmitting the disease to the ants. The pathogen is able to reduce red imported fire ant colonies from 53-100%, and may serve as an effective biological control for the ants.
A virus, SINV-1, has been found in about 20 percent of fire ant fields, where it appears to cause the slow death of infected colonies. It has proven to be self-sustaining and transmissible. Once introduced, it can eliminate a colony within three months. Researchers believe the virus has potential as a viable biopesticide to control fire ants.
In some cases, hastily adopted biological control agents can do more harm than good (such as the western mosquitofish in Australia), and it remains to be seen how much success biological control of the red imported fire ant will have.
Researchers have also been experimenting with extreme temperature change to exterminate RIFAs, such as injecting liquid nitrogen or pressurized steam into RIFA nests. Besides using hot steam, pouring boiling water into ant mounds has been found effective in exterminating their nests. Folk remedies have often sought a rapid increase in temperature by soaking the nest in gasoline or kerosene and lighting it on fire, though this is potentially dangerous. Further, the burning of the nest is ineffective due the tendency of queens to be several feet underground. This confusion stems from the observation that fuel vapor has a near instantaneous lethal effect on the ants.
In Brisbane, Australia, colonies are being eradicated or effectively controlled by ground baiting with food laced with contraceptives that render the colony's queen infertile, and toxicants. Mass baiting was undertaken following detection of the ants around the port of Brisbane and in southwestern Brisbane in 2001. Widespread public reporting of suspect colonies (by sending in samples of ants for identification) allowed mapping of the ant's locations. This was combined with satellite imagery to determine the vegetated habitats most likely to be infiltrated by the ants, and the baits were targeted in these areas. Known infested areas were declared high-risk (Restricted Areas), and any material being moved from these areas which could harbour ants (soil, mulch, potted plants, potting mix, hay bales, construction machinery, etc.) had to be inspected prior to disposal or movement, and bulk waste sent to transfer stations for examination, treatment and disposal. The infestation was initially thought to cover 270 km2, with a density of up to 600,000 colonies/km2 on highly infested sites. As program activity refined data on the infested area, overall size grew to around 80,000 ha by 2006/7. At mid-2007 in the on-going nationally funded eradication campaign, fewer than 100 active colonies were located in the entire South-East Queensland area during the six months between September 2006 and February 2007. The focus of delivering eradication has now switched largely to surveillance, while control and validation measures are expected to continue until 2009. The six-year eradication campaign has cost A$175 million to date, and has just secured funding in principle for a minimum of two more years.
A fire ant genome was sequenced in 2010. This creates new opportunities for research on fire ant behavior, and offers new opportunities for directed control measures that minimize environmental impact. The sequence can be searched and downloaded at antgenomes.org.
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