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The ant fauna of the Bahamas have been well surveyed by the early collections of Wheeler (1905) and more recently on the island of San Salvador by Deyrup (1994). It has been suggested that the fauna on San Salvador likely contain no endemic species, with Florida and Cuba being the likely source of the vast majority of ant species found on the island. Of the 44 species found on the island, 14 species are considered exotics (Deyrup, 1994) [check out my page on ants of San Salvador]. Two fire ant species were found by earlier surveys including the red imported fire ant (Solenopsis invicta Buren) and the ginger or tropical fire ant (S. geminata). This article discusses the effects of invasion and methods for control of the more aggressive and successful invader S. invicta. Research and implications regarding biocontrol of the red imported fire ant are also discussed in later sections.
Accounts have been circulating that fire ants are continuing to expand their range into areas previously thought to be unsuitable habitat. On San Salvador, fire ants are thought to be restricted to degraded and disturbed habitats, though no quantitative observations have been made (Deyrup, 1994). Some scientists have suggested that in the US, temperatures and water availability constraints keep fire ants confined to southern-most states. However, fire ant colonies have recently been found in Portsmouth, Virginia (AP, 2003) and have expanded to inhabit the northern third of the state of Georgia (Minor, 2002). Some think that there is a chance that the fire ants are adapting to survive colder winter temperatures. There is evidence that a hybrid fire ant may be able to better survive warm temperatures by burrowing deeper into the ground (Minor, 2002). However, if S. invicta is able to expand its current range the results could be disastrous [The spread of fire ants in the US].
Fire ants cause a whole host of problems for native insect and plant communities. They also have deleterious effects on agriculture and even have the potential to seriously affect human health. Agricultural damages in excess of a billion dollars a year are caused by S. invicta (Lofgren, 1986). In addition, fire ants cause millions of dollars in damages to electrical systems in the US by gnawing into electrical wires (2). Although the sting of the fire ant is generally considered only a moderate annoyance, it can cause acute allergic reactions in some individuals. In individuals with severe anaphylaxis, fatal reactions can occur. The sensitivity to fire ant stings may be more common than expected though.
A research group at the Medical College of Georgia recently found that 17% of the US population has heightened sensitivity to fire ant stings (Corwin, 2003). This is considerably higher than the 1-2% of people originally thought to be at risk. This puts an estimated 13 million people that live in areas with S. invicta at risk of allergic reaction (Corwin, 2003). Some individuals with hypersensitivity have begun taking immunotherapy shots of purified ant venom as a precautionary measure to boost immune reaction to stings. In addition to human health, pets may also be a risk. The danger of unsuspecting, curious pets nosing around fire ant nest in the yard or local parks causes concern for many pet owners. In fact, “small animals and pets are treated more frequently than any other type of animal for fire-ant related health injuries”(Rosenthal, 2003).
S. invicta individuals are relatively small and common looking ants, they are reddish-brown in color and range from 2-6 mm in size. Fire ants are aggressive predators and feed primarily on other terrestrial invertebrates and arthropods. They may also eat seeds and other plant parts as a primary component of their liquid food. Like other ants, fire ants live in colonies containing workers, males and queens. A mature colony may contain 200,000 – 300,000 individuals (Stiles & Jones, 2001). Colonies may be of two types – containing a single queen (monogynous) or those with multiple queens (polygynous). Monogyne colonies tend to be more territorial and intraspecific competition tends to limit the density of these colonies (Porter & Savignano, 1990). Fire ants build nests, or mounds, out of soil substrate and enter and exit the nest via underground tunnels that radiate out from the center of the nest. The number of mounds in an area can vary greatly but can be as many as 5,900 per hectare in some polygynous colonies (Allen et al., 1995).
The Red Imported Fire Ant (Solenopsis invicta)
The red imported fire ant is a pest of agriculture for a number of reasons. While fire ants are not primarily herbivores and do not typically eat vegetative parts of crop plants, they do use seeds as a food source. Mounds built by S. invicta can also damage farm harvesting equipment and interfere with the manual harvest of vegetable crops by stinging workers. Fire ants will also attack small, juvenile animals and may cause blindness or suffocation by stinging around the eyes or mouth (3). Due to the need for quarantining plants brought into states at risk of introductions, the landscaping industry loses over a billion dollars a year (AP, 2003). However, the fire ant is typically more of a threat to natural ecosystem function and native plant and insect survival.
Many studies have documented the decline in biodiversity, species richness and abundance of native ants and other invertebrates with the invasion of S. invicta (Porter & Savignano, 1995; Stiles & Jones, 2001). Fire ants are known to prey upon other small terrestrial organisms including lizards, frogs and birds. For example, Allen et al. (1995) found that Northern Bobwhite (Colinus virginianus) populations were negatively affected by fire ant invasion and their predation on nesting quail chicks. Populations of Bobwhite quail were consequently demonstrated to rebound over a two-year period by reducing fire ant populations with chemical control.
There are several characteristics of S. invicta, as identified by Porter & Savignano (1990), which make the fire ant such a successful invader. These characteristics are similar to those defined for countless other invasive species. One such characteristic is the wide range of climate and habitat that are suitable for colonization by this species. Another is its preference for fragmented habitat, consistent with human-altered landscapes. Without human intervention, ecosystems are relatively resistant to invasion by non-native species. The disturbance caused by forest clearing, construction, agriculture and other human-induced landscape changes make introduction and establishment exponentially more probable. Fire ants also have a very high reproductive capacity. In fact, a single queen may lie up to 800 eggs per day (Porter & Savignano, 1990). These factors, along with others such as motility and food resource adaptability, make the fire ant a very successful invader and also present numerous problems for its control. However, numerous attempts at controlling the rampant spread of this aggressive invader have been attempted.
Control of the Red Imported Fire Ant
Boiling water dumped on the nest is a common method of ant control but is also very often unsuccessful. Two primary methods of control have been employed for S. invicta – chemical and biological control. Chemical control can include the use of toxic baits and poisons, such as Amdro® (hydramethylnon), that are ingested by the ants. It can also include the use of insect hormones and growth regulators such as Extinguish® (S-methoprene). Methoprene-based control depends on the foraging fire ant workers to bring the bait back to the nest. Once fed to the queen, reproduction is affected through interference with juvenile growth hormone (4). Chemical controls can be effective but often need to be applied multiple times a year to keep the ants from returning (Porter, 2000). A new type of fire ant control is now available by the makers of Sevin™ insecticide that provides year-round control with a single application. Sold under the trade name Over ‘n Out!™ it contains fipronil, which blocks the inhibitory nervous system of ants that have come into contact with it (5). The manufacturers claim that a single application will cause year-round protection against fire ants and the treated area can be used immediately after treatment. This type of treatment may not actually permanently remove fire ants however. More recently an attempt to use biocontrol on the red imported fire ant has been promulgated.
Biocontrol is the use of an invaders natural enemies, or predators, to control or extinguish the population of invaders. In the case of the fire ant, a parasitic fly larvae (Porter, 2000) in the Pseudacteon genus seems to be the preferred option for biocontrol. Pseudacteon decapitating flies parasitize ants by laying eggs in the thorax of attacked ant workers. The developing fly larvae will migrate to the head and prevent further ingestion of food. Subsequently, the connective tissue is broken down until the ants’ head is disjoined from its body. The larva continues to develop until mature inside the decapitated head of the fire ant. Thus, the Phorid fly can stress the colony through a constant parasitism of workers thereby denying the colony access to food resources. Field trials of P. curvatus began in the spring of 2000 and releases of P. tricuspis, a similar Phorid fly already approved, have been made in Florida, Texas and other states to control fire ant populations (Porter, 2000).
Fire ant losing its head!
In addition to the decapitating fly a protozoan disease is also being employed in the fight against fire ants. It is introduced into the colony by putting infected fire ant larvae into the nest, which will facilitate the infection of the rest of the colony. This method is being assessed by the United States Department of Agriculture for efficacy and safety (Fox, 2003). This method may prove to be somewhat less safe than other biocontrol efforts. While many diseases are very species-specific, the rapid population growth of protozoans also facilitates mutation. If the introduced disease mutates and gains the ability to attack other ants the results could spell doom for native ant species. However, effective and successful biocontrol programs have been conducted using viruses, fungal and bacterial diseases. One such example is the use of Cactoblastis cactorum a moth used to control prickly pear cactus (Opuntia spp.) in Australia (6). One of the benefits of this and other types of biocontrol is that once the biocontrol agent had been established in an area the population may be self-sustaining. So, in contrast with many chemical controls – an area may only need to be inoculated with a biocontrol agent once. Many biocontrol agents may be able to spread on their own after the initial introduction.
There is little doubt that invasive fire ants and other exotic species are costly to the economy, human health and conservation of biodiversity. It seems imperative that some attempt to control them is made. Chemical controls seem to only be a temporary solution but may be useful in some cases. As for biocontrol, when weighing the potential benefits of releasing a biocontrol agent many factors need to be considered including the potential for attacks on native species. Before a biocontrol agent is released, extensive testing must be done to ensure the intended results. It is very difficult to foresee potential problems of poorly designed biocontrol but even more difficult to reverse the effects once a release has been made. The key to successful biocontrol is in a thorough understanding of the biology of the organisms, the potential impacts of a release to natural ecosystems as well as economic and social effects. If wielded properly, biocontrol can be a powerful tool in reversing some of the damage wrought on native species and the ecosystems they inhabit by the introduction of non-native species.
Perhaps efforts should be focused on how to better avoid the situation in the first place. After all, its been said that an ounce of prevention is worth a pound of cure. While strides have been made to help stem the tide of biological invasions there are still numerous loopholes for invasive non-native species to slip through. One area that could stand to improve is the landscaping industry. Many invasive plants like bush honeysuckle (Lonicera maackeii) and tree of heaven (Ailanthus altissima) were imported as landscaping plants and have now overtaken many areas and displace native vegetation in the Midwestern US. Better identification and banning of species that exhibit invasive characteristics may help. In the US, the Clinton administration set up an Invasive Species Council which has made efforts to institute these and similar measures, though much work remains to be done and budgets are tight. In Australia similar efforts are currently being made but many obstacles, such as cooperation among free trade organizations, need to be overcome. The red imported fire ant is one of many invasive species that are being addressed and if the proper measures are not taken Australia’s economy will take a $ 6.7 billion dollar hit (Faulkner, 2003). There is cause for hope however, if invasive species are taken seriously and proper preventative measures are taken than much progress can be made to avoid the damage caused by S. invicta and other invasive species.
[check out my page on ants of San Salvador]
AP- The Associated Press. September 13, 2003 (BC cycle). Officials: Fire ants may be surviving southeastern Virginia winters. The Associated Press State and Local Wire.
Allen, C. R., R. S. Lutz, S. Demarais. 1995. Red imported fire ant impacts on northern bobwhite populations. Ecological Applications 5(3): 632-638.
Cook, J. 2003. Conservation of biodiversity in an area impacted by the red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae). Biodiversity and Conservation 12: 187-195.
Corwin, T. June 25, 2003 (all ed.). Research finds fire ants are deadly to many. The Augusta Chronicle (Georgia).
Creighton, W. S. 1930. A new world species of the genus Solenopsis (Hymenoptera:Formicidae). Proceedings of the American Academy of Arts and Science 66:39 151
Faulkner, J. February, 6 2003. Noxious nasties. The Age (Melbourne, Austrialia).
Fox, G. March 15, 2003 (final ed.). 6-Legged epidemic gets people antsy. The Tampa Tribune (Florida).
Lofgren, C. S. 1986. The economic importance and control of imported fire ants in the United States. In “Economic Impact and Control of Social Insects” (S. B. Vinson, Ed.), pp. 227-256. Praeger, NY.
Minor, E. May 24, 2002 (BC cycle). Fire ants adapted to cold-weather march through north Georgia. The Associated Press State and Local Wire.
Morrison, L. W. 1998. A review of Bahamian ant (Hymenoptera: Formicidae) biogeography. Journal of Biogeography 25: 561-571.
O’ Malley, B. July 31, 2003. Fire ants break out into new territory. Courier Mail(Queensland, Australia).
Porter, S. D., D. A. Savignano. 1990. Invasion of polygyne fire ants decimates native ants and disrupts arthropod community. Ecology 71(6): 2095-2106.
Porter, S. 2000. Host specificity and risk assessment of releasing the decapitating fly Pseudacteon curvatus as a classical biocontrol agent for imported fire ants. Biological Control 19: 35-47.
Rosenthal, C. July, 6, 2003. Fire ants can be dangerous for pets. San Antonio Express-News (Texas).
Stiles, J. H., R. H. Jones. 2001. Top-down control by the red imported fire ant (Solenopsis invicta). Am. Midl. Nat. 146: 171-185.
1. Ant Colony.Org (Ant Colony Developers Association)
http://www.antcolony.org/fire_ants.htm accessed 12/3/03
2. Biology and Impact of Introduced Species in Louisiana (Tulane)
http://www.tulane.edu/~mrbc/2001/MRB%20Project/fire_ant.htm accessed 12/3/03
3. The State of Queensland (Department of Primary Industries)
http://www.dpi.qld.gov.au/fireants/8067.html accessed 12/6/03
4. PestProducts.com (Professional Pest Control Products – Pensacola,
http://www.pestproducts.com/extinguish.htm accessed 12/6/03
5. Gardentech.com (Lexington, KY)
http://www.gardentech.com/0301.asp accessed 12/9/03
6. Manaaki Whenua Land Care Research (New Zealand)
7. http://www.landcareresearch.co.nz/research/biosecurity/weeds/biocontrol/biocontrol_success_stories.asp#Prickly Pear accessed 12/9/03
For Further Info on this Topic, Check out this WWW Site: http://marineecology.wcp.miamioh.edu/antsite/index.htm.
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