Imported Fire Ants
Timothy C. Lockley
Imported Fire Ant Station
USDA/APHIS/PPQ
Gulfport, MS 39501
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HISTORY:
Map: APHIS Quarantine map showing fire ant distribution in
U.S. Four species of fire ants are currently found within the
contiguous southeastern United States. The tropical fire ant,
Solenopsis geminata Fabricius, and the southern fire ant,
S. xyloni McCook, are considered species "native" to the area.
The two imported species of fire ants were introduced into the United
States from South America at the port of Mobile, Alabama. The black
imported fire ant, Solenopsis richteri Forel, arrived sometime
around 1918 and the red imported fire ant, Solenopsis invicta
Buren, in the late 1930's. The presence of imported fire ants in the
United States was first reported in 1929 by Loding. Both species
probably came to the port in soil used as ballast in cargo ships. In
the years preceding the arrival of the red imported fire ant, the
black imported fire ant slowly spread into adjacent counties in
Alabama and Florida. Since its introduction, the red imported fire
ant, a much more aggressive species than the black imported fire ant,
has spread quickly. By the time of the first official survey carried
out by the USDA in 1953, imported fire ants had invaded 102 counties
in 10 states (Culpepper 1953). Today, the red imported fire ant has
spread throughout the southeastern United States and Puerto Rico
(Fig. 1) replacing the
two native species and displacing the black imported fire ant.
Currently, S. richteri is found only in extreme northeast
Mississippi, northwest Alabama and a few southern counties in
Tennessee.
Imported fire ants disperse naturally through mating flights, colony
movement or by rafting to new sites during periodic floods. It is
through the actions of man, however, that the dramatic spread of
imported fire ants has occurred. Beginning soon after the Second
World War, and in conjunction with the housing boom of the period,
the imported fire ant began its march across the South. The spread of
these ants was largely due to the movement of grass sod and woody
ornamental plants used in landscaping. This inadvertent movement of
S. invicta and S. richteri was noted by the U.S.
Department of Agriculture in 1953 when a direct link was established
between commercial plant nurseries and the spread of imported fire
ants. In response to mounting public pressure, the U.S. Congress
appropriated $2.4 million in 1957 for control and eradication
efforts. As part of an overall plan a quarantine was imposed to
retard or prevent the artificial dissemination of these now notorious
pests. On May 6th 1958, regulations governing the movement of nursery
stock, grass sod and some other items were instituted through the
Federal Quarantine 301.81 (Canter 1981). By that time, however,
imported fire ants had moved into eight southern states. This spread,
although slowed considerably by federal regulations and climatic
conditions, continues even today. In recent years, isolated
infestations of imported fire ants have been found as far west as
California and as far north as Kansas and Maryland.
BIOLOGY:
Ants belonging to the genus Solenopsis can readily be
distinguished from all other ant species in North America by their
10-segmented antennae with a 2-segmented club. These characteristics,
combined with the presence of a sting, a two-segmented pedicel and an
unarmed propodeum make identification of the genus relatively easy.
Identification of individuals to the species level is somewhat more
difficult; made more so by the hybridization between the two "native"
species as well as between the two imported species.
Colonies of fire ants consist of eggs, brood, polymorphic workers,
winged males, winged females and one or more reproductive queens.
Among the sterile workers, labor is divided by age (and to a much
lesser degree by size). Younger workers are assigned the job of
caring for the developing brood; middle-aged workers are tasked with
colony maintenance and protection while the eldest workers forage for
food.
Photograph: Alate fire ant. Alate, or winged forms, are most
abundant in the late spring and early summer but can be found at any
time of the year. The winged forms are reproductives. Males, easily
distinguished from female alates, are decidedly smaller, glossy black
and have a small head (Fig.
2). Although both alate males and females can be found in the
same colony; as a general rule one form will be dominant. Nuptial
flights most often occur in the middle of the morning one or two days
following a rainfall; if the temperature is above 22°C and the
wind is light. The males fly first and await the females in the air.
The female alates emerge and take flight climbing up into the cloud
of waiting males where they mate in the air. After mating, the male
dies and the newly mated female lands, sheds her now useless wings
and begins searching for a suitable nesting site. The new queen
excavates a brood cell approximately 25-50 mm below the surface
of the soil. Often more than one new queen will occupy the same cell
but only one will survive to establish a colony. Queens do not forage
for food but rely on fat reserves and the energy acquired from
absorbing her wing muscles to survive until her first workers are
ready to take on the task of colony maintenance. Initially, the new
queen will oviposit 10-15 eggs. These hatch in 7-10 days
and are fed by the queen through trophallaxis (exchange of alimentary
fluids) or from sterile, trophic eggs she has laid for that purpose.
In 6-10 days, the larvae pupate and emerge 10-15 days
later. These workers, called minims, are very small due to the
limited amount of energy the queen could devote to their development.
These small workers open the brood cell to the outside world and
begin foraging for food to feed the queen and the developing brood.
Within 30 days, larger workers have emerged and the colony begins to
grow. Workers start to emerge daily and within six months several
thousand workers can occupy the colony and a "mound" is readily
visible. As the colony matures, the polymorphic nature of the worker
becomes more apparent. The largest workers in the colony (majors) can
be as much as 10 times the size of the smallest workers (media)
(Fig. 3). The queen lives up
to seven years and produces an average of 1600 eggs per day
throughout her life. At maturity, a monogynous fire ant colony can
consist of over 250,000 ants.
Photograph: fire ant mound. One of the identifying
characteristics of a fire ant colony is the earthen nest or mound.
The mound is a conically-shaped dome of excavated soil that has a
hard, rain-resistant crust. The mound averages 0.40 m in diameter and
0.25 m in height. In heavier soils, a mound can exceed 1.0 m in
height and 1.5 m in diameter. There are usually no external openings
in the mound; tunnels approximately 25-50 mm below the surface
radiate from the mound allowing foraging workers ready egress and
ingress. The purpose of the mound is three-fold: 1) to be a flight
platform for nuptial flights; 2) to raise the colony above the water
table in saturated ground and; 3) to act as a passive solar collector
to supply warmth to the colony during the cold winter months.
Although mound size and shape differs to some extent based on soil
type, during the dry hot days of late summer and early fall, new
mounds are not formed and older mounds are not maintained. While
mounds are important to a colony, they are not essential for colony
survival. Given a dark, protected site with sufficient moisture and
an adequate supply of food, fire ants will nest in a wide variety of
sites (e.g. rotten logs, walls of buildings, under sidewalks
and roads, in automobiles, in dried cow manure).
Fire ants are omnivorous, feeding on almost any plant or animal
material; although insects seem to be their preferred food. In rural
habitats, fire ants have a major impact on ground nesting animals
from insects to reptiles to birds to mammals. The arrival of imported
fire ants into an ecosystem wrecks havoc on the local ecological
community. Studies (Allen et al. 1995) have shown that a
minimum two-fold reduction occurs among populations of field mice,
oviparous snakes, turtles and other vertebrates when imported fire
ants are allowed to establish colonies within a given area. In some
instances, the depredation by fire ants has completely eliminated
some species from an ecosystem (Porter & Savignano 1990). The
reduction or elimination of a species or group of species from a
system has repercussions throughout the local food web. Not only do
imported fire ants reduce animal populations, they also feed on
plants. Fire ants attack young saplings and seedlings. They destroy
buds and developing fruits and have been shown to feed on the seeds
of 139 species of native wildflowers and grasses (Lockley unpubl.).
Secondarily, fire ants "nurse" numerous homopteran pests of plants
such as aphids and scale insects. Although not conclusively shown,
observations indicate that their activity on the plant itself may
reduce the ability of pollinators to successfully pollinate
flowers.
In agriculture, fire ants have been identified as damaging
fifty-seven species of cultivated plants (Adams 1986). Fire ants feed
on the germinating seeds of some crops (e.g. corn, sorghum,
soybeans) and the buds and developing fruits of others (e.g.
citrus, okra). Tunneling fire ants have galleried Russet potato
tubers and have significantly damaged the subterranean pods of
peanuts. Young citrus and pecan trees have been destroyed when
imported fire ants girdled stems and trunks. Damage to plants is
exacerbated during periods of drought as fire ants seek alternate
water sources. In fields where drip irrigation is used, fire ants
will build their mounds over the emitters reducing or blocking the
flow of water to crops. In some cases, actual physical destruction of
microsprinkler pulsator assemblies has occurred (Boman et al.
1995). Finally, the mere presence of fire ants on plants and within
the field will deter hand-harvesting of fruits and vegetables.
As an urban pest, imported fire ants cause many of the same problems
experienced in rural areas as well as some problems unique to the
urban environment. As in agriculture, imported fire ants cause
significant damage to numerous plants and, as in rural habitats, fire
ants can reduce the number of birds and mammals in an urban
landscape. Fire ants nest within urban structures such as the walls
of homes and offices. They establish colonies under sidewalks and
roadways. When the site is abandoned, subsidence will cause cracks to
appear and will occasionally result in the complete collapse of
sections of these structures. The presence of fire ants can deter
outdoor activities in yards, parks and school grounds. Home invasions
can threaten small children and the elderly. House invasions are
especially prevalent during periods of heavy precipitation and
flooding. Fire ant colonies have been found inside automobiles,
trucks and recreation vehicles (Collins et al. 1993). Traffic
accidents have been caused by fire ants stinging the drivers of
automobiles. Victims of highway accidents can be attacked by fire
ants if they are thrown from their vehicles.
Imported fire ants are attracted by electrical currents and have
caused considerable damage to heat pumps, air conditioners, telephone
junction boxes, transformers, traffic lights, gasoline pumps, et
cetera.
:
Photograph: Fire ant sting. Fire ants are most notorious for
their stinging behaviour. They respond rapidly and aggressively to
any disturbance of the colony or to a food source. A single fire ant
can sting repeatedly (Fig.
5) and will continue to do so even after their venom sac has been
depleted. Initially, the sting(s) result in a localized intense
burning sensation (hence the name "fire" ant). This is followed
within 24 - 48 hours by the formation of a white pustule at the sting
site (Fig. 6). This pustule
is formed only in response to the stinging of the two imported
species. No pustule forms from the stings of native species. These
pustules can become sites of secondary infection if not kept clean
and can leave permanent scarring.
Photograph: Pustules resulting from fire ant stings. A
minority of those stung by fire ants are hypersensitive to the venom
and can react quite strongly; suffering chest pains, nausea,
dizziness, shock or, in rare cases, lapsing into coma. Some deaths
have been documented as having been caused by fire ant stings but
these cases are extremely rare.
CONTROL:
While any attempts to control imported fire ants over large areas
are currently impractical, there are two basic methods that can
successfully control fire ants within a limited area: treatment of
individual mounds and broadcast treatments.
Individual Mound Treatments - There are several proven methods that
can be employed to control individual colonies of fire ants.
Insecticidal mound drenches with common insecticides are generally
effective against fire ant colonies. The mound is flooded with a
large volume of a liquid containing a contact insecticide such as
carbaryl, diazinon, dursban, et cetera. Numerous insecticides
are currently labeled for this use. A major problem with this method
is that the queen is sometimes too deep within the colony to be
contacted by the toxicant. Care must be taken not to disturb the
mound prior to application of the drench. A disturbance will alert
the colony and the queen may be taken deeper into the mound.
Application of insecticidal surface dusts or granules have a limited
effect on a colony if they are not watered in. The dissolved granules
must come into direct contact with the ants to have any effect. As in
mound drenches, care must be taken not to disturb the colony prior to
application. The queen can be taken to a point within the mound where
she may not come in contact with the poison. Some insecticides are
marketed as injectants. These are usually more effective than surface
applications or mound drenches but are also more expensive and can be
dangerous if not handled properly. Fumigants are also commercially
available. Again, they are expensive and dangerous if not handled
properly. A number of fire ant baits are available. These can be used
for treating individual mounds or for broadcast treatment of larger
areas. The bait should be uniformly applied around the mound 0.3 to
1.0 meters away and not on the mound itself. Baits are much slower
than the control methods listed above but are generally safer,
cheaper and more effective in the long run.
Broadcast Treatments - A number of fire ant baits are commercially
available for broadcast treatments. Baits are composed of an inert
pregelled corn carrier and soybean oil. A toxicant (either a
slow-acting insecticide or an insect growth regulator) is
incorporated into the oil. Foragers find the bait and carry it back
to the colony. Once there, the ants will ingest the soybean
oil/toxicant and begin feeding other members of the colony.
Eventually, the toxicant is spread throughout the mound and all
members of the colony are affected. Broadcasting baits has a number
of advantages. Unlike individual mound treatments, colonies need not
be "located" in order for them to be treated. Foraging workers
quickly retrieve the broadcast granules and carry them back to their
nestmates. Broadcasting baits is a great deal less labor intensive
than individual treatments and per acre costs are dramatically lower
than with any of the other methods. Some of the disadvantages to
broadcasting baits include: lack of specificity to fire ants; baits
dissolve when they come in contact with water rendering them
irretrievable by fire ants; retrieval of the baits is temperature
dependent; and baits are slow-acting.
Biological Control - A large number of arthropod species have been
identified from nests of imported fire ants. Unfortunately, the vast
majority are transients and seem to have no specific relationship
with the ants (Collins 1992). However, many of these are known to be
endoparasitic, socially parasitic or predaceous (Silviera-Guido et
al. 1973, Williams 1980) and studies continue evaluating their
effectiveness. Fungal, protozoan and viral pathogens have also been
identified in association with fire ants (Jouvenaz 1983). Three
potential biological control agents have been identified in South
America for intensive study. These studies, if successful, may lead
to their introduction into the United States. The organisms under
consideration are: Pseudacteon sp. flies (Diptera: Phoridae)
ca. 17 spp.; Thelohania solenopsae a protozoan disease and;
Solenopsis dagerrei (Hymenoptera: Formicidae), a workerless
social parasite.
Phorid flies are known to be parasitic on a number of ant species.
Some are thought to be host specific to imported fire ants. The adult
flies oviposit on foraging fire ant workers outside the mound; the
maggots migrate into the ant's head capsule where they feed. This
eventually leads to decapitation of the ant. As interesting as this
phenomena is, the major effect of these flies is to cause the ants to
cease foraging. In the presence of the fly, worker ants will retreat
into the colony to prevent oviposition by the phorid. This disruptive
response to the fly restricts the ability of the colony to feed
itself and may "even the playing field" so that other ant species can
become more competitive with fire ants.
The microsporidian disease Thelohania solenopsae is an
obligate intracellular pathogen of fire ants. Preliminary field
studies on populations of the black imported fire ant, S.
richteri, carried out in Argentina indicated that decreasing
densities of fire ants were associated with increasing presence of
this pathogen. These data suggest that this pathogen may be an
important factor in reducing fire ant numbers by weakening the
colonies. Although the vertical transmission of this disease is
understood, the horizontal transmission is not.
Solenopsis (Labauchena) dagerrei is a parasitic ant that
attaches to the fire ant queen and redirects fire ant workers to tend
the brood of the parasite to the detriment of the colony's own
larvae. S. dagerrei is intriguing because it lacks a worker
caste; only queens and males are produced. The presence of this
parasitic ant has a debilitating effect on colony growth and the
proportion of sexual reproductives produced in the colony. Queens of
S. dagerrei enter fire ant colonies and attach themselves to
the mother queen. Previous studies have demonstrated that this
parasite inhibits the fire ant queen and her egg production; thus
causing the fire ant colony to collapse and eventually die out.
To date, none of these natural enemies has been sufficiently
evaluated to determine if, in and of themselves, they might produce
any true suppression of fire ant populations. In all likelihood,
parasites, predators and pathogens will be used in combinations to
reduce colony fitness. Reduction in colony vitality could cause
greater mortality under stress conditions and allow for better
competition from native ant species
LINKS TO OTHER IMPORTED FIRE ANT PAGES:
Control
of the Red Imported Fire Ant
EcoLink:
The Red Imported Fire Ant by Wendee Holtcamp
Using
phorid flies in the biocontrol of imported fire ants in Texas
REFERENCES CITED:
Adams, C.T. 1986. Agricultural and medical
impact of the imported fire ant. IN: C.S. Lofgren & R.K.
vander Meer (eds.). Fire Ants and Leaf-cutting Ants. Biology and
Management. Westview Press. pp. 4857.
Allen, C.R., R.S. Lutz, and S. Demarais. 1995. Red imported fire ant
impact on Northern Bobwhite populations. Ecol. Appl. 5(3):
632638.
Boman, B.J., R.C. Bullock and M.L. Parsons. 1995. Ant damage to
microsprinkler pulsator assemblies. Appl. Eng. Agric. 11(6):
835837.
Canter, L.W. 1981. Cooperative imported fire ant programs - Final
programmatic impact statement. USDA-APHIS-ADM8101-F. 240
p.
Collins, H.C. 1992. Control of Imported Fire Ants: a review of
current knowledge. USDA-APHIS Technical Bulletin 1807: 27 pp.
Collins, H.L., T.C. Lockley and D.J. Adams. 1993. Red imported fire
ant (Hymenoptera: Formicidae) infestation of motorized vehicles. Fla.
Entomol. 76(3): 515516.
Culpepper, G. H. 1953. The distribution of the imported fire ant in
the Southern States. Proceedings of the Association of Agricultural
Workers 50: 102.
Jouvenaz, D.P. 1983. Natural enemies of fire ants. Florida
Entomologist 66:111121.
Loding, H.P. 1929. An ant (Solenopsis saevissima richteri).
U.S. Department of Agriculture Insect Pest Survey Bulletin.
9:241.
Lofgren, C.S. and R.K. Vander Meer (eds.) 1986. Fire Ants and
Leaf-cutting Ants: biology and management. Boulder, CO: Westview. 247
pp.
Porter, S.D. and D.A. Savignano. 1990. Invasion of polygyne fire ants
decimates native ants and disrupts arthropod community. Ecology
71(6): 20952106.
Silviero-Guido, A., J. Carbonell and C. Crisci. 1973. Animals
associated with the Solenopsis complex with special reference
to Laubauchena daguerri. Proceedings Tall Timbers Conference
on Ecology and Animal Control Habitat Management. 4: 4152.
Williams, R. N. 1980. Insect natural enemies of fire ants in South
America with several new records. Proceedings Tall Timbers Conference
on Ecology and Animal Control Habitat Management. 7:
123134. Return to Radcliffe's IPM World Textbook
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