Imported Fire Ants: An Agricultural
Pest and a Human Health Hazard
Plant Protection and Quarantine - April 1999
Imported fire ants (Solenopsis invicta Buren, S. richteri Forel, and their
hybrid) were accidentally introduced to the United States. The black imported
fire ant was brought to Mobile, AL, in 1918. The red imported fire ant
arrived in the 1930's. Since then, they have become established across
the South and in parts of California and other Western States. These pests
pose serious threats to people, small animals, and agricultural equipment.
As these insects spread northward and westward, more people are asking
the U.S. Department of Agriculture's (USDA) Animal and Plant Health Inspection
Service (APHIS) for advice about how to manage imported fire ants. This
factsheet answers several frequently asked questions.
Q: Why are they called fire ants?
A: Anyone who has been stung by one or more of
these aggressive insects can answer this question. Fire ants clamp onto
their targets with powerful jaws and sting their victims repeatedly. Each
sting injects a dose of venom that causes a burning sensation. The stings
raise itching blisters that can become infected. In sensitive victims,
the stings can cause anaphylactic shock (symptoms include trouble breathing
and fainting) or even death.
Q: What damage do fire ants cause?
A: Fire ants attack and sometimes kill newborn
domestic animals as well as pets and wildlife. Fire ants can also destroy
seedling corn, soybeans, and other crops. These insects feed on buds or
fruits of many plants and may remove bands of bark from young citrus trees,
often killing them. Additionally, the hard, cone-shaped nests of fire
ants can mount as high as 2 feet, making it difficult to cultivate and
harvest crops from infested fields. These fire ant mounds are unsightly
hazards in yards, parks, and other recreational areas, where they are
especially dangerous to children and pets.
Q: Where are imported fire ants in the United
States, and where do they come from?
A: Imported fire ants were accidentally brought
to the United States from South America. Since arriving in Mobile, AL,
they have spread to 14 States and Commonwealths. They now infest all or
part of Alabama, Arkansas, California, Florida, Georgia, Louisiana, Mississippi,
New Mexico, North Carolina, Oklahoma, Puerto Rico, South Carolina, Tennessee,
Q: How can you identify fire ants?
A: Identifying fire ants is difficult because they
look much like ordinary ants. They are 1/8 to 1/4 inch long and reddish
brown to black in color. Fire ants are probably best distinguished by
their aggressive behavior and characteristic mound-shaped nests.
Q: How does a colony begin?
A: A new colony begins with a "nuptial flight"
of winged males and winged females, usually on a warm, spring day. After
mating occurs, males drop to the earth and die. Females that successfully
elude predators and survive drought and rain seek out nesting sites and
burrow underground. A new queen sheds her wings and lays 12 to 24 worker
eggs, which she tends to constantly. Upon hatching, the workers, all of
which are sterile females, take over all colony functions except reproduction,
while the queen only mates and lays eggs. She may produce more than 200
eggs per day.
Q: How large is a colony?
A: Fire ant colonies vary in size, but a mature,
3-year-old colony typically contains 250,000 workers, which are sterile
females, and hundreds of reproductive males and potentially reproductive
females. A colony population can grow to 300,000 ants. In addition to
single-queen colonies, many fire ant colonies have multiple queens, increasing
tenfold the number of mounds in a single acre.
Q: How do fire ants spread?
A: Limited local dispersal occurs during the mating
process, mass movement of colonies, and floods. Ants travel longer distances
by hitchhiking in motor vehicles and in or on soil, plants with roots
and soil attached, nursery stock, sand, gravel, grass, sod, hay, wood,
or soil-moving equipment.
Q: What is USDA doing to prevent this spread?
A: APHIS works to limit the spread of imported
fire ants by requiring that all nursery stock and other items likely to
carry the pests be inspected and treated before traveling from fire ant-infested
areas to fire ant-free areas. Regulated articles include soil, plants
with roots and soil attached (except house plants maintained indoors and
not for sale), grass sod, baled hay and straw that have been stored in
contact with soil, and used soil-moving equipment.
Q: How can these pests be controlled?
A: Control of fire ants is made difficult by the
protective behavior of the workers who guard the queen. If the colony
is disturbed, workers will hurriedly carry the queen to a safe location.
There, the queen begins a new colony. Therefore, the best control is not
a method that will merely kill workers but a bait that will be taken back
to the nest by foraging ants where it will either kill the queen or render
her sterile. Baits combine a food to attract the ants with an insecticide
or insect growth regulator (a pesticide that works by disrupting the ants'
reproductive system). Although baits are slow acting, they are often the
best way to reach the queen and eliminate the colony.
Q: How should treatments be applied?
A: Treatment options vary, depending on the type
of fire ant problem. Those applying any treatment should read insecticide
labels closely and follow all directions. Some of the baits approved by
the Environmental Protection Agency include Affirm®, Amdro®, Award®,
Clinch®, Distance®, Extinguish®, and Logic®.
For more information on imported fire ants and
Federal restrictions imposed on the movement of regulated articles, contact
one of the following:
Your county's agricultural extension agent, listed
in the county government section of your telephone directory under Cooperative
Your State's regulatory officials, usually listed under department of
agriculture, plant protection, or regulatory division in the State government
section of your telephone directory.
A Federal regulatory official, listed in the Federal Government section
of your telephone directory under USDA, APHIS, Plant Protection and Quarantine.
You can also contact APHIS through the World Wide Web by pointing your
Web browser to http://www.aphis.usda.gov/ppq and using the "Comments"
link to send a request for information.
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Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and
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