Ouch! The Fire Ant Saga
Fire ants will do anything to resist attack
by the tiny phorid fly measuring only
about one-sixteenthe of an inch. A
highly specific natural enemy, the female
pierces a fire ant's head and releases an enzyme that later decapitates it.
The latest news in the world of
fire ants: The tiny pests with a ferocious sting are spreading. Until recently,
red imported fire ants occupied more than 300 million acres in 12 southern
states and Puerto Rico. Now they ve become established in California and New
As the ants spread, the race to stop them is even more intense for a team of
Agricultural Research Service scientists
at Gainesville, Florida. There, researchers at ARS' Center for Medical,
Agricultural, and Veterinary Entomology are seeking new ways to keep this pest
"Our goal is to try to reduce their numbers so native ant species can
compete," says entomologist David F. Williams. He is the lead scientist on
the fire ant biocontrol project in the center's Imported Fire Ants and
Household Insects Research Unit that is headed by Richard J. Brenner.
Fire ants often nest where gopher
tortoises lay their eggs, in aprons
around the edge of their tunnels.
At Camp Shelby, Mississippi,
cooperator Tom Estes examines
gopher tortoise eggs for signs
of fire ant injury.
Fire ants are thought to have
spread to the United States from their native South America via contaminated
ships in the early 1930s. Since then, their spread has been slow but steady.
"We believe imported fire ants have flourished in the United States
because they have no natural enemies here. We're trying to change that by
working with state cooperators to introduce natural enemies," Williams
ARS and representatives from the Council of State Governments' Southern
Legislative Conference initiated a National Fire Ant Strategy in 1998 to help
tackle the fire ant problem. One of the group's objectives is to reduce
pesticide use by substituting biological controls. In the past 2 years, ARS
scientists have dispatched new artillery to help control fire ants: a
slow-acting disease and a decapitating fly.
Microbial Combatants on the Move
Unable to escape vicious fire ant
stings, hatching alligators are especially vulnerable to attack.
Thelohania solenopsae, a
microorganism from South America, infects fire ant colonies and causes disease.
Williams says workers probably transfer the pathogen to the queen through food
exchange. As the disease slowly reduces her weight, she lays fewer and fewer
eggs, and all are infected with the pathogen, further weakening the colony.
Williams says colonies generally die within 9 to 18 months. However, in lab
studies, he found that after 3 months the infected colonies were smaller than
healthy ones. The microorganism doesn't harm plants or native ant species. And
after years of testing, it's been found only in red and black imported fire
Williams works closely with ARS' South American Biological Control
Laboratory in Buenos Aires, Argentina, to get this and other biological
controls through quarantine and into the United States. Natural enemies of
imported fire ants are also native to South America.
In 1996, the scientists discovered that the Thelohania pathogen
wasn't confined to its native land, but had already infested fire ant colonies
in Florida, Mississippi, and Texas. This opened the door for releases in
Florida, Arkansas, Oklahoma, Mississippi, Louisiana, Tennessee, North and South
Carolina, Alabama, and Georgia.
To evaluate the allure of a fire
ant bait, biological technician
Terry Krueger positions particles
containing a queen-produced
"Since our first release in
1998 in Florida, Thelohania has spread to more than 75 percent of the
colonies we're monitoring," says Williams. "But it will take longer
to see a major impact."
Entomologist David H. Oi, who works with Williams on evaluating
Thelohania and other biological controls, says they know the organism
kills the colonies. "But we want to see how it is passed to the queen.
This is one of few microbial agents that infect mated queens and pass on to her
eggs," says Oi.
"We want to find out if infected virgin queens can mate and then spread
the disease to their offspring. If so, this will be a natural way to distribute
Thelohania, besides introducing infected brood into existing
colonies," says Oi. The scientists hope to get the answer in a study they
will start this year.
Even more powerful than Thelohania is another pathogen called
Vairimorpha, which Williams says is not that different. When combined
with Thelohania, it kills colonies in 2 to 6 weeks. "The problem
is, about 20 percent of fire ant colonies in South America contain
Thelohania, but only 1 percent contain Vairimorpha,"
Williams says. "It's so rare, hard to find, and also hard to keep alive to
study. There's a lot we don't know about it, but we're excited about finding
Fire ants will try hard to avoid the sting of a phorid fly (top, center). Once
deposited, an egg quickly hatches into a larva that eventually kills the host
by decapitation. Shown about nine times actual size.
Another line of defense that "has fire ants literally on the run is the
phorid fly, Pseudacteon tricuspus, their mortal enemies," says ARS
entomologist Sanford D. Porter. "The ants will run and hide, freeze, stop
foraging, or twist upside down so the flies don't sting them. The only way they
could have evolved these defensive behaviors is if the flies had some effect on
fire ant populations."
The phorid flies hover over the fire ant mound, then zoom in to pierce an
ant's outer cuticle and deposit an egg underneath. The egg quickly hatches into
a fly maggot, or larva, that moves into the ant's head. When the maggot is
mature, it releases an enzyme that causes the ant's head to fall
offdecapitation. Using the ant's head as a safe hideaway, the fly
completes its development inside.
"One female phorid fly usually contains a hundred or more
torpedo-shaped eggs, so she can make multiple attacks," says Porter.
Porter released thousands of the tiny flies in July 1997 in Gainesville. Since
then, he's released them in Oklahoma, Arkansas, and Alabama.
An enzyme released by the mature phorid fly larva decapitates its fire ant
"The exciting part is that
the Brazilian parasitic flies released in Florida have survived for nearly 2
years. They've gone through many generations and appear to be permanently
established," says Porter. "Fly populations are still growing, so it
may take 1 to 2 years before they have a noticeable impact."
What's next? "We have a new, smaller phorid fly species, Pseudacteon
curvatus, which will attack smaller-sized fire ants," Porter says.
"We have found at least 20 species of phorids in South America that
specifically attack fire ants."
Porter says this new species is even better, since scientists can rear it
more easily. He has completed host-specificity tests and is planning on seeking
permission to field-test P. curvatus. They have tested the flies with
different foods, animal dung, and human waste to ensure they're not attracted
to anything other than fire ants. He says the flies aren't visible unless you
kick over a fire ant mound.
This scanning electron micrograph
shows a side view of the hooked
ovipositor of Pseudacteon curvatus,
a very small fire ant-decapitating
fly from South America. Magnified
The scientists are hoping these
two flies and other natural enemies will eventually tip the ecological balance
against fire ants in the United States, so they will no longer be the dominant
species. Fire ant populations are greater here than in South America, where
natural enemies appear to keep them in check.
Still waiting to be called to duty is Solenopsis daguerrei, a parasitic
ant discovered in Argentina in 1930. This ant is unusual because it produces no
workers. Williams says the Solenopsis queen uses her mandibles to clamp
onto a fire ant queen's body. Like a wolf in sheep's clothing, the parasitic
ant queen chemically disguises herself, mimicking the natural attractants of
the fire ant colony; otherwise, she would be killed. The fire ant queen becomes
debilitated and lays fewer eggs, weakening the colony. Williams says they are
still trying to learn more about S. daguerrei.
In less than 10 seconds, an unwary
scientist was stung over 250 times
on one leg when he carelessly knelt
on a collapsed fire ant mound. The
sterile pustules developed to this
stage in 3 days.
Tracking the Enemy
It would also help scientists if they could find out where fire ants may
start new colonies. Entomologist Dana A. Focks, an expert on computer modeling,
is working on that. He's focusing on when and where winged ants, called alates,
mate and start new colonies.
"No really good models exist that can predict where alate flights might
have occurred or could possibly occur," says Focks. "We want to build
a satellite-based program to find if one has taken place."
Mating depends on temperature and weather, but most fire ants emerge in warm
weather after rainfall. Focks is working on a cooperative grant project with
IBM, Inc., Johns Hopkins University in Baltimore, Maryland, and the National
Aeronautics and Space Administration to develop a tracking system that can
predict when alates have emerged or will emerge.
Focks says this will ultimately save money for nurserymen who typically have
to treat their plants and surrounding areas with insecticides to ensure they
don't transport ants from infested areas to noninfested ones. They generally do
this before shipment.
"We're hoping that in the future a person can go to the Internet and
get site-specific information on whether a flight occurred, based on weather
conditions, temperature, and other information. Nurserymen could use this
information to keep from re-treating their stock unnecessarily," Focks
A microbial pathogen, Thelohania solenopsae infect all growth stages of
the fire ant but is most debilitating to the fire ant queen. Magnified about
Attacks on Animals
Another growing problem with fire ants is their ecological impact,
especially on endangered vertebrates and invertebrates. Entomologist Daniel P.
Wojcik is working with many environmental and conservation groups to keep fire
ants from harming endangered species like Stock Island tree snails, gopher
tortoises, Florida grasshopper sparrows, saltmarsh rabbits, and sea turtles.
"Only 300 female green sea turtles are nesting in the world, and most of
them are in Florida. Fire ants attack the immature turtles, either killing or
blinding them," he says. "Sometimes young turtles wander across a
fire ant mound. Fire ants usually sting them as a defensive response. But they
will also attack and feed on anything that doesn't move, including
Entomologists David Williams
(left) and David Oi compare a
fire ant colony infected with the
Thelohania solenopsae pathogen
(left) to an uninfected one.
An emerging anomaly and problem
with fire ants is polygyne, or multiple-queen, colonies. "These 'super
colonies' are more problematic than monogyne, or single-queen, colonies
because, collectively, they lay more eggs and are harder to control," says
chemist Robert K. Vander Meer.
Many people think if you kill the queen, you kill the colony. But this isn't
true if there are workers left, says Vander Meer. Workerless colonies will
adopt new queens, which provides one explanation as to why ants have reinfested
treated areas and may be related to how polygyne colonies form.
The queen controls adoption and other colony functions through pheromones.
Vander Meer says the ultimate goal is to find these pheromones and use them to
help decrease fire ant populations. Scientists in this research unit have
developed and applied for patents on new repellants, attractants, and
multiple-species ant baits, all of which could soon help ease the fire ant
Weaver-Missick, Agricultural Research Service Information Staff.
This research is part of Arthropod Pests of Animals and Humans, an ARS
National Program described on the World Wide Web at
The scientists mentioned in this article are in the
USDA-ARS Imported Fire
Ants and Household Insects Research Unit, 1600 SW 23rd Dr., Gainesville, FL
32608; phone (352) 374-5903, fax (352) 374-5818, e-mail
Native Habitat May Hold the Key
Most people think of Buenos
Aires as a vacation spot. But for ARS entomologist Juan Briano and biologist
Luis Calcaterra, it's a place for serious research. ARS' South American
Biological Control Laboratory in Argentina helps the agency's North American
scientists study exotic pests from Latin America.
Some pests are quarantined from import to the United States, so ARS'
international locations are vital to studying them in their original
Researchers in Buenos Aires helped ARS scientists from Gainesville, Florida,
identify biological controls for fire ants, and now they are helping to ensure
that these useful ant-busters don t harm native U.S. species.
One biocontrol agent they studied was Thelohania solenopsae, a
pathogen that weakens and eventually kills fire ant colonies. "To study
field host specificity of Thelohania, we traveled to rural areas of the
Buenos Aires Province," explains Briano. "We put 300 bait traps in 30
field sites.We captured many ant species and took them back to the lab. There
we froze them, ground them in water, and checked for the presence of
Thelohania. We found it only in fire ants."
Briano and Calcaterra traveled to the San Eladio area 80 kilometers west of
Buenos Aires to check for a parasitic ant, Solenopsis daguerrei. That
area is heavily infested with fire ants, and S. daguerrei has infested
many of the colonies.
"We checked a total of 4,131 fire ant nests and 185 nests of other ant
species and found the parasite only in fire ant nests," says
Calcaterra.By Jill Lee, formerly with ARS.
Juan Briano is with the
USDA-ARS South American Biological Control Laboratory, c/o Agriculture
Counselor, U.S. Embassy, Buenos Aires, Unit 4325, APO AA 34034-0001.
"Ouch! The Fire Ant Saga Continues" was
published in the September 1999
issue of Agricultural Research magazine.