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Red imported fire ants keep adapting to life in the
United States. They keep spreading farther and farther north from their
foothold in the Southeast, often overtaking native ants along the way to
become the dominant species.
But scientists at ARS'
Center for Medical and Veterinary Entomology (CMAVE) are hot on their
tiny trails, developing new ways to keep their numbers manageable.
Researchers with the Imported Fire Ant and Household
Insects Research Unit in Gainesville, Florida, have made
 many discoveries
since the late 1970s in their efforts to control fire ant populations.
Their strategy is to reduce the amount of chemical treatments needed by
developing self-sustaining biological control agents.
The red imported fire ant, Solenopsis invicta, was
accidentally introduced into the United States in 1929 from South
America. Fire ant population there is only 20 percent of that here,
probably because North America lacks natural enemies of the ant. They
are particularly disruptive because of their large numbers and painful
sting; about 40 percent of people in infested areas are stung each year.
Fire ants dramatically reduce populations of native ants, other insects,
and ground-nesting wildlife. They also damage many crops and much
electrical equipment. Damage and control costs are estimated to be more
than $6.5 billion a year.
Red imported fire ants currently infest more than 320 million acres
in 12 southeastern states and Puerto Rico and have recently spread to
California and New Mexico. Although fire ants keep moving into new
areas, a recent ARS model predicts that they will not be able to survive
in areas where soil temperatures drop to near freezing for more than 2
to 3 weeks. Black imported fire ants, Solenopsis richteri,
introduced to the United States in 1918, are less widespread. A hybrid
looks the same as the red ants, so laboratory tests are needed to tell
them apart.
Effective Infections
Two pathogens from South America are known to infect fire ant
colonies, and research is continuing into their effectiveness as
biological control agents. CMAVE researchers were the first to discover
Thelohania solenopsae in the United States in 1996. This
single-celled protozoan parasite infects colonies when workers transfer
it to the queen, possibly through food exchange. It reduces the queen's
weight, and she lays fewer eggs. Colonies die out after 9 to 18 months.
CMAVE researchers learned they could infect colonies by introducing
brood infected with T. solenopsae. Since fire ants will accept
brood from foreign colonies, the parasite is now established in many
regions. But because the transfer of live larvae and pupae is very
tedious and not entirely effective, experiments are under way to see
which life stages best transmit the pathogen to the queen. Entomologist
David Oi recently discovered in fire ant pupae a new T. solenopsae
spore type that he believes might be capable of initiating infection.
Some fire ant colonies have only a single queen, while others have
many. A single-queen colony can have 250,000 workers, whereas a
multiple-queen colony can have twice that number. Multiple-queen
colonies are easier to infect with T. solenopsae because they
will adopt and raise infected brood from other colonies. But, on the
downside, says Oi, they are harder to control because their large
populations limit the pathogen's impact.
"Single-queen colonies are more suspicious of outsiders, but they
will raid other colonies and take their immature ants," says Oi. "So, we
can establish an infection in single-queen colonies, but it dies down
after awhile."
The researchers are studying another pathogen, Vairimorpha
invictae, that is more lethal but also more rare than T.
solenopsae. (See "Ouch!
The Fire Ant Saga Continues," Agricultural Research,
September 1999, p. 4.) V. invictae is hard to keep alive in the
laboratory, so the scientists must occasionally have infected colonies
sent from ARS' South American Biological Control Laboratory in Buenos
Aires, Argentina—where it is being studied in the field—to examine the
pathogen under quarantine in Gainesville.
A new disease has been discovered by CMAVE insect pathologist Roberto
Pereira. It's called yellow head disease because of the yellow-orange
color ant heads and other body parts turn when infected with the
spindle-shaped spores. A protozoan from the genus Mattesia was
present in 34 percent of sites and 8 percent of nests studied in
Florida. The pathogen causes extreme changes in appearance, but its full
impact as a potential biological control agent is still under
investigation.
David Williams, head of the Imported Fire Ant and Household Insects
Research Unit, is also searching for viruses to use against the ant.
"Back in the late 1970s, a researcher looking at red imported fire ants
found a virus-like particle, but it died in the lab," Williams says.
"There are viruses in ants, but finding them is tedious and laborious
work. They can be extremely virulent if you get the right one." He
believes this untapped area has great potential for new discoveries, and
he hopes to bring a molecular biologist to their team soon to aid in the
search.
Williams is also evaluating a parasitic ant, S. daguerrei,
under quarantine. It is found only on fire ants in Argentina and Brazil.
The parasite doesn't produce workers, so it relies on its host, taking
resources from the queen while the colony feeds and maintains its brood.
Mound densities were 33 percent less in sites with the parasitic ant,
and the number of fire ant queens was 47 percent less in parasitized
colonies.
Terror from Above
Fire ant decapitating phorid flies, from the genus Pseudacteon,
are natural enemies of imported fire ants. Just their presence keeps
frightened ants from leaving their mounds. That alone is good news
because it forces a shift in when the fire ants forage and allows native
ants to compete better.
A female fly hovers over a mound before she pinpoints a victim,
dive-bombs it, and deposits one of her eggs inside it. The egg, as
researchers at the center discovered, eventually produces a larva that
eats its way to the head and causes it to fall off. Entomologist Sanford
Porter estimates there are about 20 types of Pseudacteon flies
that attack fire ants. They could be used to complement each other in
control efforts.
So far, two phorid fly species—P. tricuspis and P. curvatus—have
been released and established in the United States. P. tricuspis
took hold and is expanding at the rate of 10 to 20 miles a year. P.
curvatus works better on black imported fire ants and the hybrid
than on red ones. It was imported from Argentina and is being used in
Alabama, Mississippi, and Tennessee. A third species, P. litoralis,
works especially well on the red ants and has been approved for release
in field studies.
Porter and colleagues at ARS' Biological Control and Mass Rearing
Research Unit, located near Starkville, Mississippi, developed an attack
box that has greatly improved their ability to mass-rear phorid flies.
The boxes are held under environmental conditions that strictly mimic
the flies' natural habitat. Specially designed cups are alternately
raised up and down within the box in a cycle every 10 minutes, causing
the ants to run from one cup to the other. This action allows the phorid
flies to easily attack and parasitize them. New flies are continually
released into the main attack box from an attached holding box at one
end to maximize the amount of eggs deposited.
Because CMAVE reached its capacity to rear the phorid flies, an
initiative to mass-produce them was launched with ARS, USDA's Animal and
Plant Health Inspection Service (APHIS), and Florida's Department of
Agriculture and Consumer Services. The new facility in Gainesville will
eventually produce 6,000 to 12,000 flies a day.
Social Insects Suffer Uninvited Guests
Chemist Robert Vander Meer studies semiochemicals, such as
pheromones, in an effort to modify ant behavior and control the pests.
Social insects like fire ants use semiochemicals to communicate with
each other and influence activities. Vander Meer wants to see how the
ants use these compounds and then disrupt the processes.
One reason fire ants are so troublesome is that they commonly
short-circuit electrical equipment of all kinds. Of course, they
electrocute themselves in the process, and large numbers of dead ants
are sometimes found piled up in electrical mechanisms. Vander Meer and
Porter discovered that electrocution causes the ants to release alarm
pheromones that attract other ants as well as phorid flies. In response
to these pheromones, phorid flies lay more eggs in ants, and fly
production increases by 15 to 20 percent. This knowledge will help them
develop new ways to use alarm pheromones in biological control programs.
Vander Meer is involved in other fire ant pheromone projects. He
found that fire ants use their stinger not only for defensive purposes
but also for pheromone dispersal. His knowledge of how fire ants use
semiochemicals to recognize intruders enabled scientists to decipher how
two parasites (a beetle and a wasp) are able to infiltrate aggressive
colonies. Pioneering work revealed how fire ants locate food and recruit
other workers to the source with pheromone trails.
An ARS-funded areawide project began in March 2001 to demonstrate how
to keep fire ant populations at very low levels by combining strategic
pesticide applications with phorid flies and T. solenopsae.
Diverse demonstration sites—as large as 300 acres—in five states were
chosen for the project to represent the fire ant's infestation range,
according to Williams. ARS is coordinating the major activities of four
land-grant universities and other organizations associated with the
project for 4 to 5 years.—By
Jim Core,
Agricultural Research Service Information Staff.
David Williams
is with the USDA-ARS
Imported Fire Ant and Household Insects Research Unit, CMAVE, 1700
S.W. 23rd Drive, Gainesville, FL 32608-0000; phone (352) 374-5903, fax
(352) 374-5818. |
