Not far west of the University of Texas at Austin campus, a mosaic
of neighborhoods stretches south for miles toward Town Lake, which
runs through the heart of Austin. Tarrytown, Enfield and other neighborhoods
in this area contain closely spaced homes that range from turn-of-the-century
stone mansions to simpler, clapboard houses, all surrounded by stately
oaks and well-manicured lawns. Less visible to passersby are low
mounds of soil here and there on these lawns, evidence of what may
be the best remaining stronghold of native fire ants that have been
Texas residents for millions of years.
Fire ants in North and South America are attacked
by specific types of phorid flies based on ant size and chemical
aroma. Here, a female Brazilian phorid fly targets imported
“That area of Austin probably has the biggest patch of native
fire ants left in the southern U.S.,” said Professor Larry
Gilbert, who has taken on a foreign fire ant that is wiping out
the natives’ territory throughout Texas and nearby states.
Born in Laredo to a Presbyterian minister who kept the family moving
between small towns, Gilbert spent his youth outdoors, hunting,
collecting butterflies and spending summers helping a grandfather
tend beehives. His interest in nature led to a population biology
degree at Stanford University, his post as director of The University
of Texas at Austin’s two biology field stations and his fight
against Brazilian fire ants that were first found in Texas in the
Gilbert became concerned about imported fire ants while he was
studying butterflies and focusing on keeping armadillos, native
ants and other wildlife healthy at the College of Natural Sciences’
Brackenridge Field Laboratory on Town Lake’s northern shore.
“I’m a native Texan,” he said, “and I feel
a great loyalty to the habitat in Texas.”
So when he saw native fire ant mounds there being rapidly replaced
by those of Brazilian imports in 1981, he wanted to do something
about it. The lanky ecologist has been digging deeper into fire
ant ecology ever since.
The foreign ants that reached the southern U.S. in cargo boats
in the 1920s don’t look or act very differently from native
counterparts. But the Brazilian insects produce young more rapidly,
which means more mouths to feed. As a result, the ants will even
attack baby birds and other critters, such as Harvester ants favored
as food by the state’s dwindling number of horned lizards.
The imports also can harm livestock and objects such as underground
circuit boxes near their nests, leading to an estimated $1.2 billion
in damage annually in Texas.
biologist Larry Gilbert inspects an imported fire ant mound
at Brackenridge Field Laboratory, which abuts Town Lake, part
of the Colorado River.
Taking the “know thy enemy” approach to heart, Gilbert
and his postdoctoral students first determined what they were dealing
with in Texas: a multiple-queen form of fire ant colonies with up
to 200 queens. They also established how imported queen ants establish
new colonies so successfully.
“My postdocs dug out their nests, separated out the queens
and put fine, colored wire—unbelievably tiny stuff—around
their waists to identify them at new mounds,” Gilbert said.
Using these marked queens, the researchers demonstrated the first
example of ant colonies spreading by “budding.” Unlike
imported fire ants in many southern states and in South America,
these pests didn’t start new nests as the result of a single
queen. In Texas, the imports spread like an advancing tank brigade,
with several of a mound’s queens leaving home with a host
of worker ants to set up new mounds.
Because pesticides can harm native ants as well, Gilbert became
interested in more ecological approaches. He has now spent more
than a decade fighting the Brazilian ants with a vampire-like fly.
A phorid fly hovers over a fire ant target until the fly can dart
in and inject an egg through a gap in the ant’s outer armor.
The egg grows into a larva, which eventually moves into the ant’s
head. There, it consumes muscle tissue that causes the head to fall
off within 10 days.
Since 1994, Gilbert has been raising and testing specific types
of decapitating phorid flies in areas of Austin, southern Texas
and other parts of the state. The fly from Brazil he has favored
so far prefers to catch imported ants during fights at disturbed
Gilbert and colleagues found Brazilian phorid
flies at new sites in fall 2003 (yellow triangles). The flies
were originally released at sites prior to February 2003 (white
circles). Native fire ants still exist in part of Austin,
based on 2002-2003 sightings (blue dots). Aerial photo: Texas
Natural Resources Information System.
Years of low rainfall and temperatures that were hotter than those
in their native homeland kept the experimentally introduced phorid
fly populations from establishing and spreading. But Gilbert’s
team kept releasing the flies each year, discovering along the way
that part of this particular phorid fly’s difficulty was its
preference to produce mostly female offspring from eggs laid in
larger workers, an example of what is called environmental sex determination.
That process was unfortunate, because the multiple-queen colonies
possess relatively few larger workers. This led Gilbert to suspect
that this particular phorid in Texas is restricted to producing
eggs that mostly become male flies, limiting mating opportunities.
Despite the challenges this biology presented to phorid reproduction
here, Gilbert finally hit pay dirt in June 2003 after a string of
rainy months. He and his team found dozens of the flies circling
over imported ant mounds he kicked at Brackenridge Field Laboratory
and at the City of Austin’s Indiangrass Nature Preserve and
a private ranch along Bull Creek used for phorid release sites.
“Just their mere presence changes the scene,” he said,
referring to the jittery behavior the imported ants develop around
a predatory phorid fly.
Fire ants often establish mounds in sunny
areas, such as this Texas cow pasture. Photo: Texas A&M
The imported phorid flies in the Austin area spread so well that
they were estimated to cover from 70,000 to 150,000 acres last June.
They also took hold in Caldwell, Texas, where colleague Charles
Barr at Texas A&M University conducted independent releases.
Gilbert is hopeful that enough survived the past mild winter to
reach similar numbers this summer and fall. He continues to evaluate
and release flies at multiple Texas sites.
But most of the native fire ants are already gone from Brackenridge,
where he estimates only a single native-ant mound can be found on
the 88 acres, unlike the old urban neighborhoods of Austin a few
To make things tougher on the imported ants, Gilbert and his colleagues
are working on other South American phorid flies that will attack
the pest fire ants at sites other than disturbed mounds. Some come
from more Texas-like climates in Argentina, and some produce more
In Larry Gilbert’s
lab, imported fire ants are sorted using a sieve into proper-size
targets for stocks of various phorid flies under study in
Just as doctors attack advanced cancers with multiple treatments,
the strategy is to stress the imported ants with a flying menagerie
of various phorids that work at different times of the day to disrupt
food foraging and other activities.
That approach essentially mimics what occurs in different parts
of South America, where phorid researchers have documented up to
six types of phorid flies locally that keep their host ants scrambling
for cover during daylight hours.
“It could take decades to work out the right combination
of control agents in Texas, phorids and otherwise,” Gilbert
said, noting that his main goal at age 61 is to be alive to see
the biocontrol approach work.
Meanwhile, he hopes to find time to tackle remaining questions
about the intricate biology of his favored subjects. For instance,
he’s still curious why native ants find protection in Austin’s
urban mosaic, where some feature—perhaps the roadblock real
roads provide to the mass-invasion approach of imported fire ants—has
kept them at bay.
“At Brackenridge, the University of Texas has gone through
all this trouble to preserve the habitat, and yet the native fire
ants, and most Texas harvester ant colonies have been wiped out,”
he said, a bemused grin on his face. “The same is true on
most ranches and Nature Conservancy reserves. Then we go over into
Tarrytown, where everybody takes care of their yards and perhaps
treats any ant mound they find with pesticides, and you’ve
got the native fire ants surviving.”
The irony of that finding isn’t lost on Gilbert, but you
can see the wheels turning as he envisions urban studies that could
provide new clues for undoing the imported fire ants’ grip
on his home state.
“I’m still optimistic that we haven’t gotten
to the bottom of the barrel of possibilities,” he said. “We
may just need a subtle shift in imported fire ant competitive advantage,
reproductive rate and invasion potential to control them.”
Photos of Professor Gilbert: Marsha