Decapitating Flies Taking Hold Around Austin; Insects Could Control Imported Fire Ants

Professor Lawrence E. Gilbert, an integrative biologist, and his colleagues have found that phorid flies released near the fire ants’ mounds at three sites in the area have begun to spread. The flies are now found on between 70,000 and 150,000 acres of land locally.

Thousands of a particular kind of phorid fly, Pseudacteon tricuspis, had been released at the City of Austin’s Indian Grass Prairie Reserve, the university’s Brackenridge Field Laboratory (BFL) that Gilbert directs for the College of Natural Sciences near Town Lake, and a private ranch on Spicewood Springs Road.

The flies released at Brackenridge, for example, had been established at low levels since late 1999, but had not expanded in numbers or territory. Large numbers began to be detected in June 2003. Flies were then found by the last warm weather in early January as far as seven miles from their predecessors’ original release sites.

“Suddenly, you’ve got them all over Brackenridge Field Laboratory, but also miles away and spreading out from other release sites in central Texas,” Gilbert said.

The first species of phorid flies released here came from Brazil, just like the imported fire ants they are intended to keep in check. These parasites, which are much smaller than fruit flies, attack worker ants at disturbed mounds. There, they dart in to inject an egg into an ant’s body. The resulting larva moves into the host ant’s head, causing it to fall off in about 10 days.

The imported red fire ants likely arrived in Austin in the 1970s, about 40 years after reaching the port of Mobile, Ala., as unintentional cargo. The exotic fire ants are much more abundant than native fire ants, are pests to livestock owners and people, and may have caused the disappearance of horned lizards by annihilating one of its major food sources: Texas native Harvester ants.

The imported ants often take over land once occupied by their less prolific, native counterparts, which have their own species of decapitating phorid flies for population control. At BFL, Gilbert said, every mound of native fire ants was replaced by about six mounds of the imported ants during the initial imported fire ant invasion there in the 1980s. Many more imported ant mounds exist there now, and native fire ants are no longer present.

Although Gilbert’s team began releasing the Brazilian phorid flies in 1996, lack of consistent rainfall, severe drought and other factors for several years apparently prevented their successful establishment at most test sites. However, beginning in July 2002, the Austin area received some rainfall each month until late spring, in advance of the June 2003 rainfalls, and Gilbert’s first sighting of an upswing at BFL in phorid flies.

That June, up to 20 flies arrived to attack fire ants at about 85 percent of the mounds disturbed across the 88-acre field laboratory. Gilbert noted that that resembles what can be seen in Brazil when researchers kick into fire ant mounds.

“After all these years of most phorid introduction sites apparently failing, suddenly we’ve had a total shift,” Gilbert said, noting that similar efforts in the milder climate of Gainesville, Fla., took off more rapidly since the flies’ introduction in 1997.

While the work in Texas continues, BFL biologists are testing other imported phorid flies that prefer to attack worker ants along trails as they forage for food. These phorid species may do better at disrupting ant colonies’ attempts to maintain a food supply. Gilbert and his colleagues also are looking for other phorid species in north-central Argentina that may be more equipped to survive in the harsh, unpredictable climate of Texas. As many as eight different kinds of phorids harass fire ants in Argentina.