Greenhead horse flies give clues to marsh health after oil spills

Bruce Schultz, Foil, Lane D., Husseneder, Claudia  |  4/22/2015 1:52:54 AM

horse fly

A greenhead horse fly feeding on a visitor to the Louisiana marsh. Two LSU AgCenter scientists are studying the effects of the BP oil spill on the horse fly in affected areas. Photos by Claudia Husseneder

News Release Distributed 04/21/15

BATON ROUGE, La. – Two LSU AgCenter scientists are studying the effects of the BP oil spill on a horse fly species, using it as a bellwether to provide clues to the health of the marsh after the ecological disaster that occurred five years ago.

Claudia Husseneder and Lane Foil have studied the greenhead horse fly population in areas along the coast affected by the spill. The insect can be a biting pest for anyone who ventures into the marsh during warmer months, but it also provides a clue into the health of a marsh environment.

“The horse fly larvae are a top invertebrate predator in the marsh,” said Foil, an entomologist who has specialized in horseflies. He said the fly’s larvae feed on other invertebrates in the marsh mud.

The adults often feed on visitors to the marsh, he said, and they are an indicator of the habitat’s condition. “If you get attacked by horse flies, you are in a very healthy marsh.”

Foil set traps in the marsh to determine the horse fly population along the Louisiana coast. The number of flies caught in areas that received no oil was high compared with the low catch totals in areas hardest hit by the spilled crude.

“What we found by trapping the adult was a severe population crash,” Husseneder said.

Foil said a complicating factor was the obvious lack of data on horse fly populations before the spill, but people who frequented the marsh reported a noticeable absence of the horse flies and mosquitoes in the marsh after the spill.

Husseneder conducted genetic studies of the captured flies in the affected area and found that the DNA of the survivors showed reduced genetic diversity, indicative of a population crash that left few mating survivors, compared with horseflies from unaffected areas with a higher genetic variation.

Foil said the study is the subject of a scientific paper to be submitted to a peer-reviewed journal, and the genetic investigation is what makes the research significant.

“The census wouldn’t have meant anything without the work Claudia did showing the population had gone through bottlenecks,” Foil said.

It’s possible that the adult population sharply decreased because adult horseflies seemed to be attracted to the oil sheen and they died once they were covered in oil, according to Foil. He said this phenomenon occurred in Kuwait during the first Iraq war. Flies were attracted to oil, and it’s thought that they perceive the pools to be fresh water.

Husseneder said the study was only conducted in 2010-12, but efforts are underway to obtain grant funding to conduct more research. “We want to do a study of the long-term impact to see if the population has recovered and if the genetics have changed.”

Husseneder said people have reported that the number of horseflies in the area has recently increased. But the researchers want to scientifically verify that and check the genetic markers to see if horse flies from unaffected areas have migrated and repopulated the oiled areas or if the original populations have somewhat recovered.

She said the genetic signatures of horse fly populations will give a clue about their past loss in numbers and their present recovery potential. This might be a model for predictions for the impact of future catastrophic insults to the marsh.

Foil said a grant application has been submitted to provide funding to use drones to collect mud samples from the marsh to determine the health of the subsurface food chain available to horse fly larvae.

He said in many areas that were affected by the spill, crude oil can still be found in the mud. “You dig down into black goo.” That’s true in New England where a large oil spill hit the Massachusetts coast more than 30 years ago, he said, as well as the 1989 oil spill in Valdez, Alaska.

Bruce Schultz

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