Researchers see greenhead horse fly as indicator of marsh health

Richard Bogren, Husseneder, Claudia, Foil, Lane D.

A greenhead horse fly sits next to a drop of blood on human skin. (Photo by Claudia Husseneder, LSU AgCenter)

A greenhead horse fly feeds on human blood. (Photo by Claudia Husseneder, LSU AgCenter)

(01/26/16) BATON ROUGE, La. – The presence of a top predator of other invertebrates in Louisiana’s coastal marshes may shed light on how the marshes are recovering from the effects of the BP oil spill in April 2010.

Almost immediately after the Deepwater Horizon began spewing oil into the Gulf of Mexico, LSU AgCenter entomologists Lane Foil and Claudia Husseneder headed for the Louisiana coast to start gathering data on coastal insects.

Foil joined the LSU AgCenter in 1980, when he began studying horse flies as vectors of diseases of horses and cattle. Husseneder’s specialty is insect population genetics. They combined their specialties and Foil’s knowledge of the Louisiana coast to study the effects of the oil spill. And they chose the greenhead horse fly as their research subject.

The greenhead horse fly is one of few insects that are native to and tightly bound to specific coastal marsh habitats in Louisiana. This insect inhabits coastal marshes that range from Texas to Nova Scotia.

Foil and Husseneder assumed that the prevalence of greenhead horse fly populations would be an indicator of the relative health of the marsh.

The researchers selected six locations along the Louisiana coast and began gathering population data on adult greenhead horse flies – a biting nemesis of people and animals alike – as an indicator of the health and recovery of the oil-damaged marshes.

As it turned out, oil from the spill reached three of the locations, and three remained “pristine,” giving the researchers the opportunity to compare how oil intrusion affected the marsh.

Using funding from the National Science Foundation, the researchers began their study by comparing horse fly populations in the unaffected marsh areas with those in areas subject to the oil spill. They then used population data and genetic analysis to evaluate and compare the health of the horse fly populations in the different locations.

The oiled sites were located on Grand Bayou, Grand Isle and Elmer’s Island in southeast Louisiana. Pristine trap sites were located in southwestern Louisiana – Ship Channel in Cameron Parish, Rockefeller Wildlife Refuge at the border of Cameron and Vermilion parishes and Cypremort Point in St. Mary Parish.

They collected adult horse flies from June to October in 2010 and 2011. In addition, they collected the predatory larvae in 2011 from four of the sites.

“Their abundance is reflective of the health of the invertebrate food web around them in the mud of the tidal marshes,” Foil said. “Therefore, we hypothesized that horse fly populations could serve as bioindicators of marsh health and toxic effects of oil intrusion.”

After they started their research, Foil and Husseneder obtained a bridge grant from the Gulf of Mexico Research Initiative (GoMRI) that allowed them to continue their study. Results from the initial research were reported in a Nature journal, Scientific Reports 6, Article number: 18968 on Jan. 12, 2016. (

This initial research assembled data on population size and genetics of horse flies trapped in the various locations. Initially, the researchers discovered sharp population declines, or “crashes,” among adult horse flies.

“Our study showed decreased effective population size, number of breeders and family clusters in oiled populations that experienced genetic bottlenecks compared to populations in unaffected areas,” Husseneder said. “The severe population crash caused fewer parents to be available in oiled populations to contribute offspring.”

The most likely reasons the adult population crashed in oiled areas immediately after the oil spill were the flies’ need for fresh water and attraction to oil sheen, the researchers said.

The effective population size will probably continue to be decreased for several generations until other horse flies move into the areas to replenish the gene pool, Husseneder said.

In oiled areas, the traps captured fewer than five flies per hour, indicating they had been affected immediately after the oil reached the research sites. Horse flies captured at the unoiled sites, meanwhile, ranged from nearly 37 to more than 92 flies per hour.

The researchers recently received nearly $2 million in funding from the Gulf of Mexico Research Initiative (GoMRI) Research Board to continue their study by focusing on horse fly larvae and the invertebrate food web.

Horse fly larvae are at the top of the food chain in marsh soil, so their presence can be used to evaluate marsh health. “We know oil had an impact on the species,” Foil said. “We want to find out if the affected environments are recovering.”

The larvae live in marsh mud for three to nine months while they grow into adult flies. They are the top predators of invertebrates in that environment and feed on smaller invertebrates in the soil, Husseneder said. “As the larvae grow, their diet changes, consuming larger organisms.”

Samples of marsh soil showed far fewer larvae in the oiled areas than in the pristine sites.

“We discovered genetic bottlenecks in all but one of the oiled populations,” Husseneder said. Bottlenecks are reductions in the gene pool of a population because portions of the original population are lost, reducing genetic diversity and possibly the vitality of the populations.

“This sustained population suppression was likely due to toxic effects directly to the larvae or their food web,” Husseneder said. “Therefore, this top predator species with sediment-dwelling larvae did not show the quick recovery seen from plant-feeding insects in spartina marshes.”

The researchers are continuing with genetic studies and larval surveys to identify bioindicators to determine whether increases in populations are the result of new flies moving into the area or if the marsh sediment has detoxified sufficiently to allow the local populations to recover.

“We know oil had an impact on the species,” Foil said. “We want to find out now if these environments are recovering.”

“We want to identify genetic markers to assess and monitor the presence or absence of bioindicators,” Husseneder said. “Insects and the food chain will tell us a lot about the biology – how populations improve and how populations are replenished.”

The application of the results of the completed and future studies also will be an important element in evaluating mitigation and coastal restoration efforts, she said.

Rick Bogren
1/26/2016 11:40:01 PM
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