The Louisiana coast is again buzzing with big, green blood-hungry flies. And that's a really good thing, according to Claudia Husseneder, a biologist tracking the recovery of horse flies after the 2010 BP Deepwater Horizon oil disaster.
"This blood-sucking nemesis is a sign of a healthy coast," she said. "They're a bio-indicator of ecosystem health after an oil spill."
Husseneder's research for the Louisiana State University AgCenter shows that the green-head horse fly, a native of Louisiana's coastal marshes, was particularly hard-hit by the BP spill, considered the world's largest and most destructive oil disaster. She recently presented her findings at the Gulf of Mexico Oil Spill and Ecosystem Science Conference in New Orleans.
The disaster harmed nearly every species in the Gulf, from dolphins to shore birds, but the horse fly hints at trends in wider ecosystem recovery. They're the top predator of marsh insects and many tiny invertebrates. A marsh with few horse flies is usually a sign that the food chain is out of whack or there's something toxic in the environment.
Flies have a fatal attraction to oil. They mistake an oily sheen for fresh water, and end up stuck in the toxic substance. Oil from the BP disaster also decimated fly larvae, which live in marsh mud for several months while they grow into adults.
Husseneder and her colleague Lane Foil rushed out to the marshes just as the Deepwater Horizon rig began spewing oil in April 2010. They wanted to get a baseline of data on horse fly populations before the oil reached the marsh. Three of their test sites - Grand Bayou, Grand Isle and Elmer's Island - were soon contaminated with oil. Three other sites in Cameron and St. Mary parishes remained pristine.
"You might ask how you catch horse flies," Husseneder said. "Well, they're out for blood so we used carbon dioxide."
The scent of carbon dioxide is a horse fly's cue that blood is near. AgCenter scientists baited their fly traps with a chunk of dry ice, carbon dioxide's solid form. Traps in marshes untouched by the disaster drew swarms of flies.
"But at the oiled sites, we found there was a severe population crash right after the oil spill," Husseneder said. "Even up to two years after the spill we could hardly find any larvae."
Much of the BP oil degraded naturally from exposure to air, sunlight and oil-eating bacteria. Fly populations started to rebuild as the oil degraded, but they suffered from weak genes for years. "There was a genetic bottleneck because the population had been so reduced."
Horse flies from clean marshes eventually came to the rescue, adding numbers and diversifying the gene pool. It took six years before the contaminated marshes boasted healthy horse fly populations.
"In 2015 the numbers were rising, but there was a lot of buzzing around 2016," Husseneder said.
Other species are not faring as well. Bottlenose dolphins, sea turtles and deep-sea coral are among the marine animals still still struggling after the disaster.
The horse fly resurgence may come as bad news for anglers, duck hunters and other frequent visitors to Louisiana's marshlands.
"But look on the bright side of life," Hasseneder said. "It means you're in a much healthier and productive marsh again."
The Dutch have a new lesson: Building bigger, better infrastructure won’t be enough to overcome the threats posed by changing climate.
---
Tristan Baurick covers Louisiana's coastal environment for NOLA.com | The Times-Picayune. Email: tbaurick@nola.com * Twitter: @tristanbaurick * Facebook: Tristan Baurick and Louisiana Coastal Watch.