Insects fall silent in oiled coastal marshes

Louisiana’s coastal marshes can be noisy places with insects buzzing and chirping constantly, but that’s no longer the case in some places.

“What happened after the Deepwater Horizon is when we came to marsh impacted by the oil, they were relatively silent,” said Linda Hooper-Bui, associate professor in the Department of Entomology at LSU.

Preliminary results from field work and lab experiments point to two oil components — naphthalene and methylnaphthlane — to be at least part of an explanation for large declines in insect populations within oiled or previously oiled areas of coastal marsh, she said.

“We have results, good information, that these are increasing and that this is an emerging problem,” Hooper-Bui said of the two compounds.

The mystery is why the compounds are increasing, she said.

Eugene Turner, Boyd Professor in the LSU School of the Coast and Environment at LSU, said the two compounds are aromatics that should be venting into the atmosphere, but they’re not. Instead, some process is creating more of it in the soil than is being allowed to be released, he said.

In addition, Turner said, the problem is being found not just in areas that received the heaviest oiling but in many areas that just received some oil.

Although preliminary results point to the two compounds as a good candidate for causing or contributing to the insect reduction in oiled areas, research continues, he said. He said researchers don’t know what’s breaking down to form the compounds, but they’re working on that now.

“We’re still teasing this out,” said Ed Overton, emeritus professor at the School of the Coast and Environment at LSU. “We’re a long way from figuring this thing out.”

Hooper-Bui said the two compounds suspected as the cause for the decline in insect populations in the coastal marshes are known to be insecticides. She said they are widely used in mothballs because their toxicity to insects.

Hooper-Bui and a number of other researchers from LSU and other universities have been looking at ecosystemwide potential impacts from the Deepwater Horizon disaster in 2010 as part of the Coastal Water Consortium. Funding for the work has come from the National Science Foundation, the Northern Gulf Institute, the Gulf of Mexico Research Initiative and two grants from LSU.

Part of Hooper-Bui’s work was to look at the impacts to various insects and spiders not only for their own sake, but because they are an important part of the coastal marsh food chain.

Some insects were disappearing that didn’t have much contact with sediment or water in the marsh so the researchers set up a field experiment. Crickets were placed in small cages with food and water and then floated in a cage on the marsh so the only contact they would have with the environment would be the air, she explained.

The crickets in the oiled areas died.

“It was something in the air that was killing them,” Hooper-Bui said.

That field work was followed by some laboratory experiments using sediment collected in 2011 and again, the crickets died.

More recently, the laboratory experiment was repeated for soil collected in March 2013, and the crickets on that soil again died, she said.

There were other signs of something is wrong within the more than 100 insects and spiders the researchers examined. One example is the population decline of ants that live in the hollow stems of marsh grass starting in 2010.

“By March 2012, we were extremely hard pressed to see ants in oiled areas,” Hooper-Bui said.

In May, the ants had mating flights and she and other researchers tracked where they landed and marked the new colonies to revisit later.

“By July, all of those colonies had disappeared,” she said.

Since January, Hooper-Bui and fellow researchers have been looking for ant colonies in oiled sites but haven’t found any yet, she said. Currently, researchers are looking for mating flights of ants in these areas, but haven’t found any yet as well.

“So, the ants are gone in the oiled areas,” Hooper-Bui said.

She said there is evidence to suggest that the ants’ food sources are gone and noted that laboratory experiments suggest that ants stop foraging when there is oil around.

Hooper-Bui said the next steps for researchers include trying to figure out concentrations of the compounds in the air since they already know the concentration in the sediment. That air concentration information is needed before researchers can examine possible impacts on other organisms, or even people.

However, Hooper-Bui said when they found the two suspected compounds, she did some research on the toxicology associated with them. She said they found symptoms of naphthalene poisoning match what some residents of south Louisiana have been complaining about including skin rashes, respiratory distress, digestive distress and more.

“It’s very distressing to me, but we are lacking information,” she said.

At this point, she said, what they do know is that the levels appear to be high enough to kill insects.

“This is just one piece of the puzzle, but we don’t know what the puzzle looks like,” she said. “We just keep chipping away at it.”