St. Tammany's ditches are fouled with untreated sewage. The mosquitoes are loving it.

Nicholas DeLisi had a problem: His insecticides were killing all species of mosquitoes in St. Tammany Parish's ditches but one, which appeared not to be affected at all.

The southern house mosquito, that one species that seemed to survive the insecticides, is a virus-carrying bug that can spread West Nile and St. Louis encephalitis, two diseases that can be deadly. It’s a mosquito that DeLisi and his colleagues at the St. Tammany Parish Mosquito Abatement District really want to be able to kill.

But the district couldn't make much of a dent in its population.

DeLisi, the lab manager and entomologist at the district, figured the most obvious reason was that “our sprays aren't working,” or that the southern house mosquito had become resistant to them.

But DeLisi had another theory: That the 600 miles of drainage ditches throughout the parish were often filled with untreated sewage from malfunctioning home septic systems, which create such a fertile breeding ground that, even though the mosquitoes were getting killed off, their population was immediately rebounding. 

“When you look at our hundreds and hundreds of miles of habitat, in a polluted environment that we’ve created, it’s possible that they’re just reproducing very quickly,” DeLisi said. “It’s called ‘reinfestation.’”

It’s well-documented that this species of mosquito likes to breed in sewage-polluted water. That’s in part because where there’s sewage, the mosquitofish, which eats mosquito larvae, can’t survive. 

Through an experiment, the results of which were published earlier this year in the Journal of Medical Entomology, DeLisi confirmed that that appears to be exactly what’s happening. Fueled by the sewage pollution, the mosquitoes are breeding faster than DeLisi’s colleagues can kill them off.

The St. Tammany Parish Council previously considered an ordinance last year that sought to reduce sewage pollution coming from malfunctioning home septic systems, but delayed a vote on it indefinitely. 

How old is a mosquito?

To test his theory, DeLisi resorted to “old school” bug science.

DeLisi’s team collected southern house mosquitoes before and after they sprayed a neighborhood in Covington with insecticides. Back in the lab, they sliced open the bugs’ abdomens and examined the ovaries of female mosquitoes under a microscope, which allowed them to roughly estimate how old they were.

It’s a cheap technique that was prevalent in entomological research back in the 1960s, said Jason Rasgon, a professor of disease epidemiology at Penn State University, who was not involved in the research.

“There are other ways to age mosquitoes that are much more technically challenging,” he said. “But anybody with a cheap microscope and a couple of pins can do this.”

Before they lay eggs, mosquitoes’ ovaries are tightly coiled, DeLisi explained. But after they’ve reproduced, they look like “spaghetti that’s been boiled and thrown against a wall.”

And mosquitoes reproduce fast. “Male mosquitoes are kind of creepy,” DeLisi said. “They wait for the females to emerge on the water's surface,” right where they hatch. So if a female mosquito hasn’t mated, it’s likely that it isn’t more than a day or two old.

The older a mosquito is, the more likely it is to have contracted a virus that it can then pass along to a human through a bite. 

In areas where the district sprayed insecticides, the bugs were, on average, younger. That indicates the insecticides were likely killing off mosquitoes. But by the next day, the population appeared to have already bounced back.

They did also find that one of their insecticides wasn’t effective: the mosquitoes had developed resistance to it. The other two they use work — but weren’t able to reduce the overall mosquito population.

“What we’re doing is providing a Band-Aid,” DeLisi said. “The root problem, the pollution in the drainage ditches, is not being addressed.”

And just as there are lots of ditches in the parish, there are also a lot of home septic systems: approximately 36,000, the parish government has estimated.

DeLisi cautioned that his specific study isn't definitive in linking the sewage pollution and the inability of his insecticides to reduce the mosquito population. But the sewage is no doubt contributing to the ballooning mosquito population.

“Reducing the sewage will reduce the mosquito abundance,” Rasgon said.

Rasgon found the study convincing and hopes that additional research might measure the prevalence of viruses in the bird population before and after insecticide spraying. That can be accomplished, he noted, by putting out what scientists call “sentinel chickens” — chickens that left outside, get bit by mosquitoes, and whose blood is then tested for viruses.