NCMCD director David Lawson with an illustration of one of his subjects/ Credit: Needham Observer

It wouldn’t be summer without a mosquito or two crashing your cookout or tagging along on your hike.

More than 50 species of mosquito live in Massachusetts, but according to entomologist Kaitlyn O’Donnell of the Norfolk County Mosquito Control District, “We really are only focusing on about a dozen that bite people and spread disease.” 

Mosquitoes themselves are not the cause of disease, it’s the viruses they inject into the bloodstream when they bite. The NCMCD uses an integrated pest management approach to reduce the risk of mosquito-borne illnesses. In Massachusetts, the two viruses people worry about are Eastern Equine Encephalitis and West Nile virus. Both can cause illness, but EEE is the one with the high mortality rate. 

EEE: A bad disease

Kaitlyn O’Donnell counts mosquitoes/ Credit: Needham Observer

EEE is serious. “The brain swells, you quickly (develop) fever, convulsions,” said NCMCD director David Lawson. “One-third to half the people who get EEE die of it. Even children sometimes. It’s tragic.” 2019 was a big year for EEE. “Twelve human cases (in MA); six died,” Lawson said. “That’s high, and that’s typical for EEE.”

He emphasized that EEE is rare. “On a global scale, this is not a major risk,” he said. “The last human case of EEE was in 2020. None in ’21, none in ’22, none last year.” 

EEE is spread by birds. “Culiseta malanura (the black-tailed mosquito) bite almost exclusively birds,” said Lawson. “(When) the birds have high levels of virus in them, then it ‘goes viral’ … it spreads from bird to bird to bird. All it takes is for one random mosquito to decide to bite a bird and then bite a human.”

A 2024 EEE resurgence?

“This year we’ve ticked a lot of the boxes” suggesting the possible resurgence of EEE, Lawson said. One factor is bird immunity. “In order for EEE to get a good foothold, the bird population needs to be naïve (not immune). After 2019, a large amount of the bird population became immune to EEE. Having gone five years, there’s a bunch of new young naïve birds,” he said.

The other factor is the mosquitoes. “We had a very wet fall/winter/spring, so the habitat where the primary vector breeds has been in great shape,” said O’Donnell. “We’re already picking up high numbers of that vector this year. All of the signs are pointing to it being a EEE year.” 

West Nile never left

“We get West Nile every year,” said O’Donnell. “The hotter and drier it is, the worse West Nile is.” WNV is spread by a different mosquito, Culex pipiens, which O’Donnell describes as more of an urban/suburban mosquito. “They like to come inside and hang around people.”

WNV can be fatal, but the mortality rate is not nearly as high as EEE. “A much larger percent of people get West Nile and never know they ever had it,” said Lawson. “From 2000 to 2012, there were 103 human cases and eight fatalities.”

Controlling the source

The NCMCD, which includes Needham and surrounding towns, has a year-round integrated pest management strategy to reduce mosquito-borne illnesses. It involves much more than spraying insecticides.

“The example I use,” said Lawson, “if you found a cockroach in your kitchen, you could go crazy and spray Raid all over, and it would probably work. But the main thing to do is make sure there’s no crumbs or food on the floor or countertop. You’ve taken away the source, and you didn’t have to spray anything. That’s an integrated pest management mindset.”

The NCMCD starts its work in the winter, targeting breeding grounds where the eggs or larvae have collected. “That’s when we do all this work of clearing ditches, keeping water flowing,” said Lawson. “Mosquitoes need standing water for their life cycle to develop. Residents don’t realize they are breeding mosquitoes in their own yard — toys, tarps, birdbaths, rain barrels, gutters, flowerpots, tires.”

When the weather warms, NCMCD spreads larvicide in wetland areas. “We contract with a helicopter company for the larvicide, (which consists of) ground up corncob with some bacteria that’s normally in the soil, that are toxic just to mosquito and black fly larvae,” said Lawson.

David Lawson checks the Needham larvicide map/ Credit: Needham Observer

Spraying

Spraying for adult mosquitoes is the final stage of integrated pest management, Lawson explained. Adulticides are sprayed from trucks. “Our program is driven by surveillance of mosquito populations,” said Lawson. “Residents are also able to call our office and say, ‘Hey the mosquitoes are terrible in our yard.’ In Needham, they ask us to start after 10 p.m. Needham is always done on Tuesday evenings.”

“People will call and ask, ‘Do I need to close my windows,’” said Lawson. “We say feel free, but you don’t have to, because the amount we are putting out is so miniscule.” 

“The adulticide is a general insecticide,” said O’Donnell. “We are using it at an incredibly low rate. It has been tested against other (beneficial) insects, and the dose we’re using is not enough to kill them.” While it is not completely benign, one study showed no excess mortality in honeybees. 

Insects are not all bad

Although she works in mosquito control, O’Donnell is a bug-lover. Her favorites? Leaf cutter ants and mayflies — and some mosquitoes, too. “There are good mosquitoes out there,” she said. “The elephant mosquito gets all of their protein in their larval stage, so they don’t need to take a blood meal. They are a really beautiful mosquito, metallic blue,” she said. “They eat other mosquitoes.”

To learn more about mosquitoes, source reduction, spraying routes, and to read frequently asked questions, visit the NCMCD website. Statewide risk maps are updated regularly. 

For a history lesson on how mosquitoes have killed more humans than any other cause, read The Mosquito: a human history of our deadliest predator.

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