WELLFLEET — Barton Morris stands in the Duck Harbor parking lot and surveys the floodplain to the north. It’s a cool April day, and it’s been about two weeks since the last in a series of strong storms hit this beach. The storms pushed salt water over the dunes into this floodplain, creating puddles in the field of newly cut wood chips.
Morris is here to investigate those puddles — or, more accurately, what’s in them. He puts on his boots, gets out his dipper — a low-tech cup on the end of a pole — and marches out into the muddy plain to look for mosquito larvae.
Wellfleet is wary about the bloodsuckers. In a January 2021 storm, the coastal bank at Duck Harbor was overwashed, storm waves pushing salt water into the pine forest here, killing it. Then, the water pooled, trapped on one side by the bank and on the other side by mosquito ditches dug long ago to keep water flowing but long since abandoned and clogged. These pools of stagnant water were the perfect breeding ground for mosquitoes.
In the summer that followed, the mosquito population in Wellfleet exploded to unprecedented levels. Morris, who is the assistant superintendent of the quasi-governmental Cape Cod Mosquito Control Project (CCMCP), said that traps in the area were capturing thousands of mosquitoes every day in summer 2021 (the one-day record was 5,642), when any number more than 200 is considered a problem by the agency.
Fortunately, Morris says, Duck Harbor isn’t much of an area of concern anymore. The trees have been felled, the ditches are clear, and the gap in the dunes has widened, allowing water to flow out more easily. Plus, he notes, the larger puddles have fish in them, and they eat mosquito larvae.
He lowers his dipper into a few puddles, and they all come up empty. Besides a small second overwash creating a new stagnant wetland south of the parking lot, which Morris says could pose a problem without regular treatment, Duck Harbor is now a secondary concern. Instead of working here, he packs up his gear and heads inland, where the real problems lie.
New Floods at Bound Brook
All winter and spring, regular flooding has pushed salt water farther up the Herring River, inundating roads and often cutting off residents of Bound Brook Island from the rest of town. These floods have mostly been driven by strong storms out of the east coinciding with full or new Moons when high tides are higher than usual.
While the roads are clear for now and the town and Cape Cod National Seashore are working on plans to ensure the roads stay clear though the summer, CCMCP is worrying about the fact that the floods have created perfect mosquito breeding grounds — pockets of stagnant salt water much farther inland than ever before. These are particularly prevalent around the confluence of the Herring River and Bound Brook near the Atwood-Higgins House and around High Toss Road, Morris said.
If we get lucky and have a dry, storm-free spring, the flood water could all dry up before too many mosquitoes hatch. But both Morris and his boss, Gabrielle Sakolsky, the superintendent of CCMCP, said the conditions indicate another big outbreak of mosquitoes is possible this summer.
If this flood water sticks around, the saltwater mosquitoes getting a boost are active during the day, according to Morris, making them hard to avoid. They can also fly upwards of five miles from where they hatch, meaning an outbreak along the Herring River could affect a large area from Truro to South Wellfleet.
Morris parks his truck along Bound Brook Island Road and walks up a trail near Merrick Island. The forest is full of puddles, some nearly knee-deep. “Until Duck Harbor, there was never really water here,” he says. He sticks his dipper into the first puddle he sees and scoops up a cupful of water. In the tea-brown water are squirming mosquito larvae. Morris returns to his truck, where he fills a plastic backpack connected to a hose with a brown, milky liquid — a larvicide.
During the winter, CCMCP crews are mostly focused on water management, digging and clearing ditches to allow stagnant water to drain. Starting in mid-April, they shift their control measures to larvicide application. They use a strain of Bti (Bacillus thuringiensis subspecies israelensis), which contains spores that produce toxins that specifically target mosquitoes. Bti is found naturally in Cape Cod soil and is harmless to the vast majority of animals, including humans: besides mosquitoes, it kills only a handful of other fly larvae. Morris doesn’t wear protective gear while applying it.
Morris begins wading through the puddles, spraying larvicide in each one. Across the Cape, 10 crews of CCMCP workers apply larvicide this way, spraying each water source listed on a map of thousands of mosquito breeding sites. They repeat their applications regularly, as the larvicide wears off after a few days. Morris, however, has mostly been out here by himself. The crews are tied up with their regular routes, so the duty of spraying the flood zone has fallen almost entirely to him. Puddle by puddle, he empties his four-gallon backpack into the marsh before returning to his truck.
Even here, relatively close to the road, there are areas Morris can’t reach. He points across a channel to a thorny, overgrown patch of brush in the marsh. He has no idea whether there is water pooling there, he said, and the brush is so dense it would be extremely difficult to check.
“I was hoping they would clear that,” he said, referring to the brush clearing that the Herring River Restoration Project has been doing in the area.
Much Depends on the Weather
In the long term, the Herring River Restoration Project will allow salt water to drain from this area more easily, Morris said, hopefully making mosquitoes less of a concern. But for this year, the big unknown is whether flooding will happen again, and if not, whether the current flood water will manage to drain from the system before the mosquito population explodes.
Sakolsky said there’s reason to be hopeful: the water levels in the area are already going down, and many puddles are drying up; since the last storm in early April, there has been no further water accumulation. Also, our spring has been cold so far, which slows mosquito development. The forecast for early May looks dry, too.
Sakolsky said her team will “just have to be regularly on it, checking in, and treating what we can get to.” As the summer heats up and mosquitoes develop more rapidly, the team will apply larvicide to this area more regularly. Morris said he will likely bring in additional workers then.
A lot depends on the weather over the next few months. Sakolsky said she is hoping for limited rainfall and calm weather during the kinds of high Moon tides that caused the flooding previously.
“Nice and cool, no rain, no storms,” she said. “I could use a little bit of that.”
