As river levels fall, crews treat gnat larvae along rivers

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The Metropolitan Mosquito Control District usually kills gnat larvae in early May, but high water levels have kept treatment crews away until now.

Wednesday, workers poured bacterium into the Minnesota River in Belle Plaine to target and kill gnat or black fly larvae. MMCD is treating rivers throughout the seven-county area.

“It’s been designed to not sink to the bottom, not float to the bottom, mix with the water perfectly, and the river does the work,” said John Walz with the Metro Mosquito Control District. “It carries it downstream and gets the larvae that are attaching to tree branches and rocks in the water.”

It’s important to get rid of the larvae now before they reach adult stage and become an outdoor nuisance. These bugs don’t live in lakes or ponds, so it’s only rivers that will be treated.

“Their flight range, I’ve heard, can be up to twenty or thirty miles, so the entire metro can be effected by this river,” said Casey Herrmann with Metro Mosquito Control District.

The treatment begins to work immediately and it’s safe for everything else in the water, but for the gnat larvae - it’s poison.

“Three hours later, we’ll go downstream, we’ll pick up a stick and look at it and say, ‘This gnat larvae is dead, everything else is alive,’” said Walz. “It’s specific to gnats because they ingest it and it reacts with their gut chemistry.”

With the larvae gone, the Twin Cities metro should see less of gnats in the days ahead. Adult gnats only have a lifespan of about two weeks.

“I’ve been here, ya know, thirty years, most of the time doing black fly control, gnat control and this is I would say the worst I’ve seen it, yes,” said Walz.

Experts say fifty years ago, the gnat problem wasn't that bad, but as water quality has improved, the number of gnats has also increased.