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Researchers try caddisflies as bio control of lake milfoil

by RALPH BARTHOLDT
Staff Writer | May 3, 2020 1:30 AM

An insect that dances like smoke on the water could be the savior that destroys noxious milfoil.

Or, maybe not.

But the small, long-horned caddisfly, known as the white Millers, likely slows down the spread of the noxious aquatic plant.

In their effort to find a biological control mechanism to keep milfoil knocked down in places where people recreate, University of Idaho researchers in conjunction with the Coeur d’Alene Tribe recently began studying the long-horned caddisfly.

Although it is indigenous to slow-moving, weedy water such as the lower part of Lake Coeur d’Alene including the Round, Benewah and Chatcolet chain lakes — the same places milfoil grows — the caddisfly’s existence in Lake Coeur d’Alene had not been documented until a few years ago.

“Technically, it didn’t exist in Idaho,” said UI undergraduate biologist Beth Hoots.

When researchers began examining the bug more closely, they learned its larvae, which lives underwater, made meals of milfoil.

“You can literally see where it chews it down,” said Ben Scofield, the water resource specialist for the Tribe’s lake management department.

The news sent a ripple of excitement through the lake management community.

Eurasian milfoil is an invasive submerged aquatic plant that forms dense mats of vegetation on the water’s surface. The mats inhibit recreation and natural water flow, according to the University of Idaho. The state’s department of natural resources considers it a pernicious invader that spreads rapidly, can travel from lake to lake via plant fragments on boat trailers, bilge water or boat motors and the fragments grow into roots, stems, and leaves.

The Tribe, which had been using herbicides to slow the spread of milfoil, enlisted students to take part in a study to determine if the caddisfly could be a bio regulator.

Despite some positive returns, several years of research have showed mediocre results.

“It’s pretty much of a generalist,” Scofield said.

The bug, despite its gregarious appetite, doesn’t just target milfoil.

“They have an impact on milfoil, but also on other plants,” Scofield said.

Since around 2004, the Coeur d’Alene Tribe has taken on the odious task of controlling non-indigenous aquatics — primarily milfoil — in the southern section of Lake Coeur d’Alene, which is under its management.

Divers have been used to handpick plants as they sprout from the lake’s bottom, or they laid down sediment covers — sheets of porous fabric rolled across the lake bottom — to keep the plants from growing.

The use of herbicides including 2,4-D was tried for almost a decade, but the floating corrals that were supposed to keep the herbicides from spreading and killing native plants were difficult to control, especially on windy days. Herbicides leaked out and killed invertebrates such as insects natural to the lakes.

“We moved away from it because of its inconsistent results,” Scofield said.

That is why the discovery of the milfoil-eating caddisfly brought high hopes for its use as a biological control. In other places similar biological controls in the form of weevils, midges and caterpillars have been tested to cut down milfoil, but — just like the white Millers — they have been largely ineffective.

The caddis studies prompted the Tribe to take another avenue. Lake managers two years ago rolled out the big guns in the form of a mechanical harvester.

This summer will be the third season that the harvester rumbles around the southern lake’s milfoil fields.

“Literally, it’s a diesel tractor on the water,” Scofield said.

The tractor rides on a barge and uses a conveyor belt to move the harvested milfoil from the water to the deck, where it is piled and offloaded to decompose on shore, or to heap into burn piles.

In Lake Chatcolet, milfoil likes to grow in water around 18 feet deep.

“Just below the drawdown depth,” he said.

One way to kill milfoil is to draw down a lake or pond and subject its roots to a good freeze.

Although harvesting is messy and may contribute to the further spread of milfoil as bits are cut and float away, Scofield said the noxious weed already exists in every habitat it can occupy in Chatcolet Lake.

“The harvesting relieves the nuisance,” he said. “It’s like mowing the lawn.”

But UI researchers are not done looking at the caddisfly’s effect on milfoil. Using white Millers is part of an undergraduate study to determine how temperature fluctuations and season growth, if they coincide, could knock down the noxious weed.

“The high abundance of caddisflies we observed at the study sites in 2017 possibly may have damaged the (milfoil) sufficiently to impair ... overwinter survival and contributed to the low density we observed in 2018,” graduate student Stephanie Estell wrote as part of her thesis.

While studies continue at least for another season, managers for now are relying on the harvester to control milfoil mats in the southern lake.

“Milfoil is here to stay,” Scofield said.