Weed-Eating Weevils to Prove Their Worth With Invasive Milfoil
By Mark Prado, The Marin Independent Journal, Novato, Calif.
Jun. 15–MARIN WATER officials are considering unleashing tick-size weevils to combat an alien plant that has invaded Mount Tamalpais reservoirs.
The slim, green non-native Eurasian watermilfoil, a plant sometimes found in home aquariums, has taken root in the floor of five Marin Municipal Water District reservoirs, triggering concerns about water quality.
One solution under study is importing a weevil — an aquatic beetle — which feeds on the plant, killing it.
The plant can grow up to 12 feet and typically thrives along the shoreline. It has been an increasing problem around the state, clogging areas of Lake Tahoe, the Sacramento Delta and Central Valley canals and other waterways.
“It’s one of the biggest aquatic weed problems because it can choke out a water body,” said Greg Andrew, the water district’s fisheries manager, as he scooped the plant out of Alpine Lake with a rake.
“It grows up to the surface, then grows into a big mass,” he said.
First spotted in 2004, the plant has grown so rapidly it now rings Bon Tempe reservoir near Fairfax. Bon Tempe has been the hardest hit by the plant because of its shallow shoreline, which measures almost four miles.
“It’s astounding to me it showed up in 2004 and the problem is now that extensive,” said Alex Forman, water board vice president.
It’s not clear how the plant arrived in the Mt. Tam watershed, but it’s possible it was introduced by someone dumping aquarium contents into a reservoir.
“It is in the aquarium
trade,” said Janet Klein, vegetation ecologist with the district. “So if someone comes to the reservoir and dumps their goldfish and the plants, it can start growing.”
Because the reservoirs are interconnected, the plant can travel and it has also turned up to a lesser extent in Lagunitas, Phoenix and Kent lakes.
As it grows to the surface the plant spreads, blocking out sunlight for everything beneath it. Anglers who have visited the lakes have complained they can’t reel in their lines without getting their gear caught in the weeds.
There is also concern that as the plant dies, more carbon gets into the reservoir system, affecting water quality. And there is potential for the plant to get sucked into reservoir intake systems.
What are the solutions?
Drawing a lake down to dry and killing the plant is one technique used in other parts of the country, but that won’t work in Marin because water needs to be at designated levels to accommodate water distribution to district pipelines.
Using a chemical agent to kill the plant would work, but the technique would be a political hot potato in Marin — especially because there are uncertainties about how the water would be affected, Andrew said.
A harvester or “floating lawnmower” could be used to pull out the plant. But the process tends to break up the watermilfoil, allowing it to float and reestablish itself in other areas.
“You won’t get every last bit of it and when the tips break off they re-root somewhere else,” Andrew said. “But we are keeping that as an option.”
And then there is the biological option: the milfoil weevil, an insect that finds the aquatic plants quite useful.
“In its larval form it gets into the stem and burrows through the stem and the plant loses its structure and falls down,” Andrew said.
An analysis of the lakes in 2007 found that the weevil already exists in Bon Tempe and Lake Lagunitas, while other lakes have yet to be checked.
It was the first time the weevil has been found anywhere in the state of California.
“It was totally unexpected,” Andrew said. “We didn’t know it was here.”
It may have arrived with the watermilfoil, officials said.
While there are some weevils in Bon Tempe, there are not enough to kill all the plants. That has prompted proposals to bring in more weevils and put them into the lakes to destroy the watermilfoil.
“If you build a structure — a little weevil condo — they will mass into big numbers,” Klein said.
The weevil is native to North America, but not to California, so state agencies would have to issue a permit for the operation.
“We don’t want to release anything that could do damage to natives or get out of control,” Andrew said.
Because of the discovery of the weevils in Marin, the U.S. Department of Agriculture has taken an interest in the water district’s problem. Officials will study Bon Tempe and see how the existing weevil is doing in attacking the watermilfoil. The federal government could encourage bringing in more weevils, then duplicate the strategy in other areas of the region.
“It’s significant enough and has the potential for other Western states that it warrants attention,” said Raymond Carruthers of the USDA’s Agricultural Research Service. “We want to see if this could effectively be used elsewhere.”
The process would not be quick. It could take at least a year to do the research on the weevil and then another year to get a permit. In the meantime, water district staff will monitor the plant’s growth.
“That someone may have just dumped this from an aquarium is very frustrating,” Forman said. “I’m glad we are exploring biological controls, it’s cutting edge.”
Contact Mark Prado via e-mail at firstname.lastname@example.org
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