Researchers collect weed-eating bugs for Tahoe

7/2/2009 - By: Jim Sloan

From a distance, it was hard to figure out what these people were doing.

Some were walking through a thigh-high patch of weeds, swatting at the bushes with canvas sacks. After several minutes, they would walk over and stand patiently while another person poked a metal straw into the canvas bag and sucked out the contents.

These odd-looking activities were, in fact, part of a scientific effort by the University of Nevada Cooperative Extension (UNCE) to use weed-eating beetles to halt the costly spread of spotted knapweed in Nevada. The straws were actually simple aspirators that drew the bugs into a plastic vial. The canvas sacks are called sweep nets.

The insects were being collected Wednesday from the site of a knapweed infestation in Verdi, where they had been planted three years ago by the Nevada Department of Agriculture. The insects were then immediately transported to Glenbrook at Lake Tahoe and released in an area hit hard by a knapweed invasion.

“Spotted knapweed is out of control in some parts of the West and we want to prevent that from happening in Nevada,” said UNCE water quality specialist Sue Donaldson, who headed up the insect harvest Wednesday. “If we can use bugs to kill knapweed, that saves us a lot of money and means we don’t have to use pesticides to control the weed. That’s especially important at Lake Tahoe.”

The test site in Verdi is home to three types of knapweed-devouring insects — a seed feeder, a stem borer and a root weevil. Donaldson and her team of volunteers, working with the state agriculture department and the U.S. Animal and Plant Health Inspection Service, were collecting two of those bugs Wednesday — the seed-eating weevil and the seedhead gall fly. The root weevil will be collected at a later date.

The three insects live exclusively on knapweed, and studies have shown that they don’t bother people or attack any other type of vegetation. And their handiwork was readily apparent in Verdi; a once-thriving colony of knapweed was stunted and in many areas dying out altogether.

“The knapweed in this area would be taller and denser if the bugs weren’t here,” said Scott Marsh of the state Department of Agriculture, who helped with the bug roundup Wednesday. “You can see they’ve been busy.”

Equally encouraging were the numbers of insects the researchers found on the remaining plants. The state wants to use the Verdi site as an insect nursery so other agencies can come in and collect the knapweed-controlling bugs, and Wednesday’s harvest indicated there were plenty of weevils and flies to be had.

“This went very smoothly, thanks to the help of our Master Gardener volunteers,” Donaldson said after about 400 weevils had been collected. Among the volunteers on hand Wednesday to help out were Carol Ort, a former vice provost and biology professor from the University, and Beverly Depew, a retired law enforcement officer. Both are training with UNCE to become Master Gardeners.

Spotted knapweed — a fast-growing, noxious weed — invades pastures, open forests and rangelands and severely reduces native plants and crops. It produces more than 1,000 seeds per plant and those seeds can remain viable in the soil for five years or more. But if a natural predator can keep the weed in check, native plants have a better chance of becoming established.

Donaldson’s work promises to save Nevadans money by reducing the amount of pesticide needed to control knapweed and protecting wildlife forage and domestic crops. It also demonstrates a natural, nontoxic approach to controlling noxious weeds. Moreover, knapweed can cause increased rates of erosion, so controlling the weed at Lake Tahoe will help reduce the amount of algae-fueling runoff that is threatening the clarity of the famed alpine lake.


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