Get the dirt on tuber moths

07/01/2006 02:00:00 AM

Combine insecticides and cultural practices to help manage this profit-robbing potato pest

By Vicky Boyd

With three years of field trials under their belts, researchers say they are beginning to gain an understanding of the tuber moth, a tropical pest plaguing Pacific Northwestern potato growers.

Insecticides combined with cultural practices appear to control the moth and its offspring, the tuber worm, but scientists admit they still have several questions, including treatment thresholds, that they hope to answer.

"We are in our infancy in understanding how to control this pest,” says Alan Schreiber, president of the Agricultural Development Group of Eltopia, Wash., and a consultant to the National Potato Council in Washington, D.C..

Tuber moths have been the most serious potato pest in the tropics and subtropics for decades. More recently, they have plagued California potato growers in the Kern County area.

But they had rarely been found in the Pacific Northwest until 2002, when several Hermiston, Ore.-area growers were surprised to find fields with significant tuber worm damage.

Tony Amstad, a Hermiston potato grower who had a couple of circles damaged by tuber worms in 2002, says he learned a lesson. He now deploys pheromone traps near every circle to monitor for the pest, and he treats when traps pick up significant moth numbers.

“The tuber moth is a real issue for us,” Amstad says. “It’s moving farther up the [Columbia River] basin.

An expanding problem
Since it’s original discovery near Hermiston, the moth has expanded its range and now is found as far north as Wilbur, Wash., and as far south as the Klamath River Basin bordering Oregon and California. During summer 2005, University of Idaho researchers and Idaho State Department of Agriculture inspectors also found the moth in about two dozen fields in the southwestern part of the state.

Subsequent surveys by University of Idaho scientists that involved looking at foliage for larval feeding and examining thousands of tubers for damage yielded no tuber worms, says Eric Dotseth, an entomology support scientist with the university’s Aberdeen Research and Extension Center.

What does this pest look like?
The nocturnal, 1/2-inch-long, gray tuber moth causes little potato damage, feeding on nectar. Females prefer to lay eggs on the underside of green foliage. If no leaves are available, or if the leaves are dry, they’ll search for tubers exposed by cracked soil or a lack of soil covering.

In those potatoes, females will deposit eggs in the eyes. They can lay from 60 to 100 eggs during a lifetime. Once the eggs hatch, tuber worms burrow into plant stems to feed on the cavities. They also feed on potato leaves, leaving telltale mines.

The real damage is caused when the worms burrow into tubers, leaving dirty tunnels and rendering the spuds unmarketable. The holes also provide entryways for secondary disease organisms, such as fungi.

An early-warning network
For the past three years, Washington and Oregon researchers and Extension agents have maintained a series of delta pheromone traps, which attract male moths, within the Columbia River basin to monitor tuber moth populations.

Oregon State University posts its trap count online weekly at to keep growers abreast. In addition, university Extension entomologist Silvia Rondon recommends growers put one delta trapper field for detection or four traps per field to monitor the pest’s phenology and see how populations fluctuate during the season.

Dorseth and his colleagues at Aberdeen have been conducting grower educational sessions on the importance of this new foundpest.

"We hope to get more growers involved with this survey, as well as [educate] people about identifying these things,” Dotseth says. “There are characteristics about them you can see with a microscope. I still see situations where I have been training Extension educators, who are looking with a hand lens, and they may have a question. I tell them, ‘If you think you may have it, send it to us for confirmation.’”

Throughout the Columbia Basin this year, the number of moths trapped is fewer than at the same time last year. But Andy Jensen, research director for the Washington Potato Commission in Moses Lake, says he can’t explain the population decrease.

“The trap catches were down dramatically last fall, so you would expect trap catches to be down this spring as well,” Jensen says. “It’s totally different than what happened the previous fall when we had large trap catches.”

Rondon, who maintains traps around the Hermiston research station where she works, agrees. During the 2004-05 season, each trap in the Hermiston region averaged 80 moths per week, she says. Even in 2003-04, traps were picking up moths in December, when most insects hibernate.

This year, however, traps in what Rondon describes as the hot area were averaging fewer than one moth per trap per week.

Cultural and chemical controls
Since 2003, Washington and Oregon university researchers have conducted field and laboratory research looking at the pest’s biology and control measures. They say that although they plan to continue trials examining chemical and cultural controls this summer, the past two years of field trials have yielded some encouraging trends.

"A combination of chemicals and irrigation seems to have an effect on tuber moth,” Rondon says. “The drier it is, the more likely there are to be cracks in the soil where tuber moths can find tubers.”

During 2005 field trials, Rondon and her colleagues applied 0.1 inch of water daily between vine kill and harvest—just enough to keep the soil moist.

"It’s very critical right before harvest because after the vines die, the tuber moth seems to prefer the tubers rather than the foliage,” Rondon says. She and her group plan to repeat the trials this season.

Ensuring the tubers are covered with at least 2 inches of soil is another strategy to minimize tuber moth egg laying, says Mark Pavek, a Washington State University Extension potato specialist in Pullman.

"What we have found is that tuber moth typically will infest tubers that are within 2 inches or less of the soil surface,” Pavek says. “The thinking is, if you manage to reduce green tubers, you are managing to reduce tuber moth damage.”

Growers can try to plant deeper, use broader hills or use a combination of the two methods, he says. Depending on the variety, deeper seed placement may be a challenge, since some varieties have difficulty germinating under too much soil.

Schreiber also recommends eliminating cull piles, since the worms thrive on potatoes. Some growers feed culls to cattle, but Schreiber warns the practice won’t eliminate the pest threat unless the animals consume the potatoes immediately.

In addition, he says growers should be vigilant about eliminating volunteer potatoes, since they can harbor the pest.

Several registered treatments
Researchers still don’t know the population threshold that should trigger pesticide application, Schreiber says. The current recommendation calls for treating a field when traps start collecting tuber moths.

Trials conducted last summer show that several registered products are effective against tuber moth. But Schreiber says growers should carefully choose insecticides, because some, such as pyrethroids, may flare other pests, such as aphids and mites.

Schreiber plans to conduct additional trials this summer to study, looking at the best time to treat before harvest. He currently recommends treating no later than four weeks before desiccation, should tuber moth be detected.

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