In a war being waged against an exotic moth laying waste to cherry trees in the western part of the state, Washington State University Cooperative Extension entomologist Lynell Tanigoshi is point man.
The moth in question is the cherry bark tortix, a Eurasian invader first found on the North American Continent in British Columbia in 1989. The tortrix has since spread down the I-5 corridor into the suburbs of Portland.
Counted in the war’s casualties are an unknown number of dead and weakened cherry trees caused by the feeding of the moth’s larvae. But, cherry trees are not the only plants at risk. Research shows it will attack other members of a large plant family, including cherries, apples, pears and mountain ash.
While the insect constitutes a serious concern for homeowners in western Washington, it is regarded as a threat to the multi-million dollar ornamental landscape plant industries of western Washington, Oregon and the Fraser Valley of British Columbia.
“Oregon is especially concerned because its biggest agricultural industry is ornamental woody plants,” said Tanigoshi, who is stationed at the WSU Vancouver Research and Extension Unit. “They produce a tremendous amount of both sweet and sour cherry rootstock plus a variety of ornamental flowering fruitless cherries for landscapes.”
Over the past five years, Tanigoshi, along with colleagues at the Oregon Department of Agriculture; Agriculture and Agri-Food Canada; the CABI European station for biological control in Delemont, Switzerland; the University of Washington; and Seattle’s Department of Parks and Recreation have been studying the biology and ecology of the insect.
Agriculture and Agri-Food Canada is the Canadian equivalent of the U.S. Department of Agriculture. CABI is a global non-profit organization specializing in sustainable solutions for agricultural and environmental solutions.
Tanigoshi’s work has uncovered some good news, at least for the state’s orchard industry in eastern Washington. “We think the limiting factor stopping their spread eastward could be humidity,” he said. “We’re doing some trials now to see if the eggs will hatch at 30 percent to 40 percent relative humidity. Preliminary studies show they don’t hatch. They dessicate.”
The insect’s tidiness may be its weakest link. “The little larvae are always cleaning out their domiciles,” Tanigoshi said. “They take out their fecal pellets usually in the evening. Instead of just kicking them out, they spin a hollow silk frass tube. We think the frass odor attracts parasites.”
The orange frass tubes—a sure sign of the infestation—accumulate on the bark in the spring and fall when the larvae are most active.
The insect can be controlled effectively with spot treatments of any of a number of common organophosphate or pyrethroid chemicals available at garden stores. “Just spot treat the tree where the frass tubes are,” Tanigoshi said. “The frass will absorb the toxicant and when the little critter comes out at night to kick out the frass, it contacts the toxicant and dies.”
\Timing is critical. “One of the other things we have learned is that fall is probably the best time to treat them. The larvae are really active in September and October as they build up food stocks in their tissues to survive the winter. You don’t have to cover the whole tree. Just get the frass tubes wet.”
While pesticides work well to control infestations of trees in the backyard, Tanigoshi believes that biological controls are the ultimate solution. “You can’t treat adult moths because they’re too elusive. They are in flight from April to September.” Compounding that obstacle is a giant reservoir of potential host trees and shrubs in the wild which could never be treated. “You are always going to get infestations from these wild trees.”
Tanigoshi and his research team have been combing the Northwest and Europe for natural predators, which could restore the balance of nature. “We hope to get enough parasites that over time they’ll multiply and do what they are doing in Europe, keeping the native populations of cherry bark tortrix at fairly low levels.
“When the cherry bark tortix came in we had a couple native wasps switch over and attack at not very high rates of parasitism,” said Terry Miller, manager of WSU’s Pacific Northwest Biocontrol Insectary and Quarantine facility in Pullman.
The most promising so far is a pinhead-size trichogrammatid, Trichogramma cacoaeciae, found in Bellingham. In some places it has been found, it will parasitize up to 90 percent of the eggs laid by a tortix, Miller said.
“Since they are native, we don’t have to go through the permitting process. When they come from overseas, we set up a protocol with the U.S. Fish and Wildlife Service and the U.S. Department of Agriculture where we test them on a number of organisms to make certain they won’t switch over to say some rare and endangered butterfly.”
The trichogrammatid is not touted to be a silver bullet. “It’s best to have a mixture of different natural enemies attacking the host at different stages,” Miller said. “If you don’t get all the eggs, then you’ve got to wait another generation before hitting them again.”
But, before you can even test parasitoids brought in from overseas, you have to breed them in captivity. So far, this has been a problem.
“To breed the parasitoids, we have to have the host,” Miller said. “The cherry bark tortix feeds on wood. This creates the problem of producing an environment in an artificial situation where a moth that eats wood can be replicated in large enough numbers to supply it as a food source for exotic organisms to come in from overseas.”
“We’ve tried a whole bunch of artificial diets and have come up with one we are satisfied with as a starting point, but we need to fine- tune it to get better performance out of it,” Miller said.
“At this point we can rear small numbers of moths on our artificial diet, but the wasps don’t recognize them. They just walk over them. They don’t smell right.”
"The challenge right now is to develop an artificial diet to substitute for cherry wood,” Tanigoshi said, “so that we can rear cherry bark tortrix all year round as hosts for various species of exotic para-satoids. An effective rearing system will allow us to eventually propagate candidate natural enemy species for augmentive field releases in native habitats.”
