Courtesy Mark Gardiner/Agri-Food Canada
OROVILLE, Wash. — The apple clearwing moth, a European apple tree killer, is taking a toll in British Columbia and probably has been on the U.S. side of the border for a number of years, a Canadian entomologist says.
Apple trees on hundreds of acres in the British Columbia Cawston and Okanagan valleys have died in the last five years and thousands more are infested, said Gary Judd, research entomologist with Agriculture and Agri-Food Canada in Summerland, B.C. The federal agency is roughly the equivalent of USDA.
The Washington State Department of Agriculture first trapped a clearwing in Western Washington in 2008 but none have been trapped in the Washington Okanogan. The region, which straddles the border, is spelled Okanagan in B.C. and Okanogan in Washington.
However, Judd said he’s seen damage to trees in the Oroville area, about 20 miles southeast of Cawston, that he’s sure is from clearwing although he’s not seen the moth there.
Apple clearwing moth was first recorded in North America near Cawston in 2005, according to the B.C. Ministry of Agriculture.
A student working in an apple packing house and daughter of a Cawston orchardist found “a cool looking moth” and took it to local authorities who thought it was a native insect, Judd said.
She then brought it to Judd who thought it was an apple clearwing moth. Testing by the California Department of Agriculture and DNA testing in Ottawa proved him correct.
“In the early 2000s, the local apple industry was heavy into replanting to convert to high-density, dwarfing orchards. We ran into a shortage of dwarfing rootstock, so went to the Netherlands and Belgium to import rootstocks,” Judd said.
After checking records, he became “99 percent sure” that’s how the pest arrived.
It’s present, in vary degrees, in all 1,200 acres of apple orchard in Cawston Valley and spread over the mountains to the Okanagan Valley by people moving rootstock, Judd said. From there is has spread north and south as far as the moth might fly, he said.
It’s been most prevalent in Gala and Ambrosia apple trees likely because they’ve been most recently planted in large numbers in the Okanagan, he said.
For a number of years, the moth, and the cytospora disease it transmits, seemed relatively stable, causing about 20 percent reduction in yields but compensated for with fertilizer, Judd said.
But in recent years, a number of factors including hotter summers, water use and combining with other diseases has accelerated mortality in young trees, he said. It’s reached an alarming point and is a “hot topic” in the Canadian Okanagan apple industry, he said.
The adult moth lays its eggs in cracks or wounds in bark, commonly the grafting union, but also pruning scars and bud spurs, Judd said. It’s been found in tops of Gala apple trees that are top pruned to increase sunshine to fruit.
Larvae hatch and feed under the bark, girdling the tree and disrupting flow of water and nutrients, he said. Eventually, the tree dies when heavily infested.
Pheromone disruption doesn’t work because the clearwing relies more on sight than smell for mating. Researchers have been unable to design traps that work to attract large numbers.
That leaves pesticides, also not totally effective.
“Organic sprays of Entrust are not very effective because it requires repeated applications that are very expensive,” Judd said. “It requires high water volume, so hand gunning which you can’t do on a big scale. We don’t have good data on airblast (tractor towed sprayer) efficacy.”
Judd and other entomologists, including Elizabeth Beers at the Washington State University Tree Fruit Research and Extension Center, in Wenatchee, are discussing how best to combat the pest and the best fungicides for the cytospora disease which has several species.
“We don’t have a lot of good chemicals to do it and we’re limited in the control timing,” Judd said.
Beers said clearwings can be detected in April and May when they leave orange-brown frass on tree bark near larval feeding areas. It can be controlled with only part of a tree damaged, she said.
“It’s a real concern,” Judd said, “because neither the insect nor disease are easy to control.”