B.C. battles apple borer

Apple growers in British Columbia, Canada, are battling a new and lethal pest called the apple clearwing moth. Its larvae bore into the wood of apple trees, usually at the base, and can quickly kill a small tree by girdling it.

The pest has also shown up on backyard trees in Whatcom County in northwestern Washington.

Linda Edwards, an integrated pest management consultant whose own orchard at Cawston, British Columbia, is infested, said it’s going to demand a different approach from other pests that apple growers are used to dealing with.

"We have never had a borer in apples before, and now we do," she said. "That’s truly, from an orchardist’s point of view, the worst pest in apples because they can kill your trees."

Dr. Gary Judd, entomologist at the Pacific Agri-Food Research Centre in Summerland, British Columbia, said identification of the pest was ­confirmed in 2005 after larvae were found in an apple tree in Osoyoos, British Columbia, the previous fall. Edwards said the pest was reported in one organic and one conventional orchard in Cawston in British Columbia’s Similkameen Valley in 2005, and its populations have been quickly building in the area.

The apple clearwing moth (Synanthedon myopaeformis) is in the same genus as the peach tree borer, and is also known as the small red-banded clearwing moth. Both the male and female apple clearwing moths look similar to a female peach tree borer, but are smaller. They have a bluish-black body with a bright orange stripe. Their size varies between half an inch and an inch long, Judd said.

From Europe?

The insect has been a pest in Europe for more than 70 years, but had never been seen before in North America. A national survey conducted by the Canadian Food Inspection Agency in 2006 and 2007 turned up more apple clearwing moths in Abbottsford, near the Fraser River delta in British Columbia, not far from the U.S. border, and in London, Ontario, but showed that it was concentrated in the Similkameen Valley.

Like its relative, the peach tree borer, the pest has a two-year lifecycle in northern regions. Judd said that it must have been in Canada by 2003 or earlier. He suspects that it arrived on European nursery stock when Cawston area orchards were heavily replanted between about 1998 and 2000.

Edwards agreed. "I’ve been a consultant in the tree fruit industry since the early 1980s, and we did not have this pest," she said emphatically.

The pest has a wide geographic range. It is found in Denmark and throughout western Europe, and as far south as Egypt and Jordan, Judd said. In Europe, it used to be considered a secondary pest, but it became a greater concern in the late 1970s and 1980s when standard orchards were replanted with trees on dwarfing rootstocks. Whereas a standard peach tree can withstand a certain amount of damage from the peach tree borer, an apple tree on a Malling 9 rootstock in a high-density orchard is quite vulnerable to the apple clearwing moth.

Eric LaGasa, chief entomologist with the Washington State Department of Agriculture, said the apple clearwing moth has been detected at about a dozen sites in Whatcom County in Washington State, south of Abbotsford. It was first found there in 2006.

Trapping for the pest has been done in conjunction with apple maggot trapping, using a trap designed for the peach tree borer (which has a similar pheromone to the apple clearwing moth).

So far, it has only been found in roadside or backyard apple trees, LaGasa reported, even though some infested trees have been close to or between commercial apple orchards.

This year, LaGasa has funding from the U.S. Department of Agriculture to look for infested trees. He’s not sure yet what kind of threat the pest is to commercial orchards, but he fears it will be a serious problem in trees that are not managed.

"I actually think it has some very scary potential for impacts on unmanaged trees," he said.

It’s particularly worrisome for the area west of the Cascade Mountains, which has been invaded by another exotic pest called the cherry bark tortrix, he said. The cherry bark tortrix feeds in the bark of apple, cherry, and other rosaceous host trees. In European literature, it’s been reported that the cherry bark tortrix and the apple clearwing moth can work synergistically in attacking fruit trees. As the cherry bark tortrix feeds in the bark of a tree, it exits to defecate outside the opening, and so maintains the opening in the bark. That creates an entry point for the apple clearwing moth.

"Particularly in trees that are not managed, they could really take out a lot of apple trees on the West side," LaGasa said.

Many trees have been seriously affected by the cherry bark tortrix since it was first discovered in northwestern ­Washington in 1991, he said. It has been moving south from British Columbia and has reached the Willamette Valley in Oregon.

This season, LaGasa planned to trap for the apple clearwing moth in northwestern Washington, from the Canadian border to Seattle, and in Okanogan County in eastern Washington. Roadside and backyard trees will be trapped in conjunction with apple maggot trapping, he said. Although it has been found just over the border at Osoyoos in Canada, the apple clearwing moth has not yet been found in Okanogan County.

Quarantine

Dr. Mike Willett, vice president for scientific affairs at the Northwest Horticultural Council, said a request has been made to the USDA for special funding to conduct a survey of nursery-stock postentry quarantine sites. Nursery stock that is imported from other countries is held at a quarantine site for two years. According to the USDA, there are 2.3 million trees in postentry quarantine in Washington, and 500 acres would need to be trapped. Most of the acreage is in the Columbia Basin.

Willett said the pest seems to fare well in warm, arid conditions as well as in the cooler coastal areas. When he visited orchards in the Similkameen Valley early this year, he was surprised by the high populations of apple clearwing moths. "It seems to do well in that kind of a climate," he said.

Willett said that in peach and nectarine orchards in Washington, even when fairly broad-spectrum pesticides were used, the peach tree borer generally required pesticides to be applied specifically for that pest. Insecticides aren’t normally applied to the trunk of the tree, and the adult moths weren’t killed by insecticides before they laid their eggs.

He noted that many of the newer pesticides used in orchards today aren’t designed to kill adult moths, so if the apple clearwing moth were to become a pest in commercial orchards, it might require additional trips through the orchard with pesticides, resulting in additional costs.