Healthy cherry trees will try to bleed out the intruding pest, creating shothole “pigtails.” Photos by Dr. Michael Bush, WSU Extension
Shothole borer infestations are a sign of the times. The pest tends to show up in neglected trees, trees that have recently been pulled out, or woodpiles.
“When things are going good and everything’s healthy and taken care of, we don’t see a lot of these pests,” Mike Doerr, research assistant with Washington State University in Wenatchee, reported at the Okanogan County Horticultural Day.
Shothole borer, Scolytus rugulosus, is one of two species of bark beetle commonly found in Pacific Northwest orchards.
The beetles make holes in the bark of trees and burrow underneath. The female builds a maternal chamber in the wood’s cambium layer where she lays her eggs. Larvae that hatch from the eggs excavate small galleries out from the chamber. When mature, they burrow their way out through holes in the bark.
Doerr said the pest’s lifecycle does not appear to match up with reports in the scientific literature, most of which are from California, so there is still much to learn about the pest in the Northwest. The Ambrosia beetle also infests fruit trees, though it doesn’t move as readily to healthy trees. It is attracted to the aroma of dying trees.
Shothole borer develops through two generations in the Northwest. The first generation of beetles begins at about bloom and peaks in the last two weeks of May. In cherries, first-generation beetles don’t cause much damage, perhaps because they’re controlled by cherry fruit fly sprays. The second generation starts in July and peaks in late July or early August. This generation can cause problems in healthy trees, whether apples, pears, or cherries.
Doerr recommends brightly colored sticky traps for monitoring the pest, as they attract far more shothole borers than the larger and more expensive funnel traps, he said. About 75,000 beetles were found in one trap in Okanogan County in one week.
Woodpiles stacked next to orchards can be a source of borers. Doerr recommended that piles be moved before the winter.
“If you leave them there, you’re inviting shothole borer to set up shop,” he said.
The borers will colonize wood from the time that it’s cut to about 18 months later. After that, other beetles will move in and take over. For example, powder-post beetles will munch on the wood until it’s a pile of sawdust.
Shothole borer will attack other types of smooth-barked trees, such as hawthorn, elm, ash, and poplar, as well as fruit trees. Doerr said he knew a grower who cut out a poplar windbreak and stacked it next to the orchard. Within three weeks the shothole borers had moved into it.
If the pest stayed in dead trees or wood, it would not be an issue, Doerr said, but the beetles can quickly move into nearby healthy trees. Young plantings are more vulnerable than older ones.
The insects attack the bases of the shoots, and sap oozes out. Other evidence of shothole borer attack includes stressed or weakened trees, flagging branches, and blind wood. As soon as the sap flow is blocked, the shothole borer is able to reproduce in the tree, and the next step is a dead tree, Doerr said.
Usually, the shothole borer moves only one or two rows into the orchard from the source, regardless of how severe the infestation is. If the borers are found throughout the orchard, they are probably coming from internal sources, such as dead limbs.
Seventy-five percent of all the damaged limbs associated with shothole borer are in the row next to the source, and another 20 percent are in the second row, he reported.
The shothole borer can be parasitized by a pteromalid wasp, but even parasitism levels of 25 to 50 percent are not enough to suppress damage in orchards.
The insecticides Asana (esfenvalerate) and Guthion (azinphos-methyl) will control shothole borer but are not compatible with integrated pest management programs, Doerr said, and Malathion is too short lived. That’s why it’s important to focus on the source, Doerr said.
Growers should check their woodpiles to see if they’re infested, or put out sticky traps.
“If you get any significant captures, take care of the woodpile,” he urged. “You can really turn it around immediately.”