Now that we have almost closed the book on 2015, let us reflect a bit on the extraordinary weather of this past year. It is no surprise that summer 2015 was the hottest on record in the region, with a total annual precipitation of 3.72 inches. That is not much considering that 9.16 inches of moisture were lost through evapotranspiration during August. Plus, some areas experienced limited irrigation water.
This extraordinary year was compounded by the fact that until 2015, 2014 was the hottest on record with 3.44 inches of precipitation, a total evapotranspiration of 9.02 inches in August, and an unusually mild December. Also consider that 2013 was the second hottest summer on record until 2015, with a total annual precipitation of 2.79 inches. Plus, summer 2013 was the sixth year of summer weather with near or above average temperatures.
That is a great deal of summer heat. Heat stresses plants, especially ones not well adapted to our summer climate. Heat stress is compounded by drought stress that results from limited irrigation water or improper watering practices. Drought stress may also be inflicted on a tree because of a lack of an adequate root system, physical injury to the trunk, restricted or girdling roots, compacted soil, or other factors that affect the tree’s ability to absorb or transport water to the top of the tree.
Stress on trees makes them more vulnerable to attack by certain insects, especially boring insects. Because of several years of summer heat stress that local trees have endured, we are seeing an increasing number of wood borers attacking ornamental trees.
Even if a tree looks fairly healthy to you and me, it may become fodder for borers because it is stressed. Stressed trees are more vulnerable to attack by borers because stress causes them to produce volatile chemicals, such as terpenes, that attract boring insects.
In addition, some borers (such as bark beetles) emit an aggregation pheromone (a chemical compound that elicits insect behavior) once it finds a stressed tree. This insect pheromone lets other bark beetles in the area know of the tree’s presence. One bark beetle might not significantly harm a tree, but a bunch of them feeding spells trouble.
Once the volatile chemicals emitted by the tree and the aggregation pheromone get a gang of bark beetles to the tree, they still have to get under the bark. As they start to eat their way into the tree, the sap pressure in a healthy tree will often drown or push them back out. However, in a stressed tree, the sap flow is lower, and they can successfully eat their way in.
The quote of “the best defense is a good offense” may or may not be true in sports, but it is true when it comes to protecting your trees from boring insects. Keeping a tree healthy is not just its best defense against borers, in many cases it is the only defense.
We no longer have the long-term residual insecticides that were once used to protect trees from borers. Insecticide applications as a spray to the trunk are generally not a good offense, and even the systemic insecticides applied to the soil or injected into the trunk are only effective against a few types of borers.
Hopefully, next year we will not experience another summer of extraordinary heat and limited irrigation water. If we do, make every effort to keep your trees healthy to protect them from attack by borers.
Marianne C. Ophardt is a horticulturist for Washington State University Benton County Extension.