Healthy trees don't attract borer attacks

KENNEWICK -- "Belly up to the bar, boys!" I remember this phrase from the movies that I watched as a teenager.

When I think about tree borers finding a stressed shade tree in our area, this phrase comes to mind.

An increasing number of shade trees in our region are being attacked by boring beetles and moths.

The most common boring culprits are bronze birch borer, locust borer, ash borer, and carpenter worm and willow borers which also attack cottonwoods, and poplars. There's also the redheaded ash borer.

Back in the "old" days, long-term residual pesticides, such as lindane, were relied upon to control boring insects in trees, but lindane and dursban (lindane's replacement) have been banned because of their detrimental impact on humans and the environment.

Since the loss of Dursban in 2001, gardeners haven't had chemicals available that provide a cost-effective, reliable control of borers.

The sprays of the past (and present) were aimed at controlling adult borers as they emerged from trees and newly hatched borer larvae before they found their way under the bark. This was done by applying spray applications to the bark and trunk of the trees.

Many of the borers that attack shade trees emerge over a relatively long period of time. The older, banned chemicals worked well for borer control because they had a long residual, but it's also what made them bad for us and the environment.

Since the banning of Dursban there have been a few shorter residual materials labeled for home garden use against borers, but this necessitates multiple spray applications over the span of emergence.

With the entry of imidacloprid landscape professionals and home gardeners have been wondering if this systemic drench applied to the soil at the base of a tree could be effective controlling borers within a tree.

Entomologists doubted it because most borers eat their way into the heartwood of the tree. The heartwood is nonliving woody tissues that no longer transport water or nutrients. Once borers are in the heartwood, neither surface sprays applied to the bark or systemic materials applied to the soil will reach them.

However, flatheaded borers (a type of beetle) feed mostly in tissues right under the bark. Research has shown that an imidacloprid soil drench can provide reliable control of flatheaded borers if applied early enough in the season.

Colorado State University Extension entomologist W. S. Cranshaw indicates that imidacloprid "is not effective against any of the borers that develop into moths (e.g. clearwing borers, carpenterworms, Dioryctria spp.) nor against species that tunnel into the heartwood of the tree (e.g., roundheaded borers)" despite these borers being listed on the product label.

The best bet is to keep trees vigorous and healthy because most borers only attack stressed trees.

Various causes of stress, such as drought, sunburned bark or improper planting, weaken a tree and make it more vulnerable to borer attack.

Planting and watering correctly can help prevent a borer attack.

Trees also become stressed if they're planted in soils or sites where they aren't well adapted.

The best bet is to keep trees healthy before borers "belly up."

* Marianne C. Ophardt is a horticulturist for the Washington State University Benton County Extension Office.