Gardening With Allen: Birch trees dying from beetle damage

Borers have been attacking my two birch trees this year. Our arborist says this problem is pretty wide spread in the Portland/Vancouver areas this year. How can I control this pest?

Attacks of the bronze birch borer on white bark birch trees have increased this year due to our hot, dry summer weather. Water stressed trees are especially susceptible to borer attack.

The bronze birch borer is a slender, dark bronze beetle about 1/2 inches long. The adult female lays eggs in the cracks and crevices of susceptible birch trees in May. The light brown 3/4 -inch larvae bore into the wood and begin feeding on the interior tissue of the bark. They mine intricate tunnels, feeding actively until fall, where they overwinter. Larvae pupate the following spring and emerge through D-shaped holes cut into the bark. Winged adult beetles feed on tender young foliage for about a week and then move around trees laying their eggs.

Other borers can attack other tree species.

Symptoms

Spring and early summer warning signs are yellowing and thinning of foliage in the upper part of trees. Tunnels made by the larvae inside the bark girdle the trunk and branches reducing the flow of sap within trees. These tunnels appear as a raised or rippled bumpy surface of the bark.

In late summer and fall, the foliage turns brown and falls to the ground. Symptoms begin in the younger and smaller branches and progress downward to the larger branches and trunk and will kill the tree in about three years.

Prevention

The Cully River Birch (Betula nigra ‘Cully’) is the least susceptible to the bronze birch borer. If you are planting or replanting, use this variety.

The best way to protect existing trees with little or no damage is to irrigate trees deeply at least once a month during dry weather. It takes two inches of water to reach the deeper tree roots. The best way to apply this is to lay a soaker hose in a circle under the outer reach of the branches. Let the water run for several hours, even overnight. A three inch organic mulch under trees is also effective. In addition to bark dust or bark chips, most arborists will deliver mulch chips from pruning at no charge.

Borer insecticides sprayed on the bark in May and June can prevent attack.

Control of infected trees

Since the larvae are feeding under the bark it is difficult to reach them with pesticide. The most effective way to reach the feeding larvae is by insecticide injection. Some arborists have injection equipment. Tree injection kits can be purchased online. Imidocloprid systemic insecticide such as Monterey Once a Year Insect Control can be applied as a drench around the trunk of trees. Tree roots absorb the insecticide and take it up to the inner bark of branches.

Symptoms

Prevention

Control of infected trees

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