With the fading of blooms on Vanhoutte spirea, it is time to apply controls for bronze birch borer. Bronze birch borer attacks white-barked birches, tunneling as larvae in the cambium beneath the bark and eventually girdling the tree. Damage first appears as dead twigs at the top of the tree. If more than one-third of the branches on the tree have died, control efforts are unlikely to save it. Adults emerge at this time of year through 1/8-inch-wide, D-shaped holes in the trunk. The 1/2-inch long, elongate, bronze-black beetles mate, and then females lay eggs in niches that they chew in the bark. The hatching larvae tunnel into the cambium to feed.
Chemical control consists of spraying the trunk and larger branches three times at two-week intervals with chlorpyrifos (Dursban). An alternate method is to apply a band of dimethoate (Cygon) as a concentrate to the trunk. Treat only once in a band that is equal to the diameter of the trunk but no wider than six inches. Make sure that you buy a product that has this application on the label; it will be listed as a control method for birch leafminer.
Dimethoate acts systemically through the bark to kill the larvae in the cambium. Because of this systemic action, it can be applied any time within the next month to achieve control. However, immediate applications should provide a higher level of control. Realize that this insecticide is effective due to the birch tree’s thin bark and the activity of the bronze birch borer larvae just under the bark. On other trees and borers, the bark is usually thicker and many other borers feed deeper into the wood, making this application ineffective. Also, dimethoate is labeled for use on very few trees and tends to be phytotoxic if over- applied or applied to unlabeled plants.