Many kinds of trees including dogwood, white birch, paper birch, yellow birch and peach, particularly those with low vitality, are killed or seriouvly damaged by borers every. Borers - white, worm-like immature stages of moths or beetles - feed on plant tissue beneath the bark.
Several kinds of shrubs are also subject to attack, including rhododendron and lilac.
To some degree the trees and shrubs can be given protection at this time of the year. Certain insecticides can be sprayed on the bark to kill the borers after they hatch and before they can work their way into the tree.
But in most cases, unless it's done by someone who knows how, this won't be successful. The adults emerge over a period of two or three months to lay eggs, and two or three or more sprays, at the right time and in the right way, are needed for complete protection. The adults may emerge at different times and in varying numbers from year to year. Monitoring traps have been developed to help decide if and when insecticidal sprays are needed. Synthetic sex attractants (pheromones) draw males into the traps, where they are caught in a sticky substance; by analyzing the catch, spraying can be timed to catch the insects at their most vulnerable stage.
One thing most borers have in common is their preference for weakened plants, according to John A. Davidson, University of Maryland entomologist. The best way to prevent them is to keep the trees and shrubs as healthy as possible, he says. This can be done by proper watering, especially newly planted trees, proper fertilizing, controlling leaf-eating insects, using precautions with heavy equipment to avoid damage to the trees, and giving desirable trees shade by wrapping the trunk with a suitable material to the first brahch.
Good sap flow is essential: newly planted trees are especially susceptible before they establish good root systems. Two or three years of low rainfall are often followed by large numbers of borers.
Young plants grown in partial shade should not be transplanted into full sun without some type of protection to avoid sunscald. Trees and shrubs that require shade should not be planted in full sun.
Concentrations of air pollution, salts applied to roads and sidewalks in winter, fumes from factories - all weaken plants.