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The Dogwood Borer is a pest which threatens not just dogwoods but also members of the hickory, elm, willow, oak chestnut and apple families among others. It is common in the United States west of the Rockies and in Canada. Damage is caused by insects in the larval or caterpillar stage burrowing beneath the bark of trees to eat healthy tissue. Moderate infestations slow growth while heavy or repeated infestations can kill trees by girdling and disrupting the flow of sap.
The Dogwood Borer has been around for a long time and is generally attracted to tree wounds caused by storms, lawnmowers or other factors. However, the incidence of the insect has increased dramatically in recent years along with the use of nursery-grown clonal tree stock. Borers are attracted to the ‘burr knots’ found on most nursery stock. Burr knots are composed of root tissue and appear as rough patches on the lower part of the trunk close to the ground. Borer infestations of the burr knots themselves cause little damage. It is when the larvae move beyond the knots that the real problems occur.
The Lifecycle of the Dogwood Borer
There are four stages in the lifecycle of the dogwood borer. Small, light brown, oval eggs are generally laid on the tree trunk near burr knots or wound areas in June or July. The eggs hatch after about 8 or 9 days and the larvae immediately begin to burrow into burr knots or other vulnerable areas and consume tissue.
Borer larvae, cream in color with a reddish head, have seven moulting stages or instars and can exceed half an inch (1.6 cm) in length at maturity. They do not feed during the winter but hibernate in their tunnels until around May when they waken to continue feeding.
In June, the larvae enter cocoons composed of spun silk where pupation or the final transformation to moth takes place.
After 25 days in the pupation stage, the Dogwood Borer moth emerges. It is slim and blue-black in color with yellow abdominal stripes and clear wings which can reach almost an inch (2.2 cm) across. Moths live only long enough to mate, 6 to 8 days, before dying.
Dogwoods and other trees with burr knots or other vulnerable areas are susceptible to Dogwood Borer infestation. Apart from visual identification of the insects themselves, tell-tale signs include fine reddish dust-like ‘frass’ (excrement and boring material) on burr knots and damaged areas.
How to control the Dogwood Borer
As Dogwood Borers are attracted to damaged areas of trees, it is important to avoid damaging trees and to treat tree damage promptly. In particular, use caution with lawnmowers and other implements
There are a variety of insecticides and other chemical agents available to combat existing Dogwood Borer infestations. Consult your local nursery or tree care specialist for details. As always in the use of chemicals, follow the directions carefully.