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Shade Tree Borers

By Whitney Cranshaw, Colorado State University Cooperative Extension Entomologist

Wood borers are among the most damaging insects to shade trees, shrubs and fruit trees. They develop by feeding beneath the bark of woody plants, producing girdling wounds that weaken and kill plants. Some borers also tunnel deeply into plants, making them susceptible to breakage during high winds or heavy snows.

All of the important wood boring insects in Colorado are the larvel (grub) stage of either beetles or moths. These include:

Flatheaded Borers: Flatheads (seen above) feed on the cambium just beneath the bark and produce winding tunnels filled with tightly packed sawdust. These girdling wounds cause plants to decline and die back, often over a period of several years.

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Winding tunnels produced by flathead borers.

The bronze birch borer is the most damaging of the flatheaded borer insects, contributing to problems that lead to the birch tree's decline. Aspen, honeylocust, and pines also host flatheaded borers. In the adult stage, these insects are known as metallic wood borers, because of the metallic color of the beetles.

Roundheaded borers: These borers often gouge deeply into wood while feeding, and produce stringy sawdust that may be pushed out of openings. Poplar borer, roundheaded apple tree borer and black locust borer are among species that damage plants. In abundance, these borers can injure and weaken trees, causing limbs or even trunks to break.

Roundheaded borers can be long-lived, and some can survive several years in firewood or lumber. The adult stage is known as the longhorned beetle, based on the long antennae that characterizes this insect.

Clearwing borers: The clearwings are among the most aggressive of the wood borers, capable of attacking healthy plants. Peach tree borer is widespread in Colorado, attacking most of the stone fruits (peach, plum, cherry, apricot). Raspberry crown borer commonly destroys raspberry plantings; the ash borer damages ash and lilac.

Most of the clearwing borers tunnel near the base of the plant, although others attack trunks and branches. In their adult stage, clearwing borers are day-flying moths that closely mimic wasps and bees.

The best defense against borers is a good offense: Keep plants healthy. Vigorous plants often resist borer attacks, but weakend or dying plants are vulnerable. Avoid wounding plants, as borers can attack at weaker areas around wounds.

Borer larvae are difficult to control once they enter the plant. Borers that make an opening to the outside sometimes can be killed with a thin wire, or they can be fumigated with a moth crystal inserted in the hole. Most borers, however, remain well protected beneath the bark.

Properly timed treatment with insecticides can control future attacks. All borers emerge from the plant in their adult stage to mate and lay eggs. Adult insects and newly hatched eggs, usually laid in crevices on the bark, can be killed to prevent the next cycle. Depending on the plant and label directions, insecticides such as permethrin and carbaryl can control borers at this stage.

The trick is in the timing. Although most borers emerge from late May to early July, emergence is highly variable. Each borer species tends to come out at different times of the year with considerable seasonal variation. Correct identification of the suspected borer is the key to successful treatment.

Photos: Judy Sedbrook

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Date last revised: 01/05/2010