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Which Pest Is It?

By Marty Shaw, Happy Tree & Lawn

Digital cameras and email can help professionals figure out unresolved problems. This digital microscope can take detailed images of pests and disease symptoms. Photo: Spectrum Technologies, Inc.

There are hundreds of different kinds of wood boring insects. Some are the caterpillars of moths or butterflies, some are larvae of beetles, some are sawfly larvae, and still others are weevil grubs. Identifying and controlling these pests is one of the most challenging tasks that professional arborists face.

One of the fundamental elements of any integrated pest management program is understanding the life cycle of the target pest. By understanding the life cycle we can then look for weak stages of the insect's development or habits, and thereby design treatments that hit the insect hard when and where it is most vulnerable.

The brilliantly-colored emerald ash borer, or EAB, is wreaking havoc on ash trees in Ohio, Michigan and adjacent areas of Canada. Photo:

Some insects feed on tree leaves. Dealing with borers is different. Except for bark beetles, wood borer insects can feed on any woody part (i.e., twigs, branches, trunk, or roots). Therefore, as tree professionals, scientific classification of borers, is a necessary skill in determining life cycles and the proper method of treatment. Correct identification is paramount in order to apply the best treatment at the optimum time. One size does not fit all when it comes to treatment for borers.

Pine Bark Beetles are the most common and the most economically damaging to the drought-stressed pine forests across the west and south of the country. Pine Bark Beetles are easily recognized by the damage they produce. The larvae hatch and begin to burrow through the inner bark where the pine tree responds by filling the injured area with resin or pitch. Fresh attacks are seen as droplets of clear, sticky resin on the bark. Soon, a mass of white or reddish looking pitch forms around the entry hole as feeding begins on the tender cambial layer and inner bark.

Boring insects may be a tree's worst enemy, but other species attack the foliage of trees and other plants. This image shows the appropriately-named unicorn caterpillar, which will grow into a moth during the adult stage. Photo:

Of all the bark beetles, the most famous is the elm bark beetle. This notorious bug is the insect that carries the deadly Dutch Elm Disease (DED). Control of this beetle slows down the infection rate of DED on American Elm.

Ambrosia beetles, shothole borers, and other Bark Beetles have similar life cycles and attack a wide range of plant and tree hosts. They most commonly attack weakened or dying trees (all too common in the urban landscape).

Wood-boring caterpillars are the larvae of moths and butterflies. This group also includes the clear-wing moths that mimic the actions and appearances of certain wasps. These borers attack a wide range of trees, especially oak, dogwood, elm, beech, ash, walnut, chestnut, poplar, willow, crabapple and fruit trees. Life cycles vary somewhat but generally speaking the adults take flight in April and continue through September. In many species, the larvae are laid on the bark and must enter through an open wound in the tree.

The notorious elm bark beetle is the source of Dutch elm disease, or DED. The insect carries and transmits the disease to elms and has devastated what was one of the most popular trees in eastern states. Photo:

Twig girdlers and twig pruners present a unique problem because the adults help the larvae survive inside the branch or twig. They do this by interrupting sap flow to the terminal where the grubs feed unencumbered inside. Individually, the damage is of little consequence to the tree, but numerous infestations can cause considerable damage, increase stress and reduce tree vigor.

Flatheaded borers and roundheaded borers are beetle larvae that can brood for one to three years inside a tree. They can attack any woody structure of trees and are often associated with white frothy sap, discolored and saturated bark, swollen areas under the bark, cankers, frass, adventitious growth in the lower crown, dieback in the upper crown, and/or oval, round or D-shaped emergence holes. The insects initially feed on xylem and cambium tissues, burrow into the sap or heartwood, and overwinter. In spring, shallow feeding resumes. After completing their larval development, the adults emerge and leave behind distinctive exit holes.

The adult beetles are usually very colorful with distinctive black, white, red or yellow markings. Most common are bronze birch borer, locust borer, long horned beetles, red headed ash borer, emerald ash borer, round headed apple borer, sugar maple borer, cottonwood borer, two lined chestnut borer, and metallic wood borer. Oak, hickory and poplar borers are not as colorful but cause similar damage. Many of the shoot and twig borers are the larvae of inconspicuous looking beetles while a few are sawflies.

The recently-introduced Asian longhorned beetle is becoming a serious problem for many hardwood trees in the New York, New Jersey, Chicago and other eastern areas. More information:

Root-feeding beetles are even more insidious because the early symptoms are often mistaken as a nutrient deficiency. Root rot disease, thinning and decline continue as the beetle grubs destroy more and more root mass. Similar to construction damage, the damage that root borers cause can easily create hazards in an urban setting.

Over the years that I have been caring for trees, borers have been among the most challenging pest problems to solve. These insects are difficult because once the larvae penetrate the bark and begin feeding, no amount of spraying will stop or even slow them down. Another reason that borers are so difficult is that when borer attacks are successful, the victorious borer often emits powerful chemicals (pheromones) that attract other borers to the scene. One might call this the insect world's version of a dinner/orgy invitation. The only way to treat borers that are actively feeding is to use a systemic insecticide. If you don't want to wait three or four months for soil applied systemics like Merit(R) to take effect, the only option for you is micro-injected systemic insecticides.
--Marty Shaw wrote the above text while with TIPCO, or Tree Injection Products Co. He now runs Happy Tree & Lawn in Nashville, Tenn.

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June 18, 2019, 8:50 am PDT

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