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This month’s Tree Tip was researched and written by Emil Peter, ISA Certified Arborist #SO-6363A and Forester.  He may be contacted directly via email with any questions concerning this article or any tree related issues.

Some of the most devastating tree pests we encounter are the boring insects. These are insects, much like their names suggest, that tunnel, or bore, under the bark or deep into the wood of trees. Some famous examples of borer damage in recent history are the great elm tree die-off caused by Dutch Elm Disease (DED); and more recently, the threat to North America’s ash forests from Emerald Ash Borer (EAB). Both epidemics can be directly traced back to boring insects. Because of their damage to the moisture and nutrient-carrying layers of the sapwood, borers are usually fatal to the tree. However, if infestations are found early enough, are not too severe, and the pest is not a deep-boring species, the tree may be saved.

Commonly, adult borers lay eggs on or in the bark of the tree or shrub. These eggs hatch into larvae, the immature, grub-like stage where they chew their way into the wood under the bark or inside the trunk. Here they continue to feed for a period that can range from a few days to several years.  When the larvae mature, the adult insects chew their way out of the tree or shrub and begin the cycle again.  It is this tunneling activity inside the wood that causes the damage and eventual death of trees. Trees that are weakened stresses like extensive defoliation, drought, construction damage, or other causes may be more susceptible to borer attack. The longer a tree is stressed, the greater the chances that borers will find it. Oak, cottonwood, willow, birch, ash, lilac, juniper, pine, spruce, and fir are most commonly attacked by borers.

To check for infestations of borers, look for small holes, usually 1/4 inch or less in diameter, that have sawdust falling from them or that are oozing sap or pitch. Sawdust on the tree trunk or ground should also alert you. This sawdust is called frass and, while technically sawdust, it has been “processed” by the insect chewing its way through the tree’s vascular system. Clean holes that are not packed with sawdust are generally signs that the adult insect has already emerged and is gone, perhaps after laying another batch of eggs on the tree.  Holes with frass indicate the immature borer is still feeding inside the tree.  It can be helpful to inspect tree trunks and limbs in early spring before leaves of deciduous trees obscure the signs of borer activity.  Evergreens can show similar signs, or the holes may be hidden under globs of pitch.  Remove the pitch with a pointed object and probe to see if there is a hole underneath.  On spruce leaders, which are the new year's terminal growth, small pinpoint holes oozing pitch can indicate feeding or egg-laying activity of the white pine weevil, which is not as serious as other borer species, and which will not cause tree mortality. Differing boring pests attack different parts of trees. White pine weevils attack only the new leader on a tree, while the flatheaded apple tree borer attacks some 30 species of woody plants including maple, hickory, oak, and linden as well as apple trees in places all over the trees!

Many times, if a tree is heavily infested, it can be impossible to save it. The best course of action for such trees should be to remove them before the pests spread to healthy trees.  Identification of the species of borer will help determine if this will be necessary.  If there are only a few borers or if they only affect the new leaders, chemical control to prevent egg laying and damage from the new generation of insects may be an option.  Effective chemical control is difficult however, because the borer is vulnerable to insecticides only during the time it hatches from the egg and before it has bored into the tree. Once the immature insects have tunneled into the wood they are unaffected by pesticides applied on the bark because the chemicals will not penetrate deeply enough to reach the pests.  Residual protection offered by the chemical applications will last several weeks, but one, or possibly two additional applications may be needed, spaced throughout the growing season.  If borers are discovered in mid-season, a single application of insecticide may be sufficient.  Our licensed applicators can also apply Restricted Use insecticides; one application should provide season-long protection.

When applying insecticides for borers, thorough coverage of the entire trunk and all main limbs is difficult, but essential.  If the hatching larvae do not come in contact with the insecticide, the chemical application will have little to no effect. 

A second chemical treatment option is to have our applicators use a method that injects or implants insecticides directly into the tree’s vascular system, where they are carried throughout the entire tree, killing borers which ingest the material. Injections and implants can be somewhat effective for bark beetles and borers that feed in the cambium and sapwood (xylem) layers; they are ineffective for species that bore deeper than the active outer layers of sapwood.  There is no movement of materials in heartwood (layers of tissue that are deeper than the sapwood), so materials injected deeper than one inch do not move beyond the drill hole and can be damaging to the tree. Pipes which are pounded into trees then filled with insecticide are particularly damaging and totally ineffective.  Injections and implants should only be used by professionals who have received the appropriate training to prevent serious damage to the tree. Chemical treatments of any kind should not be used unless you find signs of borer activity or know for a fact that damage is occurring to other trees in the neighborhood.

Keeping trees strong and healthy is the best defense against borers. Some of the easiest ways are listed here:

  • Right tree in the right place. If trees are suited to the site where they are planted, stress is reduced, lessening possibility of attack by borers and other insects and diseases.
  • Mulch your trees. Mulch protects from damage such as physical injuries from machinery such as lawnmowers or vehicles in addition to helping regulate soil moisture and lessen competition from grasses and other plants. 
  • Properly fertilize your tree. Overly fertilized trees & trees with nutrient deficiencies can develop weaknesses that boring insects will utilize as a vector to enter your tree.
  • Keep trees properly watered. Drought stressed trees will quickly be found by borers and colonized first.

Our arborists want to keep your trees healthy for years for your enjoyment. If you have any questions or need any assistance with your trees, please contact us.  Please email Joanna@woodlandtree.com or call us at (901) 309-6779.

Terran Arwood
President

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