Oak Decline and the Future of Missouri’s Forests
for ridgetops and south and west-facing slopes than are scarlet and black oaks. White oak might be an appropriate choice for mid-slopes.
Managing Wood Borers In Landscape Trees
- Wood borers are attracted to trees stressed by injury, drought, soil compaction and limitation of root growth due to nearby buildings and pavement.
- Healthy trees can defend themselves against wood borers and are able to recover after a limited number of borer attacks.
- Avoid damaging tree trunks and roots with mowing equipment and string weed trimmers.
- Avoid pruning oaks in spring and summer, when beetles carrying the oak wilt fungus and wood borer adults are active.
- Do not have trees topped (cutting branches back to stubs in a hat-rack manner.) Topping stresses trees, increasing their risk of tree decline and death.
- Place mulch around the base of trees to conserve moisture. Mulch should be distributed 2 to 3 inches deep in a ring around the tree, but should not touch the trunk or be piled against it in a volcano shape.
- Provide supplemental water during summer droughts by allowing a hose to drip slowly on the soil above the root zone at approximately two-week intervals.
- Wood borers are difficult to control with insecticides, and few insecticides are registered for treating wood borers in landscape trees. Attempts to use insecticides are likely to be ineffective and may have harmful effects for humans or the environment.
Dealing With Oak Decline
At sites where insect borer larvae attack, you might see oozing sap or a sap stain on the bark. Oozing sap can sometimes also be a symptom of a fungal infection, sapsucker attack or a mechanical injury.
Frass (granular or powdery mix of insect excrement and wood particles) often accumulates in bark crevices or around the base of a tree attacked by wood borers. Exit holes made by various kinds of borers can range in size from about 1/16 inch to 1/2 inch in diameter. Their larval tunneling also degrades the quality of oak lumber.
- Remove trees that have 30 percent or greater die-back (dead outer branches) in the upper and mid-sections of the crown.
- Remove smaller scarlet and black oaks that are overtopped by larger trees.
- If the stand does not respond with improved tree growth and vigor, you may consider regenerating it (harvesting mature trees and allowing seedlings and sprouts to re-stock the site.)
- After regenerating a stand, consider removing some sprouts before they are 20 years old. This will allow sprouts that originate low in the stump to become dominant. This will reduce competition and lower decay losses for future stands.
- There are no pesticides that are effective in treating oak decline or wood borer infestations in forest stands, and attempts to use pesticides in these situations are likely to have harmful environmental effects.
Dead Tree Alert
- Be alert for dead, weakened trees, and dead limbs, especially hanging broken limbs and branches along roads and trails. Dead trees and branches can fall at any time.
- Keep a safe distance away from dead trees, especially in windy weather. One tree height distance (often 70 to 80 feet) may be needed.
- Be careful where you set up camp and park a vehicle. Dead oaks may fall or large branches may break off, especially under windy conditions.
- As oaks die and fall, logs and limbs could blanket the ground, possibly fueling wildfires or making prescribed burns more difficult to control. Be careful with camp fires.
- Report any dangerous trees or limbs to the responsible land management agency.