The Boring Effects of Drought

Headaches, tense muscles, or a general feeling of stress can clue us into the health of our bodies. Often, I find the solution is to re-hydrate with more water. Trees are like us in this way. While some tree species can tolerate water depletion, almost all trees in our urban landscape can become stressed during a drought period.  Wisconsin was nearly eight inches behind our average rainfall in 2021. Trees in our area are likely to be stressed by this deficit, which increases the likelihood of borer attacks.

A common misconception is that all trees have deep tap roots, pulling from underground water sources during dry spells.  Most tree roots grow in the top 20” of the soil and in urban soils they rarely penetrate deeper than a few feet. This means that trees exhaust their water supply quickly which can lead to an SOS response.

When stress occurs, trees can become susceptible to various ailments much like humans. For trees, this includes attacks from insects and root loss. Usually, trees can tolerate some level of insect borer activity because they are healthy enough to “fight off” the attack. However, when their defenses are weakened, insects can take advantage of the living host.

When trees become stressed, they release pheromones from cracks or openings in the bark that can attract wood-boring insects to lay their eggs. The eggs will hatch into larvae, which tunnel and feed under the living wood, destroying the tree’s conductive tissues. This can cause the tree’s water supply to be cut off. If damage is severe enough, it can result in branch die-back, structural compromise, decline, and eventual death. On top of it all, these borer holes can also provide entry points for fungal, bacterial, and viral pathogens.

On the other hand, a tree with sufficient water supply will produce a healthy level of carbohydrates, increasing its energy reserves. These energy reserves are used to help the tree grow and remain beautiful.  Plus, they are essential to the tree’s production of defense chemicals that ward off pathogens.

Lilac Borer Holes

Tips for Optimizing Trees’ defenses:

• Select well-adapted tree cultivars for the planting location
• Ensure proper planting. Transplanted trees need additional water to establish roots.
• Avoid wounding the bark or roots when mowing or string trimming. Fresh wounds are attractive to egg-laying borers.
• Plant for biodiversity to reduce disrupt the spread of any given borer
• Keep the root zone healthy by avoiding soil compaction and deeply watering the root zone during dry conditions. The tree is better able to use underground sources of water when it has a prolific and healthy root system
• A healthy layer of 2 to 4 inches of organic mulch (such as wood chips) goes a long way to buffer soil moisture and temperature. It also encourages a greater density of fine roots.
• If storm damage occurs, contact your Certified Arborist to determine what steps to take for protection and prevention.

Often, the damage done by borers is noticed after it is too late to act. If you suspect that your tree may be hosting these boring insects, contact your Certified Arborist to determine the right action steps to take.

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