Correcting Borer Damage in Italian Cypress

Q. My Italian cypress started to lose their color and then die. Upon inspection of the trunk, I saw borers in them. Borers seemed to take place within two months or so and killed a few mature trees. I treated them with Bayer Tree and Shrub Insect Control but wanted to know if there is anything else I can do to help save them.

Borer damage found in Italian cypress trunk

Bora damage found in another Italian cypress trunk

Suspected board damage found in Italian Cypress

A. Damage from borers or boring insects is typically a slower progression than a couple of months unless the plants are small. You are right, the progression in many plants is a change in color from a vibrant green to a dull, gray green color as the limb and leaves are dying from a lack of water.

            My guess is borer damage to your trees started before this year. Internal damage to the trunk from borers can be present for several years before enough damage has accumulated to result in the death of older trees. If trees are growing vigorously, they can recover from light borer damage on an annual basis.

            This color change due to damage starts in midsummer when air temperatures begin to heat up and plants require more water. Damage from boring insects begins in mid-spring (March) but get progressively more intense as these insects become larger and more voracious feeders.  By midsummer (June and later) internal damage to limbs and the trunk can be extensive, unrecoverable by the plant, and the supply of water from the roots is cut off.

Homeowners have this product available for borer control containing Imidacloprid in the active ingredients. It comes in other names as well. It is always best to check the active ingredients to make sure. Apply it late spring after any flowering has occured to avaid possible problems with honeybees. This chemical has been implicated, not proven, in colony collapse of honeybees. The safest way to apply it in our environment is a soil drench. Always read the label before applying any chemical.

            You chose the correct insecticide to use but it was probably applied too late. The best time to control boring insects with these types of insecticides is in mid spring when they first become active.

            Consider using pesticides as a last resort because they can be a bit like “whack-a-mole” and contribute to other pest problems. Death of boring insects can happen in a few days when systemic insecticides are applied as a liquid drench to the soil.