What Are Tree Borers?

Life Cycle

• Adult moths or beetles either emerge from infested trees during spring and summer or fly in to host in a healthy tree. Following mating, the females start to lay eggs in the crevices or wounds on the bark. It takes anywhere between 10 days and two weeks for the eggs to start hatching. As soon as the larvae emerge, they start to eat and tunnel their way beneath the bark where they continue to feed and mature. Fully developed adults emerge from oval holes chewed in the bark.

Symptoms

• Early signs of infestation include the wilting of terminal shoots, discolored foliage and crown dieback. The main sites of infestation develop calluses, cankers and cracked bark. As the larvae feed, they produce frass or waste material that is pushed out from the infestation holes. Larvae frass resembles fine sawdust. Large limbs start to die and are often weak enough to break easily in strong winds. It is possible for stronger trees to get infested year after year before finally dying.

Control

• Trees with bark that shows signs of extensive damage cannot be saved and need to be removed entirely. This will help save the surrounding trees from possible infestation. Control is most effective when insecticides are used on adults as they are laying eggs on the branches and trunk. There is no way to reach the larvae once they have penetrated the wood. Timing is hence critical for adequate control. The University of California Extension recommends pyrethroids and carbaryl as insecticides for adult pests.

Prevention

• Keep trees in good health with adequate pruning, fertilizing and irrigation. Plant trees in accordance with their requirements. Do not plant native understory trees such as rhododendrons or dogwoods in areas of full sun. Choose varieties that are adapted to your growing zone. Inspect trees regularly to control insects and diseases that weaken trees and invite secondary pests such as borers. Place lawn mower guards or mulch around tree bases to avoid wounding trees unnecessarily. Use preventative sprays especially on younger trees as these are more prone to borer attacks.