Lauri Jean Crowe explains oak tree borers, the larvae of beetles and moths. Learn how to prevent and control these insect infestations and damage.
What is it?
There are several types of borers which attack oak trees. Oak tree borers are the larvae of moths and beetles which are leaf feeding insects. regardless of the type of borer which attacks your oak trees, they still all borers do the same types of damage and inflict the same sort of wounds to the oak tree.
What does it look like?
Damage from oak tree borers manifests as foliage at the top of the tree being sparse and thin looking. The damage from the oak tree borers begins at the top of the trees and migrates downward, with foliage turning yellow or brown and then dying off. As damage continues the individual oak twigs and branches of the oak tree will become weakened due to borer holes. The borer holes are usually in the trunk of the oak tree or on the larger branches. As the borers feed, they leave behind a sawdust like material. Usually the areas of the oak tree which have the largest number of holes and sawdust like material will become the weakest. These branches and their twigs will then begin to die off and the oak tree may lost both foliage and branches. Often the oak tree bark will slough off in areas of extreme populations of oak tree borers. The areas beneath the sloughed off bark will show holes and tunnels filled with sawdust like material. Usually borers attack older branches on trees, but when they infest a younger tree or a newly transplanted one such as a sapling, the oak tree can quickly be killed off entirely if the borer population is large and active.
How does it manifest?
Female borers lay their eggs in oak tree bark and crevices throughout the summer growing season. When the borer eggs hatch, the young borers emerge to feed on the oak tree. The areas the borers feed most on are the bark, heartwood and sapwood. When the oak tree borers tunnel and feed they damage the oak tree and its flow of nutrients throughout the tree. This starves the foliage, and areas toward the tips of the oak tree branches and twigs which causes the discoloration and death of these areas of the tree. Borers damage the nourishment conducting vessels within the oak tree and along with twig, branch and foliage dieback, the oak tree will leak sap as a defense mechanism to this damage. However, sap only flows as a defense mechanism in a health oak tree. If the oak tree is already severely damaged or dying you will not see the sap leakage occur. Often the oak tree borers are killed when they burrow into the wood and the tree sap flows into the tunneled hole. The sap coats the borer which quickly kills it, and seals up the damage. Usually trees which already have wounds in the bark or twigs are the ones which become easily infested with borers because the wounds are attractive to the adult female as areas to lay eggs in the summer. Therefore, oak trees with existing damage from pruning and other mechanical injuries, poor growing conditions or damage from other leaf growing insects are those most prone to borer populations and infestations.
What can you do about it?
Once borers have tunneled into your oak tree they can be difficult to control, especially if the insect population is large. You can help to control borer damage by pruning out and destroying all dead or dying branches or those which show signs of tunneling such as sloughed off bark areas. If you have transplants or young saplings which become infested it is best to dig them out and destroy them entirely rather than try to salvage them because these trees will be prime targets for adult females to lay eggs if they make it through to the following season, because of the wounds from the initial infestation. You can also use a lindane containing insecticide in mid to late May on the trunk of the oak tree. This will help control the borer populations on the healthy branches and twigs. The lindane insecticide treatment should be repeated again in two weeks after the first application. You will also want to repeat applications of the insecticide in July and August when new eggs will hatch into young borers and females will be looking for sites to lay eggs. However, the best defense against borer infestations is to keep the oak tree healthy to begin with so that there are no obvious mechanical or insect wounds which will make the oak tree attractive as a place to lay eggs.
