Identifying Their Damage and Controlling Borers

Q. I am having a problem with a bug infesting my trees. I haven't identified the pest but I wanted to first ask how did this infestation happen or if it's common in the Vegas Valley and second how is the cycle of reproduction of this bug?

Flat headed borer taken from tree

A. Thanks for sending the pictures. You have taken the larva or “worms” of a beetle from this tree. These larva or worms are called borers. You can notice the flattened head of this worm so it is commonly called a flat-headed borer.

I think it is either the Flatheaded Apple Tree Borer or Pacific Flat Headed Borer. I have been following this insect and its damage for many years in this valley. The adult of this “worm” is a beetle about ½ inch long.

Borer damage on Apple when first seen

The adult female beetle flies around searching for a mate so that it can lay its eggs. It finds a male by “smelling” the male’s pheromone it releases. Most likely this happens any time from about April through August. Once the female mates and prepares to lay eggs, she searches for suitable woody plants that are injured or unhealthy.

Extensive borer damage to Locust

This insect is part of an “ecological recycling cycle” helping plants that are on the decline continue in a downward spiral toward death. However, I have found this insect in trees that appear to be very healthy as well. The female laden with fertilized eggs searches for suitable plants through her sense of “smell”. One of the “smells” she identifies is damage of plants from sunburn.

Discoloration and canker to scaffold limb because of sunburn

High light intensity of our desert sun shining directly on the bark of trees that are thin and don’t provide much insulation become damaged and frequently die in the damaged area. Death in localized areas of the branches and trunk of woody plants leaves behind a “canker” in the surface.

Sunburn damage to Japanese blueberry

The smells and aromas of damaged plants attract the egg-laying of flat headed borers. She lays her eggs in the general vicinity of the damage but on top of healthy tissue. The very tiny worm or larva “hatches” from this egg and tunnels inside the plant just under the bark.

Borer damage to flowering plum

The wood just under the bark is laden with wet, sugary juices from the plant which the flatheaded borer feeds upon. As this flat headed borer feeds, it inches slowly forward just under the bark making a convoluted tunnel. At the beginning of this tunnel is the flattened head and it leaves a trail of sawdust and feces behind it as it feeds, moves and creates the tunnel. I can sometimes push on the bark covering the surface of these tunnels and feel a difference in the “hardness” of the stem.

Borer feeding under the bark inside peach tree limb

This borer matures (pupates or turn into an adult) inside the tree, exits the tree as a winged adult and searches for a mate to repeat the lifecycle. Sometimes, if it is late in the season, this borer will tunnel into the center of a limb to survive the winter and exit as an adult the following spring.

Borer feeding just under the bark of peach tree limb

Borers attack dozens of different trees and shrubs, many in the Rose family. This includes many of our fruit trees, landscape plants in the Rose family like Pyracantha and flowering plum and numerous others as well.

What to do?

Dig it out. Using a sharp and sanitized knife and laying it almost parallel with the branch, I will cut away the surface of the branch to expose the tunneling. I will remove all of the surface bark where there is damage. I will clean the surface bark all away until I have nothing but fresh wood remaining. In this way, I expose the tunneling and the source of protection for the boring insect. This exposes the hiding place to predators of this soft bodied “worm”.

Borer control using a sharp, sanitized knife

Prune it out. If the damage is more than 50% of the branch, I will remove the branch. If removing the branch seems excessive, I will not remove it and give it a chance to heal.

Using insecticide. In a last ditch effort I will use is a soil applied systemic insecticide. I never use these on fruit bearing trees but on ornamentals only. I will use these on non-flowering ornamentals because it is suspected that this systemic may impact bees visiting the flowers. If the tree produces flowers, I time the application of the insecticide just after the time of flowering.

One of the insecticides containing imidicloprid systemic for borer control

Whitewash to prevent. Whitewashing the upper surfaces of limbs, scaffold limbs and the trunk reduces sunburn which should also reduce problems from borers. I make the whitewash by mixing white latex paint 1:1 with water. You can also buy commercial whitewash. In the old days, it was made from lime.

Whitewashing fruit trees in the winter to help prevent sunburn