Plant Health - Wood-Boring Insects

Scott Ludwig

Wood-boring insects can include long-horned beetles, flatheaded beetles, ambrosia beetles and even caterpillars. Not all of these pests can be controlled using the same management techniques. You need to know which borer group your trees are susceptible to and develop your management plans accordingly.

Long-horned beetles
Long-horned beetles get their name from their antennae, which are as long as or longer than their bodies. They lay their eggs on the bark, and newly hatched larvae tunnel underneath bark and into the heartwood. The tunnels are oval to almost round in cross section because of the round shape of the larvae.

While tunneling, larvae may pack their tunnels with excrement (frass), which looks like compressed wood fibers, or push frass out of the holes they produce. You can often detect an infestation by the frass that may be visible on the bark.

This group is rarely a problem in nursery production. Since the larvae burrow into the heartwood of the tree, the larvae can’t be managed once they infest the tree. Preventive trunk sprays can be used to protect the trees from infestation.

Metallic wood-boring beetles
Adult beetles are flat and boat-shaped. They have large eyes, short antennae and distinctive metallic colors. They can be green, blue, bronze, black and copper. Their larvae have widened, flattened body segments just behind their head. As the larvae tunnel beneath bark, they produce oval or flattened tunnels in cross sections.

Metallic wood-boring beetle feeding usually girdles the trunk or branches of the infested tree. Girdling can result in disfigured trees or even death.

Preventive trunk sprays are effective in preventing attack. Systemic insecticdes have also been shown to prevent larvae development and can be used to kill larvae once they start developing. Neonicotinoids have been shown to effectively manage flat-headed borers. However, label rates are often appropriate for landscape trees and not nursery produced trees.

If you decide to treat trees at your nursery, check your application rate and volume to make sure you do not exceed the maximum application rates per acre.

Clearwing moths

The caterpillars can be identified by their “false legs” on the undersides of some of the abdominal segments. These are not found on beetle larvae.

Peach tree borer is probably one of the most important pests in this group. Larvae tunnel into the roots and lower trunks of peach, cherry, plum, nectarine and apricot. They feed on growing tissue and inner bark. Infested trees can be identified by masses of sap around damage sites at the base of the trunk.

Other clearwing borers include lilac or ash borer, dogwood borer and lesser peach tree borer.

Ambrosia beetles
Ambrosia beetles have become an important pest of nursery production in many parts of the country. Adult female beetles bore into the trunk and stems of young trees. These insects make galleries directly into the heartwood of the tree, which they inoculate with an ambrosia fungus.

The ambrosia fungus, which adults and larvae feed on, blocks the trees xylem vessels. This is often the cause of death in the trees. In addition they can introduce or create entry points for pathogenic fungi such as Fusarium spp.

Visible symptoms include wilted foliage and strands of toothpick-like spines protruding from small holes. The spines are strings of boring dust produced by the female beetle as she excavates her gallery.

There are no known curative treatments available. Preventive trunk sprays with products registered for ambrosia beetles are the only known way of preventing an infestation.

Managing wood-boring insects
Most wood-boring insects are secondary invaders. If trees are being attacked they are probably under stress.

If you have more than the random plant being attacked then you probably need to assess your production practices. Proper tree care discourages most borer pests and will help the plants survive an attack. Sap flow from healthy trees is the tree’s primary defense from damage by many borer pests.

There are some practices that you want to avoid to prevent borer attack. Delayed potting of trees to a larger container size can cause undue stress to trees. Unfortunately, with the current financial situation, many growers are holding onto plants longer than they should before bumping them up to a larger size.

If you are trellising your trees, make sure the support wire is not damaging the tree. I have observed a strong relationship between trees damaged by trellises and borer attacks.

Under- and over-watering and fertilization can also stress the trees. If you use slow-release fertilizer that releases based on temperature, try to avoid putting it out in the heat of the summer when you could get a large release of nitrogen into the pot. My recommendation is to top dress in the spring or after the heat of summer is over.

Effective treatment for borers requires that all surfaces be covered with the insecticide. There are a few cases where the treatment can be targeted to a specific location on the tree, such as the lower trunk for peach tree borers.

If you are using an airblast sprayer make sure the insecticide reaches all sides of the trees. A spray that only covers half the trunk will not be 100 percent effective since it is leaving part of the tree exposed.

Systemic insecticides
There are only a few systemic insecticides currently registered for borer control (e.g., acetamiprid, dinotefuran and imidacloprid). These can be used for preventive and curative control of flat-headed borers. Due to the cost and label restriction associated with trying to treat all the trees on your nursery, consider using these insecticides on infested trees when you detect damage.

Scott W. Ludwig is an extension program specialist at Texas AgriLife Extension Service.