Insects

originally prepared by Jim Newman, Extension Forestry Specialist
updated by Jeff Stringer, Extension Forestry Specialist, and Lee Townsend, Extension Entomologist, University of Kentucky

Wood Boring Insects

Bark Beetles

Bark beetles use trees as breeding sites and have an important natural role in killing weak or old trees or aiding the decomposition of dead wood. Odors from damaged trees attract bark beetles so initial attacks in an area often occur on stressed or injured trees. Beetles that develop in these trees emerge through small round holes in the trunk and move to other trees in the area.

Trees typically die in a directional pattern from
southern pine beetle attack
(Ronald F. Billings, Texas A&M Forest Service, Bugwood.org)

Adults enter trees to lay their eggs, often creating distinctive tunnels or brood chambers. The grub-like larvae bore extensively into the wood feeding on tree tissue or fungi brought to the tree by colonizing individuals. Most bark beetle species in Kentucky attack trees that have been significantly weakened by disease, smog, competition, or physical damage. Extensive tunneling by the larvae can girdle and weaken or eventually kill the tree. Healthy trees have defensive compounds that can kill or injure attacking insects, or simply immobilize and suffocate them with the sticky fluid. However, under outbreak conditions, large numbers of beetles can successfully attack healthy trees with disastrous results for the lumber industry.

Pine bark beetle life cycle (barkbeetles.org)

Some bark beetles (ambrosia beetles) carry a fungus with them that grows within their galleries in the tree. These bark beetle larvae bore in the tree but feed on the fungal growth. The fungal growth clogs the vascular system of the tree and causes death.

Stained sapwood is an indication of ambrosia beetle attack (entne.dept.ufl.edu)

Good silvicultural practices reduce the potential for bark beetle attack by reducing stress and keeping trees actively growing. When feasible, prompt removal of damaged trees significantly reduces the likelihood of successful bark beetle attacks.

Preventive truck spray to protect against attack by wood boring insects (klyq.com)

Insecticides used in bark beetle control do not penetrate the tree and kill the developing larvae so trees that have been successfully attacked by bark beetles cannot be saved by insecticide applications. However, uninfested high value trees, judged to be at high risk, can be sprayed with an insecticide as a preventative measure against attack. The area of the tree requiring insecticide treatment depends upon the insect species for which the application is being made. The appropriate area of the tree should be thoroughly wetted with the insecticide spray mixture.

Wood Borers

Round headed (left) and flat headed (right) wood borer larvae
(extentopubs.tamu.edu)

Roundheaded and flat headed wood borers are mostly beetles, and a few caterpillars, that infest terminals, shoots, twigs and roots of living trees. Terminal and shoot insects are of particular importance in the initial stages of forest regeneration and early stand growth. These insects are very importance in forest nurseries and ornamental trees. Other species in this category damage or destroy trees that would otherwise produce quality lumber or other wood products. Most insects that cause this damage are borers as adults, larvae, or both.

Attack by a carpenterworm, the larva of a moth,
typically begins near a wound.
(James Solomon, USDA Forest Service, Bugwood.org)

As with bark beetles, most borers are secondary invaders that attack bark and wood of trees that are seriously weakened, dying, or recently cut. Examples include carpenterworms, oak clearwing borers, metallic wood borers, and pine sawyers. Trees attacked by these pests are usually scattered so that most control measures are difficult and not economically feasible. Prevention is the best management practice to reduce losses to wood borers. Keeping trees healthy and vigorous will allow them to fight off invading borers.

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Leaf Feeders or Defoliators

This diverse group of insects, which includes many species of caterpillars, sawflies, and beetles, eats leaves and needles. Trees attacked by defoliators can be recognized by missing foliage and uneaten leaf parts such as veins and petioles. Some members of this group feed within a leaf, mining between the upper and lower epidermis. Correct identification of the leaf feeders gives information needed to assess the problem.

Defoliation reduces photosynthesis, interferes with transpiration and translocation within the tree. Light defoliation normally has little effect on the tree but moderate-to-heavy or repeated defoliation can reduce tree vigor. The impact on a tree varies with time of attack, tree species and health, and single or repeated defoliations.

Sawflies

Several species feed on conifers or deciduous trees in forest and plantation stands. The adults are small broad-waisted wasps. Larvae resemble caterpillars but are usually without hairs and have pairs of fleshy prolegs on the underside of every segment on their abdomen (caterpillars normally have four or fewer pairs). Larvae of the more commonly found sawflies vary from 2/3 to 1 ¼ inches long, are usually greenish to dusky gray, and have conspicuous stripes or spots. Outbreaks occur periodically, sometimes over large areas, and can result in loss of tree growth and sometimes tree mortality.

Caterpillars

Many species of caterpillars feed on deciduous trees in forest and a few feed on conifers. Adults are usually moths but a few are butterflies. Most caterpillars have five pairs of fleshy prolegs – four along the abdomen and one pair at the end. Full grown larvae of the more common caterpillars range from 3/4 inch-long to more than 3 inches. Color and markings are variable. Outbreaks occur periodically, sometimes over large areas, and can result in loss of tree growth and sometimes tree mortality.

Forest tent caterpillars
(James Solomon, USDA Forest Service, Bugwood.org)

Forest Tent Caterpillar

Forest tent caterpillars are very similar to eastern tent caterpillars but have a row of light keyhole-shaped spots down the center of the back rather than a stripe. They feed on a wide range of trees including sweetgum, oak, birch, ash, maple, elm and basswood. Like the eastern tent caterpillar, there is one generation in the spring.

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Potential Problem Species

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Insecticides for Forest Pest Control

Insecticide applications are rarely practical against forest insect pests. However, they may be useful in specific situations, such as limited infestations of an invasive species. Insecticidal soaps, horticultural oils, Bt- insecticides, and systemics are common choices because they have very low potential to harm the environment, non-target species, and applicators.

Insecticidal soaps are made from of salts of fatty acids. When sprayed directly on vulnerable stages of softbodied insects, such as aphids and adelgids, they kill by damaging individual cells. However, they are not very effective against stages of the life cycle that are inactive or hidden, or against larger insects such as caterpillars and beetles.

Spraying insecticidal soap to control hemlock woolly adelgid (archive.knoxnews.com)

Insecticidal soap sprays must come into direct contact with the target pest and often results are best against specific life stages. Timing and thorough spray coverage are essential for best results. If the vulnerable stage is active over a long period of time, several applications are needed to control most insects.

Do not apply insecticidal soaps directly to water or use near a water source. Hard water is not effective for mixing soap sprays, so use softened or distilled water for best results. Tender young growth of evergreens and shrubs in the spring can be sensitive to insecticidal soaps. Insecticide applications are rarely practical against forest insect pests. However, they may be useful in specific situations, such as limited infestations of an invasive species. Insecticidal soaps, horticultural oils, Bt-insecticides, and systemics are common choices because they have very low potential to harm the environment, non-target species, and applicators.

Bt insecticides (Bacillus thuringiensis) are protein toxins that are produced by a common soil bacterium. Many provide specific control of caterpillars without affecting other types of insects (beetles, sawflies, etc.). Bt insecticides disrupt the gut wall cells in the caterpillar digestive tract so a formulation of the insecticide must be sprayed on foliage that will be eaten by the caterpillars. Caterpillars stop feeding soon after eating the insecticide but usually do not die for several days. Bt insecticides work best against young caterpillars that are less than half-grown. These insecticides are relatively non-toxic to mammals and other animals.

Imidacloprid is the common name of a systemic insecticide that is used to control sap-feeding insects, such as the hemlock woolly adelgid. It is a nerve poison but it is much more toxic to insects than to warm-blooded animals. Imidacloprid can be diluted in water and applied as a drench poured around the base of a tree, or injected into the soil. The insecticide is taken up by the roots and moved throughout the tree.

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References

Brooks, R. Forest Herbicides and their mode of action. U of Idaho CES Tree Planting and Care No. 15. Forest pest control. U Ga Special Bulletin 16 http://www.bugwood.org/pestcontrol/ Invasive plants of the eastern United States: Identification and Control. http://www.invasive.org/eastern Kentucky Woodlands Magazine Vol 1. SAF.

Herbicide use in forest management – a position of the Society of American Foresters. This publication, originally prepared by Jim Newman, Extension Forestry Specialist, is provided by the Pesticide Safety Education Program of the UK College of Agriculture. This version has been updated by Jeff Stringer, Extension Forestry Specialist, and Lee Townsend, Extension Entomologist.

http://www.ext.colostate.edu/mg/Gardennotes/331.pdf