BORING INSECTS

General Description

Many species of boring insects attack the trunk, stems, bark, buds and roots of woody ornamental plants and trees in the nursery and landscape. Some bore deeply into the wood (buprestid, cerambycid and ambrosia beetles), many bore just under the bark (dogwood borer, elm borer and bark beetles). There are twig beetles which are found only in smaller terminal branches (black twig beetles and dogwood twig borer). Others attack the wood near ground level as well as the roots (magnolia borer). The giant palm weevil feeds in the buds of palms.

The two major groups of boring insects attacking woody plants include various beetles and clear-wing moths. The primary families of beetles include Cerambycidae (long-horn or round-headed borers), Buprestidae (metallic wood borers or flat-headed borers) and Scolytidae (bark beetles, wood-boring bark beetles and ambrosia beetles). The clear-wing moths all belong to the family Sesiidae.

Important Species

The main families of boring insects include:

Other families of borers commonly attack hardwood and conifer trees. The metallic wood borers such as the flatheaded appletree borer, ambrosia beetles, such as Xyleborus spp. and Xylosandrus spp., and engraver beetles, Dendroctonus spp. and Ips spp., may attack trees growing under stressed or injured conditions. Healthy trees are also occasionally attacked. Typically, borers must be controlled with preventative insecticide treatments, because, once the insects are under the bark, they are difficult to control. Pay particular attention to plants showing signs of stress such as wilting, disease infection or injury from equipment. Borers of one species or another will successfully attack such trees usually during the summer months. Small holes in the trunk or limbs with fine sawdust indicate the presence of these beetles. The dogwood twig borer, Oberea tripunctata, is a longhorn beetle that infests elm, viburnum, azalea, sourwood and fruit trees (Prunus spp., Malus spp.), but dogwood is the most common host. The larva tunnel through the terminal twigs and cut small holes to the outside from which they expel sawdust and feces. Infested twigs are girdled and killed by the female when she oviposits. Infested twigs wilt and hang on the tree or drop to the ground. Natural enemies attack boring beetles, but do not reduce them to low levels in nursery stock.

Biology

Buprestid beetles deposit their eggs on the bark, in crevices in bark or under the bark at the edge of wounds. Most spend the winter as adults in pupal cells. The life cycle usually requires one to three years, however some species have several generations per year. Cerambycid beetles deposit eggs in protected places on the bark. The larvae feed on the inner bark, then bore into the sapwood or heartwood. In some species, the life cycle requires two to three years, other species have several generations per year. Scolytid beetles usually have two to six generations per year. The Scolytid bark beetles feed on the inner bark of trees and the ambrosia beetles bore into the sapwood and heartwood and feed on an ambrosia fungus which they cultivate. Bark beetles have a large spine at the apex of the front tibia. Ambrosia beetles lack the spine. Clear-winged moths typically have one to two generations per year.

Host Plants

Virtually all woody plants and trees are subject to borer attacks. Borers nearly always attack unhealthy or stressed plants or trees. The preferred and most effective management practice is to keep plants and trees healthy. Prevention is the best cure.

Seasonal Abundance

Borers may attack woody plants anytime during the year, but predominately during the spring, summer and fall months.

Damage

Buprestid larvae tunnel under the bark and some species excavate winding tunnels through the sapwood. These tunnel are often filled with frass, and oval in cross section, more than 3 mm wide and usually enter the wood at an angle. Foliage discoloration occurs and there is usually noticeable sap-staining of bark around holes.

Sap-staining of bark around holes

Cerambycidae larvae tunnels, are circular in cross section, more than 3 mm wide, go straight into the wood for a short distance before turning and are free of frass. Foliage discoloration occurs and like buprestid damage, there is sap-staining of bark around holes. There are also usually cone-shaped holes in bark made by the female beetle for egg deposition.

Scolytid bark beetles mine between the bark and wood, often engraving both. They attack twigs, branches, trunks and roots. When the larvae hatch, they feed away from the egg tunnel at right angles. Conifers that are attacked form pitch or resin tubes at the site of the beetle's entry into the bark.

Scolytid larvae damage
Pitch on resin tubes

Scolytid wood-boring bark beetles bore directly into the wood and feed on woody material in both adult and larval stages. Scolytid ambrosia beetles bore directly and deeply into the sapwood. They introduce and feed on ambrosia fungi. The fungi, not the beetles or their larvae, feed on the wood fiber. Their boring activities cause white fluffy boring dust to accumulate at the base of the infested tree or woody plant. Small holes, 1/2 to 3 mm wide, will be noticed in the bark. The galleries are full of frass and stained a bluish or brownish color from the fungi.

Clear-winged moths characteristically deposit eggs near wounds on the trunk. They usually tunnel beneath the bark causing blistering, peeling and spongy areas. Some leave the pupal skin sticking out of their emergence hole.

Clear-winged moth larvae damage

The dogwood borer is the most destructive pest of dogwood. Damage by this borer is related to trunk injury and sun exposure. Larvae tunnel under the bark in the phloem and feeding damage may result in branch dieback, unthrifty or dead trees. There is one generation a year and adults emerge in spring or early summer.

The magnolia borer infests container and field grown magnolia. The larvae mine the roots and destroy the phloem tissue. The eggs are laid on the tree and the larvae move into the media to find a root. As they feed they move toward the root collar. At maturity they can be found feeding in the zone of the trunk about 2 inches from the media. The bark will appear spongy to the touch and frass may be observed in the area. Two to three generations occur per year.

The peachtree borer, along with the lesser peachtree borer attack most species of Prunus. They feed in the phloem under the bark and girdle the trunk or branches of these fruit trees.

The elm borer attacks a variety of hardwood trees including Chinese elm, pecan, hickory, oak, pine and dogwood. It can be an important pest of elm nursery stock where it feeds under the bark and girdles the trunk.

Sampling

Observe trees and woody plants for sap stains, holes in the bark, blistered, peeling or spongy bark areas, especially on plants that are stressed or have been subjected to trunk injury. Pheromones have been identified for many clear-winged moths and can be used to monitor for them.

Cultural Control

Preventing borers from infesting trees and other woody plants is the preferred and most effective method of control. Stressed plants are especially susceptible to borer attack. Carry out all approved horticultural practices to promote plant vigor and health to minimize borer infestations.

Adult clear-winged borers can attack healthy as well as weak or injured trees, but are most commonly found associated with stressed plants.

Biological Control

Natural enemies attack borers, but do not regulate them to low levels.

Chemical Control

For the most current insecticide recommendations to control these pests, please contact the local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at

http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide

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