Wood-Boring Insects of Trees and Shrubs
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B-5086 01-08 Wood-boring Insects of Trees and Shrubs Bastiaan M. Drees, John A. Jackman and Michael E. Merchant* produces square holes in rows around a trunk or any insects feed and make their homes branch. (See photo on page 6.) in the bark, trunks and branches of Borers tunnel in the inner bark layer (cambium), shade trees and shrubs in Texas. Insect which transports nutrients and water to the leaves. Mborers belong to several different insect groups When the cambium layer is completely girdled the including a variety of beetles, moths and horntail plant eventually dies above or beyond the damage wasps. site. Partial girdling reduces plant growth and vigor Most insect borers are attracted to weakened, above the site of attack. On occasion, tunneling damaged, dying or dead plants. These are referred to makes the tree weak, causing limbs and branches to as “secondary invaders” because they attack only fall. Borer damage can severely affect the quality of after a plant has been weakened by another stress. lumber and can make trees susceptible to disease. Secondary invaders are a symptom of other prob- lems with the health of the tree or shrub, but may contribute to its decline. Secondary invaders include Wood-boring Insects species from groups already mentioned, but also Long-horned beetles or round-headed may include termites, carpenter bees and carpenter borers (Coleoptera: Cerambycidae) ants. Adults are called long-horned beetles (Fig. 1) Many other insects live in dying or dead trees, because their antennae are occasionally longer than including natural enemies (predators and parasites) their bodies. Larvae tunnel underneath bark and of the insect borers, sap or fungi feeders, or species into the heartwood. The tunnels are oval to almost which merely use the spaces provided by the tunnels round in cross section because of the round shape of and galleries as living quarters. the larvae (Fig. 2). Larvae of some species are leg- Wood-boring insects that attack healthy trees less, but most have three pairs of small legs on the and shrubs are called “primary invaders.” Primary first three segments behind the head capsule. While invaders may eventually kill trees. tunneling, larvae continually pack their tunnels with excrement (frass), which looks like compressed wood fibers, or push frass out of the holes they pro- Damage duce. This excrement, along with the sap exuded by Borer infestations often go unnoticed until plants the plant in response to the damage, is often visible or parts of plants begin to die or show external signs on the outside of infested trunks or branches. Many of damage. Wood-boring insects often produce saw- species of beetles belong to this group, but most are dust-like frass (excrement). Their holes are normally secondary invaders. Some examples of long-horned round, oval or semicircular and are found in a ran- beetles are described below. dom pattern on the plant. Woodpecker damage is Locust borer (Megacyllene robiniae) adults are sometimes confused with that of wood-boring bee- 3 medium-sized ( ⁄4 inch long) long-horned beetles tles; however, woodpecker damage will not produce frequently found feeding on goldenrod or other frass. One woodpecker, the yellow-bellied sapsucker, flowers in the fall. They are dark brown to black with distinctive gold-yellow markings. Larvae *Professor and Extension Entomologist; Professor and Extension hatch from eggs laid in bark crevices. Visible Entomologist; and Professor and Extension Urban Entomologist, symptoms of infestation are wet spots The Texas A&M University System. and frass on the bark of black locusts. Later, lar - and maple trees and can be a serious pest in nurs - 5 1 vae tunnel into the inner bark and construct cells eries. The reddish-brown adults ( ⁄8 to 1 ⁄8 inches in which they spend the winter months. In a year long) lay eggs individually in bark crevices dur - the larvae are fully grown and about an inch in ing July and August. Larvae tunnel under the length. bark and into the heartwood. Infested sites can be Cottonwood borer (Plectrodera scalator ) is fre - recognized by the frass around the buckled bark quently found on cottonwood, poplar or willow near the gallery entrance. Larvae often tunnel 1 completely around the trunk or branches they trees. Adult beetles are large (1 ⁄4 inches long) with an attractive black and whitish-yellow pattern. infest, producing noticeable scars or girdling. Red They are active from May through August. The oak borers feed for more than a year before larvae (1.75 to 2 inches long) tunnel at the base of pupating in chambers tunnelled into the heart - the trunk or below ground level. They require wood. Damage kills limbs or terminals and about 2 years to develop. increases the risk from secondary invaders and diseases. Red-headed ash borer (Neoclytus acuminatus ) is one of the most common wood-boring beetles. It Twig girdler (Oncideres species) damage occurs has a narrow body with a reddish thorax and primarily from egg laying. This insect attacks light brown wing covers marked with four yellow pecan, mimosa, chinaberry and huisache. The 1 lines on each. The yellow lines are slanted down - grayish-brown adults (1 ⁄16 inch long) girdle limbs ward toward the middle, giving the appearance during the fall (late August through mid- of a “V” across the back. The antennae are rather November) by chewing a V-shaped groove entire - short and the long legs are thin and fragile. Red- ly around twigs, branches or terminals. Eggs are headed ash borers feed in many species of wood inserted into the bark on the girdled part of the including ash, oak, elm and even grapes. Adults branch away from the tree. Girdled limbs eventu - can be found on dead log piles and frequently ally break and fall to the ground, particularly dur - emerge from firewood. ing high winds and storms. Damage can disfigure a young tree and leads to secondary branching, Red oak borer (Enaphalodes rufulus ) attacks oak particularly if the terminal is attacked. Larvae 7 reach up to ⁄8 inch long and are unable to develop in healthy sapwood. Removing the girdled twigs and branches from the ground during winter and spring and destroying them can reduce the popu - lation of these insects. Twig and branch pruners (Elaphidionoides and Agrilus species) produce damage superficially similar to that of twig girdlers on elm, hackberry, hickory, maple, oak, pecan, persimmon, redbud, sweetgum and other trees. In these species, how - ever, it is the larvae that girdle twigs and branch - es underneath the bark. The surface of the sev - ered end of the twig is smooth. The insect usually Figure 1. Long-horned beetles or roundheaded borers: locust borer severs branches where small twigs branch from (left); cottonwood borer (center); and red-headed ash borer (right). the main, girdled branch. Red oak borer larvae girdle main trunks and Female twig girdler beetles chew a V-shaped Twig and branch pruner larvae girdle from branches during the second year of larval groove entirely around twigs, branches or ter - underneath the bark. development. minals. (Photo by M. E. Rice) 2 Metallic wood-boring beetles completed within a year. When abundant, they (or flat-headed borers) can attack healthy trees. Larvae tunnel beneath the bark producing tunnels or galleries in patterns (Coleoptera: Buprestidae ) resembling the letter “S” (Fig. 5). This tunnelling Adult beetles are flattened, hard-bodied and boat- quickly disrupts the cambium layer, girdling the shaped with short antennae. These are beautiful bee - tree. Infested trees can have numerous masses of tles with distinctive metallic colors (green, blue, resin called "pitch tubes" on the tree trunk. bronze, copper). Larvae are cream-colored and leg - Needles of newly attacked trees turn reddish- less with widened, flattened body segments just brown 1 to 2 months after infestation during the behind the heads. Consequently, when these larvae summer, and up to 3 months afterward in the tunnel beneath bark or into the sapwood they pro - duce oval or flattened tunnels in cross section (Fig. 2). Galleries are often winding and packed with frass. Tunneling can girdle trunks and branches. Many species of flat-headed borers occur in the state. Most are secondary invaders. Examples of flat-headed borers include the bronze birch borer (Agrilus anxius ), uncommon in Texas because of the lack of host trees; Agrilus species found on oak and raspberry ( A. bilineatus and A. ruficollis , respectively); flat-headed appletree borer (Chrysobothris femorata ) and a closely related Figure 3. Metallic wood-boring beetles or flat-headed borer adults: species that attacks recently transplanted or stressed Agrillus bilineatus (left); flat-headed appletree borer (right). shade, pecan and fruit trees. Bark beetles (Coleoptera: Curculionidae ) Beetles in this group tunnel below the bark of trees and/or into the wood. Adult beetles are small and reddish-brown to black. Larvae are cream- colored grubs without legs. One member of this group, the European elm bark beetle ( Scolytus multis - triatus ), is the carrier of Dutch elm disease. It occurs Figure 4. Bark or engraver beetles: Southern pine beetle (left); Ips in the Texas Panhandle, but is infrequently encoun - engraver beetle (center) and granulate ambrosia beetle (right). tered in other parts of Texas. Other members of this group are described below. Southern pine beetle (Dendroctonus frontalis ) is a primary pest of southern pine forests. Adult bee - tles are active during warmer months (when tem - peratures are above 58 degrees F), and disperse widely to injured, weakened or stressed trees in the spring. Seven or more generations may be Figure 2. Larvae of round-headed borer (left) and flat-headed borer (right) with cross sections of tunnels (above).