By E. A. Buss and J. L. Foltz
Insect borers can be serious aesthetic, economic, and structural pests of trees and shrubs. Their tunneling damages wood, creates “hazard” trees, and lowers the wood’s value for lumber and veneer. Infested nursery stock may have poor form, reduced growth rates, or be impossible to sell.
Borer larvae (immatures) and adults make tunnels in the shoots, branches, trunks, or roots of woody plants of all ages and sizes. Eggs of most borer species are laid on or in the bark, and larvae chew into the plant tissue. Most borers are larvae of beetles or moths, but some are wasps or flies. Most insect borers are considered “secondary pests” because they attack only after a plant has been weakened or killed by another stress. Some, however, are “primary pests” and are able to attack and develop in fairly healthy trees and shrubs. Knowing whether insects are primary or secondary pests is critical to assessing and treating plant problems.
The presence of borers is hard to detect until plants or plant parts become damaged or die. Sawdust-like frass (excrement) may be around an exit hole or in a pile on the ground. Sap may ooze from the wounded site. There may be round, oval, or D-shaped holes randomly located on the plant. Insect exit holes can be distinguished from woodpecker holes by the absence of frass. The chewing of some species may be heard by someone standing near the tree.
The severity of plant damage depends on the number and location of insects in the plant tissue. Phloem feeders destroy tissues that transport food and produce new wood and bark. Feeding by a few individuals may produce necrotic lesions, whereas feeding that encircles the stem may kill a branch or the entire tree. Xylem borers make holes in the sapwood that disrupt the flow of nutrients and water, as well as structurally weakening the plant. Twig and shoot borers decrease fruit, nut and seed production by causing branch dieback. Borers may also feed in the succulent callus tissue around grafts, thus preventing the connection of scion and stock. Dead branches, pitch masses, and wood and bark riddled with holes decrease the aesthetic value of plants.
* Bark beetles (Coleoptera, Curculionidae, Scolytinae): Beetles in this group tunnel beneath the bark of trees. Adult beetles are small, somewhat cylindrical, and reddish-brown to black. Larvae are cream-colored and legless. Signs of infestation include a red or white powdery dust that collects around the entrance holes and in bark crevices. Sap may flow from entrance holes and form small “pitch masses.” Over time, the bark looks like its been riddled with “shot” from the small beetle exit holes.
* Southern pine beetle (Dendroctonus frontalis): Southern pine beetle is one of the most serious pests of pines in the United States, but normally is a scavenger of dying pines. It becomes a pest when its populations increase. Dendroctonus beetles are distinguished by round posteriors, in contrast to Ips beetles, the abdomens of which are concave posteriorly and have small spines on the wing covers. Adult beetles, active throughout the year whenever temperatures are above 58 degrees Fahrenheit, disperse widely to infest injured, weakened or stressed trees. When abundant, they can attack and overwhelm healthy trees. The presence of pitch masses on the tree trunk may indicate an attack. Adults tunnel beneath the bark constructing frass-packed egg galleries in patterns resembling the letter “S.” The tunneling quickly destroys the phloem and kills the tree. One to three months later, the needles of the dead tree turn reddish-brown. Seven or more generations may occur each year.
* Black turpentine beetles (Dendroctonus terebrans): Black turpentine beetles attack fresh stumps and the lower trunk of living pines, usually below a height of 10 feet. Adult beetles bore into the cambium and make galleries that extend downward. Galleries in the inner bark may be vertical, D-shaped, or fan-shaped. All pine species native to the southeastern United States are known to be hosts. Attacks are identified by white to reddish-brown pitch masses about the size of a half dollar. The adult insect is dark brown to black and 3/8 inch long. The posterior end is rounded. The life cycle takes from two-and-a-half to four months, and there are two to four generations per year.
* Ips engravers (Ips spp.): Ips engravers are normally secondary pests, attacking pines stressed by factors such as lightning strikes, root damage and drought. Occasionally, however, large populations attack and kill young, healthy pines and the tops of older pines. Signs of infestation also include pitch masses and fading foliage color, similar to that of the southern pine beetle. However, their frass-free egg galleries radiate out from a common entrance hole. Ips beetles also transmit bluestain fungi, which disrupt the water transport system of trees. Egg-to-adult development occurs in as little as four weeks, so these beetles may also have seven to 10 generations per year.
* Ambrosia Beetles (Coleoptera, Curculionidae, Scolytinae and Platypodinae): Ambrosia beetle females bore into twigs, branches or trunks of apparently healthy, stressed or freshly cut host material. Attacks on living plants are often near ground level on saplings or at bark wounds on larger trees. Visible symptoms of an infestation include wilted foliage and strings of boring dust from numerous small holes. Ambrosia beetles feed on symbiotic fungi that they introduce into their tunnels and cultivate. Eggs, larvae and pupae occur together in the tunnels there are no individual egg niches, larval tunnels or pupal chambers.
* Long-horned beetles or round-headed borers (Coleoptera, Cerambycidae): Adults of this family are called long-horned beetles because their antennae are usually long. Larvae (round-headed borers) tunnel underneath bark and into the wood. The tunnels are oval to almost round in cross section, conforming to the cylindrical shape of the larvae. Larvae of some species are legless, but most have three pairs of small legs. While tunneling, larvae continually pack their tunnels with frass, which looks like compressed wood fibers, or push frass out of the holes they make. This frass, along with the sap exuded by the plant in response to the damage, is often visible on the outside of infested trunks or branches.
* Twig girdler (Oncideres cingulata): Twig girdlers are important long-horned beetles. Their damage occurs primarily from egg laying. The grayish-brown adult females (1-1/16 inch long) are active from September to November. They girdle limbs by chewing a V-shaped groove entirely around twigs, branches or terminals. Eggs are inserted into the bark on the girdled part of the branch away from the tree. Girdled limbs eventually break and fall to the ground. Larvae cannot develop in healthy sapwood. Damage can disfigure a young tree and leads to secondary branching, especially if the terminal is attacked. Oak, persimmon, hickory and pecan are common hosts.
* Metallic wood-boring beetles of flat-headed borers (Coleoptera: Buprestidae): Adult beetles are flattened, hard-bodied and bullet-shaped with short antennae. These beetles often have distinctive metallic colors (green, blue, bronze or copper). Larvae are cream-colored and legless with widened, flattened body segments just behind the head. Thus, when these larvae tunnel beneath bark or in the sapwood they make oval or flattened tunnels in cross section. Galleries are often winding and packed with frass. These beetles are often associated with stressed or wounded trees.
* Carpenterworms (Cossidae): Carpenterworms are large caterpillars that tunnel through the trunks of many tree species. These larvae develop over one or two years, initially feeding underneath the bark but later tunneling into the sapwood. Outward signs of attack include large piles of sawdust and frass. In the spring, large adult moths with mottled wings emerge.
These daytime fliers are called clearwing moths, and they look superficially like wasps. Eggs are laid at the margins of insect holes, lawnmower cuts, on galls and other wounds. Newly hatched larvae bore into the phloem and feed beneath the bark. Signs of infestation include wet spots, ejected frass, and masses of sap around damage sites. Infestations can kill branches or entire trees.
Keeping plants healthy can minimize damage from secondary pests like insect borers. Avoid other physical injury or stress to tree trunks or roots (e.g., lawn mowers, weed trimmers, digging building foundations, septic tanks, soil compaction, soil added or removed above the roots, drought, flooding, or lightening). Mulch around the trunk to increase the distance between machinery and the plant. Because many borers are attracted to recent wounds, avoid pruning during adult activity periods. Place trees and shrubs in properly prepared areas protected from extreme weather conditions. Follow recommended irrigation and fertilization guidelines.
After trees and shrubs are infested with borers, non-chemical controls are limited. Remove and destroy (burn or chip) infested, dying or dead plants or plant parts, including fallen limbs. Severely infested trees (“brood trees”) only produce more pests that can attack neighboring trees. Several natural enemies attack insect borers, including predatory beetles, parasitic wasps or flies, and birds especially woodpeckers. Insert a flexible, small gauge wire into borer entry holes to puncture and kill the tunneling insect. Several attempts may be needed to be successful. Use pheromone traps to monitor adult activity, and disrupt the mating and egg-laying of clearwing borers. These traps often only attract males. Use ultraviolet blacklight traps to monitor adult beetles, which are drawn to the light and die in the bucket. These traps attract both males and females.
Most insecticide products registered for borer control are applied as sprays to the trunks and branches, and are contact, residual insecticides (e.g., carbaryl, chlorpyrifos, lindane, permethrin). Although these products do not kill larvae that have already penetrated the sapwood or heartwood, they will kill adults and larvae tunneling through the treated bark layer. These products are applied preventively and may be effective for three to ten weeks. Complete spray coverage of all trunk and branch surfaces is necessary for preventive control of borers. Treating only the base of the tree trunk is enough to protect the tree only from a few insects such as the black turpentine beetle and the peachtree borer. Thorough coverage may be difficult on large trees and may result in drift to non-target areas. To minimize drift, spray only on non-windy days. Read the insecticide lable for the proper protective clothing requirements.
Fumigants (e.g., paradichloro-benzene (PDB) moth crystals) can kill caterpillar larvae at the base of trunks if the crystals are inserted into tunnels with external openings, such as for carpenterworms or clearwing borers.
Eileen A. Buss, Ph.D., is associate professor/extension entomologist, Entomology and Nematology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.
John L. Foltz, is associate professor, emeritus, Entomology and Nematology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville.
This document is ENY-327 (MG007), one of a series of the Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Revised: June 2006. Visit the EDIS Web site at http://edis.ifas.ufl.edu.
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* Featured Creatures Web site: http://creatures.ifas.ufl.edu/
* Southern pine beetle in Florida (EDIS publication): http://edis.ifas.ufl.edu/IG147
* WoodyBug Web site: http://woodypest.ifas.ufl.edu/