Of all the insect pests that attack woody plants, borers must be the most pernicious. In my opinion, collectively, borers have the greatest potential to kill trees and shrubs of any guild of arthropod pests. Granted, repeated defoliation by caterpillars can weaken trees and predispose them to attack by other pests. Scale insects and mites can cause plants to drop leaves prematurely, dieback and, in extreme cases, die. But boring insects have the potential to kill trees in a hurry.

I will deal with three broad issues:

• How to recognize borer attack

• Which borer is the culprit

• What can be done to prevent and manage borer attack

Diagnosis

The primary symptom of borer attack is dieback of shoots, limbs, or portions of the canopy. In the worst case, the entire tree may die. As a rule, when you see dieback you should also look for signs of mechanical injury or problems with roots and vascular elements. Dieback means the tree has problems moving nutrients and water up or the products of photosynthesis down. Root injury associated with construction or soil compaction often results in canopy dieback. Diseases and insects that attack and kill roots can cause similar symptoms to occur up top. Prolonged periods of drought or water - logging can produce the same result.

How do you differentiate borer attack from other causes of dieback? Dieback involving borers is usually accompanied or preceded by round, oval, or D - shaped holes in the bark, sawdust-like material around the base of the tree, resin flow or pitch tubes, or sap flows. Borer entry or exit holes may exude insect feces commonly referred to as frass. In some cases the frass will be dry and have the appearance of sawdust.

In an attempt to repel their attackers, trees produce pitch, resin, sap, or gum that drip from the site of the bore gallery. These products are a component of the generalized wound response of many plants. Pressurized resin canals in pines may be part of the tree's way of physically forcing the attacker out. If trees are healthy, environmental conditions favorable, and attackers few, the tree may win and the borer is repelled. If plant defenses are compromised by drought, compacted soils, restricted root zones, construction, or mass attacks, the borers may win and the tree dies.

Who's who?

The dieback of this birch tree was caused by a bronze birch borer, a flathead that is the bane of many paper bark birches. Flatheads are some of the most deadly of all the beetle borers. Their larvae bore into the vascular cambium of their host tree, girdling and killing it.

Who are these borers? In general, borers belong to two of the most diverse groups of insects on earth - the Coleoptera (beetles) and Lepidoptera (moths and butterflies). There are a few primitive Hymenoptera (sawflies) and Diptera (flies) that bore into plants, but these are of minor importance to woody plants. Beetle borers are remarkably diverse with varied lifestyles and devious methods of attack.

Among the most deadly of all the beetle borers are the flatheaded borers. Larvae of these beetles are flattened - top to bottom, hence the name. They bore in the vascular cambium of their hapless host, thereby girdling and killing them. A characteristic D-shaped hole is created as the adult insect exits the plant. Flatheads include the bronze birch borer - the bane of many paper bark birches, the Pacific flathead borer - a west coast relative that attacks alders, the flatheaded apple tree borer, and the twolined chestnut borer, both pests of fruit trees and hardwoods. Unlike many borers, these may target trees that appear healthy. However, insidious conditions may predispose trees to flatheaded borer attack.

On the coastal plain of the mid-Atlantic, bronze birch borers readily attack white birches planted outside of their mountain zone of adaptation. The Pacific flathead assaults alders used in hot open locations that may have restricted root zones such as planting islands in parking lots. The two-lined chestnut borer infests oaks that may have been weakened by gypsy moth defoliation. The flatheaded apple tree borer attacks hardwoods wounded in the nursery damaged at installation, or those planted too deep.

Close relatives to the flatheaded borers are the roundheaded borers. These too may damage vascular cambium and heartwood. Larvae are round in cross section and their presence is recognized by round or oval exit holes on the bark. Common roundheaded borers include the Asian longhorned beetle, the Eucalyptus longhorned beetle, roundheaded apple tree borer, dogwood twig borer, twig pruner and twig girdler. The pine sawyer, a large species, is a vector for the nematodes that cause pine wilt disease.

A huge clan of beetle-borers are collectively known as bark beetles. Their small, round exit holes earn them the common name of shot hole borers, but engravers, ambrosia beetles, and turpentine beetles all belong to this group. Shot hole borers may damage trees directly by boring in and through vascular tissues and heartwood.

In addition, several bark beetles vector important fungal pathogens. The smaller European Elm Bark Beetle and Native Elm Bark Beetle are the principal vectors of Dutch Elm disease. The fungus is carried from infested to healthy hosts by the beetle in a specialized structure called the mycetangium. Once introduced, the fungus multiplies and destroys normal vascular function of the tree. In this macabre relationship, the bark beetle may benefit by diminished tree defenses and the fungus benefits by hitching a ride to a new victim.

Bark beetles such as the mountain pine beetle, black and red turpentine beetle, and southern pine beetle kill thousands of trees in urban and natural forests each year. Some species may have several generations each year. Drought, lightning strikes, construction, restricted root zones, and soil compaction contribute to bark beetle outbreaks.

Weevils also attack landscape and nursery trees in many parts of the country. The larvae of the white pine weevil are notorious for killing the central leader of white pines and spruces. The female emerges from its overwintering site in the leaf litter early in the spring, nibbles a hole in the central leader of the host, and deposits a clutch of eggs in the hole. Legless, C-shaped larvae tunnel down through the leader and kill it. Relatives of the white pine weevil attack other species such as cedar and may bore into the bole.

Caterpillars, the immature stages of moths and butterflies, usually graze on leaves. However, a large, sinister element of the moth clan chews through the shoots, branches, and trunks of their hosts. The caterpillars, or larvae, of shoot borers such as the Nantucket pine tip moth, pitch pine moth, and European pine shoot moth produce copious resin flows and dieback in the tips on their coniferous hosts. Zimmerman pine moth creates large pitch masses on the branches of trees. In these species, winter is usually spent as a larva or pupa with the adult moth emerging in the spring. The females mate, lay eggs on needles or terminal buds and the hatchling larvae enter the tissues of their host. In the case of tip moths, there may be several generations each year.

A fascinating group of moths have evolved to look like or mimic bees and wasps. By resembling stinging insects, these moths gain protection from their own predators. However, the larvae of many of these clearwings are deadly borers of prized woody plants. The banded ash clearwing is the primary culprit behind dieback and death in green and white ash in landscape plantings.

A close relative, the lilac-ash borer attacks ash, privet, and lilac as well. The peach tree borer and lesser peach tree borer hit species of Prunus low (usually within 2 meters of the crown) and high (2 meters up and on scaffold branches), respectively. Other clearwings attack Rhododendron, Viburnum, and in the west clearwing borers are important pests of Platanus and Sequoia. The dogwood borer has a broad host range attacking not only Cornus but also many fruit trees and hardwoods.

In general, clearwing borers overwinter as larvae and depending on thespecies, emerge in spring, summer or fall. Each species has a unique emergence time. Females attract mates using an airborne chemical message called a pheromone. Pheromones have been characterized, synthesized, and incorporated into sticky traps. These traps are not used to control the borers; they only catch the males. Male flight indicates mating and the deposition of fertile eggs that will soon hatch into larvae ready to enter the tree. The capture of males signals an excellent time to apply residual bark insecticides for control of the tiny borer larvae.

Other caterpillar borers include the larvae of leopard moths that create huge holes in the base of hardwood and fruit trees and the American plum borer that feeds beneath the bark of damaged fruit and plane trees.

Managing Borers

This Eucalyptus tree was killed by the Eucalyptus longhorned borer, a roundheaded beetle that damages vascular cambium and heartwood. Its larvae are round in cross-section and their presence is recognized by round or oval exit holes on the bark.

Borer management begins with plant selection, proper installation, and good maintenance practices. Start by selecting resistant varieties whenever possible. For example, the Heritage River birch is highly resistant to bronze birch borer. The Japanese dogwood, Cornus kousa, is far more resistant to dogwood borer than our native Cornus florida. Do not buy stock that has been damaged in the nursery during delivery. Bark wounds may be the perfect entry sites for borers.

Avoid planting too deep or too shallow. Both activities will impair normal root function. The same is true where compaction, restricted root zones, or non-porous soils prevent roots from receiving air or water. Mulch your trees correctly to reduce soil temperatures, conserve soil moisture, and provide a buffer to reduce mechanical injury during lawn mowing or weed removal. Avoid practices such as trenching that will sever roots and design pedestrian traffic patterns away from trees. During periods of drought, supplemental irrigation may be necessary to alleviate stress.

Once borers have colonized a tree, a successful outcome is in doubt. In severe infestations it may be wisest and most ethical to remove the tree rather than attempting a heroic rescue. In my experience, heavily infested trees such as pines attacked by turpentine beetle have little chance of recovery. They have lost their aesthetic value and serve as a factory for beetles that will move out and attack other trees.

When borer infestations are detected early in the attack cycle several chemical options are available. Potent systemic insecticides may be applied through the bark of the tree either as injections or implants. These chemicals move into vascular tissues where they kill borers. They are most effective against beetle borers. A debate exists as to how often these may be used. At issue is the fact that the tree is wounded and tissue is damaged each time an injection is made.

Newer systemic materials are applied with soil probes, as drenches, or in granular formulations. No wounding is involved but it may take several weeks for uptake from the soil. The trick here is to plan ahead and be sure the soil is moist. The new soil systemics have proven most effective against the flatheaded borers but do not work well against caterpillars.

Traditionally, wood boring caterpillars were controlled with residual insecticides applied to the bark of the tree. When tiny larvae hatch from their eggs, they encountered the poison as they entered the plant and were killed. With the loss of materials such as cloropyrifos (Dursban) for landscape use, options for this approach are somewhat limited. However, some of the synthetic pyrethroids such as permethrin, are remarkably effective in controlling boring caterpillars and beetle borers as well.

The trick here is to time the application of the insecticide as closely to egg hatch as possible. Pheromone traps may be used to help with the timing for many species of clearwing borers and some boring beetles as well. Predictive models based on flowering plant phenology or degree-day accumulations may also help with timing.

One new and somewhat novel approach is the use of nematodes that attack boring caterpillars under the bark. This has proven successful for clearwing larvae in sycamore, cherry laurel, dogwood, and ash. Biological control has also been used successfully to manage the Eucalyptus longhorned borer in California.