General Overview

Borers and miners are the larvae of various insects, particularly beetles, flies, and moths. Beetles comprise the largest order of borers. These larvae construct tunnels in terminal shoots, branches, trunks, and roots of woody plants of all sizes. Borers can aesthetically and/or fatally damage shade, ornamental, and fruit trees and shrubs. They are commonly concealed beneath the bark or in the wood, making them difficult to detect and control. 

Damage caused by borers often begins in the upper branches of tree crowns and progresses downwards as the tree weakens. Common symptoms include sudden wilting and discoloration of foliage in the tree tops and branch die-back. Infested trees will probably also exhibit D-shaped emergence holes and sinuous or zig-zag patterns in their bark.

Since borers usually prefer to attack weakened trees, good cultural practices that promote high tree vigor and reduce the incidence of disease help to minimize infestations and the resulting damage. Such practices include preventing injury to young trees, watering during dry periods, and appropriate pruning techniques (avoid topping your trees).

Woodpeckers are known to consume borers in large quantities. Attract this natural pest control to your yard by providing suet during cooler months (suet can go rancid in warmer weather and will mat the birds' feathers).

If borer damage occurs, cut and destroy infested wood by chipping or burning. Since infested trees may die during the first season of attack, you may need to consider the use of chemicals to protect particularly valuable trees.

Sawfly Borers & Miners

Many woody plants are vulnerable to sawflies, of which there are two main types: gall sawflies and stem sawflies. Gall sawflies are mostly leaf feeders, although some burrow internally in buds, petioles, twigs, or stems, usually producing galls. Susceptible trees include maples and willows. Several natural predators help to keep these pests in check, including several species of birds as well as ants and grasshoppers. Some commercially available parasites have also proven to be effective controls.

Stem sawflies bore into tender shoots of trees and shrubs, occasionally inflicting serious damage in localized infestations; however, injury is seldom widespread. Vulnerable woody plants include blackberry, currant, raspberry, and roses, as well as some oak species, poplar, viburnum, and willows. The most effective control entails pruning and destroying infested growth. 

Other images

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Larva of the flatheaded appletree borer larva (Coleoptera). J. Solomon, USFS, Bugwood.org
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Adult flatheaded appletree borer (Coleoptera). N. Wright, FDACS, Bugwood.org
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Larvae and galleries of the twolined chestnut borer (Coleoptera) on oak (Quercus). J. Solomon, USFS, Bugwood.org
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Dieback due to feeding damage of the twolined chestnut borer (Coleoptera). J. O'Brien, USFS, Bugwood.org
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Mandibles are allthat is showing of this beetle borer (Coleoptera) feeding on a dead white pine (Pinus strobus)
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Abdomen and feeding tunnel of a beetle borer (Coleoptera) in a dead white pine (Pinus strobus)
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Not all borers are pests; many borers are part of the decay process, like this beetle borer (Coleoptera) feeding on a dead white pine (Pinus strobus)
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Not all borers are pests; many borers are part of the decay process, like this beetle borer (Coleoptera) that was found feeding on a dead white pine (Pinus strobus)
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An adult blackhorned juniper borer (Coleoptera), Callidium texanum, on its preferred host, freshly cut eastern red cedar (Juniperus) with the bark still on
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An adult blackhorned juniper borer (Coleoptera), Callidium texanum, on eastern red cedar (Juniperus). Note: the holes from which the adult beetles emerged.
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Emergence hole of the adult blackhorned juniper borer (Coleoptera) from eastern red cedar (Juniperus)
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Cutaway showing tunneling in eastern red cedar (Juniperus) by the larvae of the blackhorned juniper borer (Coleoptera)
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View of the tunnels in eastern red cedar (Juniperus) made by feeding of the larvae of the blackhorned juniper borer (Coleoptera)
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Red milkweed beetle (Coleoptera) on milkweed (Asclepias), a common borer of milkweed roots
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Red milkweed beetle (Coleoptera) on milkweed (Asclepias), a common borer of milkweed roots
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Red-headed ash borer (Coleoptera) from ash (Fraxinus)
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Red-headed ash borer (Coleoptera) from ash (Fraxinus)
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Red-headed ash borer (Coleoptera) from ash (Fraxinus)
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Flatheaded borer adult (Coleoptera) found on an oak tree (Quercus)
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This adult spined oak borer Elaphidion lays its eggs on dead oak trees Quercus); it is obviously a type of longhorn beetle (Coleoptera)
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This adult spined oak borer Elaphidion lays its eggs on dead oak trees Quercus); it is obviously a type of longhorn beetle (Coleoptera)
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Buprestis rufipes (Coleoptera), a beetle borer that feeds primarily on maples (Acer), birches (Betula), sourgum (Nyssa), oak (Quercus) and elm (Ulmus)
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Buprestis rufipes (Coleoptera), a metallic beetle borer; note the reddish colored legs
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Underside of Buprestis rufipes (Coleoptera), a metallic beetle borer; note the reddish colored legs
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Honey locust (Gleditsia), pruned and trained to resemble wisteria, is infested with borers (Coleoptera)
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Borer holes (Coleoptera) in a honey locust (Gleditsia) that has been pruned and trained to resemble wisteria
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The leafminer damage on the leaflets of this trumpet vine (Campsis radicans) was caused by beetle larvae (Coleoptera)
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When a twig girdler larva (Coleoptera) is ready to pupate, it chews through the sapwood of twigs, causing the twigs, with the pupa inside, to fall to the ground; shown here on pin oak (Quercus)
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A twig girdler larva (Coleoptera) girdled this pin oak twig (Quercus) then tunneled into the heartwood, plugging the hole behind it.
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Girdled pin oak twig (Quercus) caused by feeding of a beetle known as a twig girdler (Coleoptera)
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Locust borers (Coleoptera) attack only black locust (Robinia pseudoacacia) and its cultivars. Honey locusts are not attacked.
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Dead top (flagging) caused by borers (probably beetles) in Chinese juniper (Juniperus chinensis ‘Fairview’)
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Borer damage in Chinese juniper (Juniperus chinensis ‘Fairview’)