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.

Damage

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.

Prevention

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.

Non-Chemical Control

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.

Chemical Control

Stressed, unhealthy trees may be repeatedly attacked and need repeated insecticide treatments. This is often expensive and not environmentally friendly. The first priority is to improve overall tree health, and use insecticides as a last resort. In addition, most chemicals can only be obtained and applied by licensed professionals with specialized equipment.

Insecticide products registered for borer control are listed in Table 1 . Most of these products are applied as sprays to the trunks and branches, and are contact, residual insecticides (e.g., carbaryl, chlorpyrifos, lindane, permethrin). While 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 3 to 10 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.

Systemic insecticides are often ineffective for borer control and few are labeled for this purpose. Systemics may be applied as foliar sprays, root drenches, or trunk injections. Trunk injections work by delivering pressurized and concentrated insecticides into the tree ( Figure 9 ). However, these injections are most effective against sap-feeding insects and rarely affect woodborer larvae. The process of injecting the insecticide through a narrow tube and into the drilled hole may result in sap staining, and could allow pathogens an entry point. Translocation of the insecticide is unlikely in partially girdled areas. The use of these products has not been studied in Florida.