Now is the time of year to take appropriate measures to minimize problems with wood-boring insects in landscapes and nurseries. The two main groups of wood-boring insects are beetles (order: Coleoptera) and moths (order: Lepidoptera). The wood-boring beetles active at this time of year include bronze birch borer (Agrilus anxius), flat-headed appletree borer (Chrysobothris femorata), and round-headed apple-tree borer (Saperda candida). Moth borers active at this time include lilac/ash borer (Podosesia syringae), peachtree borer (Synanthedon exitiosa), and viburnum borer (Synanthedon fatifera).
Many of these wood-boring insects feed on a variety of plant types. Bronze birch borer attacks European white birch, gray birch, paper birch, and yellow birch. The flat-headed appletree borer primarily attacks plants in the rose family (Rosaceae), including crabapple, cotoneaster, hawthorn, pyracantha, and rose. Lilac/ash borer attacks ash, lilac, and privet. In general, adult females lay eggs on exposed bark; they hatch into larvae that tunnel through the cambium. Larvae feed within the sapwood or heartwood, whereas adults feed on leaves or flower nectar.
The key to managing wood-boring insects is to keep plants healthy and avoid stress by using proper cultural practices, including watering, fertilizing, mulching, and pruning. Avoid lawn-mower or weed-trimmer injury to the base of trees and shrubs as this removes essential cambium tissue that is responsible for transporting food upwards to leaves. This injury places undue stress on plants. Many wood-boring insects are opportunistic and thrive on stressed plants; healthy plants are less susceptible. Pruning trees and shrubs at certain times of the year may increase problems with certain wood-boring insects. For example, it is generally suggested to avoid pruning birch trees, especially white birch, from May through August because bronze birch borer female adults are flying around looking for places to lay eggs. Pruning then creates wounds that emit odors that attract females.
Newly planted trees and shrubs are highly susceptible to borer attack. For example, the flat-headed appletree borer attacks recently planted shrubs and trees because they are stressed, thus increasing their susceptibility. It is important to properly water plants, provide adequate drainage, and mulch young plants to minimize stress. Avoid placing a thick mulch layer (over 6 inches deep) against the crown because it suffocates the plant; this is especially important with finely decomposed mulches. Place a 3-to-4-inch layer of mulch around trees and shrubs, and leave a 2-to-3-inch gap between it and the base of the plant. Finally, avoid overfertilizing as this may cause plants to divert resources from production of defensive compounds and increase susceptibility to wood-boring insects.
Insecticides may be used to minimize problems with wood-boring insects. The loss of landscape use of chlorpyrifos (Dursban), dimethoate (Cygon), and lindane due to the Food Quality Protection Act has created concerns on availability of effective insecticides. Chlorpyrifos is no longer available to homeowners, as it cannot be used in the urban landscape; it is still available to commercial operators for use in nurseries and golf courses. Permethrin (Astro) is the product of choice for managing moth borers. Imidacloprid (Merit, Imicide, and Pointer) has some activity on beetle borers. Research has shown that imidacloprid is effective in controlling bronze birch borer.
The residual activity of insecticides applied to plants often depends on bark characteristics, with activity generally lower on smooth bark (such as birch bark) compared to ridged or furrowed bark. Insecticide binds more easily to rough bark, with less potential for wash-off from rain or irrigation. It is important to thoroughly soak the bark up to 5 feet from the tree base as adult borers tend to lay eggs there.
The timing of insecticide applications to the bark is critical. Apply before eggs hatch or when adults emerge because most insecticides do not penetrate bark after insect entry. After larvae are inside the tree, very little can be done except to maintain plant health.