Now is the time of year to take the appropriate measures to minimize problems with wood-boring insects in nurseries and landscapes. The two main groups of wood-boring insects are beetles (order: Coleoptera) and moths (order: Lepidoptera). The wood-boring insects active this time of year include bronze birch borer (Agrilus anxius), flat-headed appletree borer (Chrysobothris femorata), lilac/ash borer (Podosesia syringae), round-headed appletree borer (Saperda candida), peachtree borer (Synanthedon exitiosa), and viburnum crown borer (Syn anthedon 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 pri-marily 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. The eggs 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 any type of stress by implementing proper cultural practices including watering, fertility, mulching, and pruning. In addition, avoid lawn mower or weed-whacker injury to the base of trees and shrubs because 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 or shrubs at certain times of the year may increase problems with certain wood-boring insects. For example, it is generally suggested that you avoid pruning birch trees, especially white birch, from May to August because bronze birch borer adults are flying around looking for places to lay eggs. Pruning during this time creates wounds that emit odors, which attract females.
Newly planted trees or shrubs are highly susceptible to borer attack. For example, the flat-headed appletree borer attacks recently planted trees or shrubs because they are stressed, thus increasing their sus-ceptibility. It is important to properly water plants (and provide adequate drainage) and mulch young plants to minimize stress. Avoid placing a thick mulch layer (over 6 inches deep) against the plant crown because this will suffocate 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 the mulch and the base of the plant. Finally, avoid overfertilizing plants as this may cause plants to divert resources away from the production of defensive compounds and increase their susceptibility to wood boring insects.
Pest-control materials may be used to minimize problems with wood boring insects. The phasing out of chlorpyrifos (Dursban) due to the Food Quality Protection Act (FQPA) is causing some concern about the availability of effective pest-control materials for managing wood-boring insects. Chlorpyrifos is no longer available to homeowners; however, it is still available to commercial operators. At present, the alternative pest-control materials that have demonstrated some level of efficacy against various beetle and moth wood-boring insects include dimethoate (Cygon), lindane, and permethrin (Astro).
Dimethoate and lindane will most likely not be available in the next 2 to 3 years; both products are undergoing re-registration by the Environmental Protection Agency (EPA). Permethrin will probably be the product of choice for managing moth borers. Recent studies show that imidacloprid (Merit, Imicide, and Pointer) may have some activity on wood-boring beetles, but more data is needed.
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. The insec-ticide binds more easily to rough bark, and there is 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 because adult borers tend to lay eggs there.
The timing of insecticide applications to the bark is critical. Make applications before eggs hatch or when adults emerge because most insecticides do not pene-trate bark after insect entry. After larvae are inside the tree, very little can be done except to maintain plant health.