Managing Stem Boring Insects on Ornamentals

Adult ash lilac borer

The term "borer" strikes fear in the heart of the average person, bringing to mind the image of a larva secretly chewing away under the bark of his or her favorite tree. While there are hundreds of species of wood boring insects, fortunately, most cause little damage to ornamentals. Among the most troublesome for landscape plants are the roundheaded, the flatheaded borers, which are larvae of longhorned (Cerambycid) and shorthorn (Buprestid) beetles, respectively. Larvae of "clear wing moths" such as ash lilac borer and dogwood borer are also troublesome. With the phasing out of some old standby insecticides like Dursban (chlorpyriphos), Lindane and Cygon (Dimethoate) for homeowner use, effective borer management may require a little more thought about the natures of these little beasts.

Damage to Missouri's state tree caused by dogwood borer

Perhaps the most important thing to remember about borers is that they generally cause little damage to vigorously growing trees. Plants that are suffering stress from drought or transplanting are prime candidates for borer infestation. Newly planted maples are very commonly attacked by flatheaded appletree borers which may tunnel under the bark for most of the transplanting season. Often the damage only becomes apparent the following season as bark peels away from the trunk in a zig-zag pattern. Newly planted or stressed ash trees are frequently attacked by the larvae of ash lilac borer. A heavy infestation may cause dieback of branches and basal sprouting.

When considering borer control using an insecticide, it is important to note that most borers have a distinct weak point in their life cycles when they are most vulnerable to insecticide. Adults emerge from the trunk in spring or summer, depending on the species. They then mate and, soon after, females deposit eggs in bark crevices on the trunk and branches. The insect is most vulnerable when the eggs hatch and the tiny larvae begin to enter the trunk. Once the larvae have penetrated the bark, chemical control is difficult or, in some cases, impossible. For this reason, effective chemical control of borers usually requires that insecticide be present on or in the bark when the eggs hatch.

Chemical control of borers has become somewhat more difficult with the loss of Dursban and Lindane for homeowner use. The reason why these materials are so effective is that they persist when sprayed on the bark for several weeks. Two or three trunk sprays with Dursban during spring and early summer generally work well for borer control. While there are a number of products labeled for "borers" on trees in home landscapes, it is unclear just how effective these materials will be on specific insects like ash lilac, flatheaded and roundheaded appletree borers. One product that lists both flatheaded and roundheaded borers on its label is Bayer Advanced Tree and Shrub Insect Control. The active ingredient in this product is imidacloprid, a systemic, soil applied insecticide, which is taken up by tree roots. Although imidacloprid is long acting once inside the plant, it must be applied well in advance of the expected infestation to be effective. In tests in the Chicago area, imidacloprid was found to be effective in preventing infestation of maples with the Asian Longhorn Beetle, a longhorned beetle. It was not, however, effective in eliminating existing infestations. For control of lilac ash borer and other clearwing moths, there are several products on the market with permethrin as the active ingredient. In a recent nursery test in Missouri, trunk sprays with a permethrin product provided good control of ash lilac borer when timed correctly. Adult emergence of this species is usually from late April through May. Other products with the word "borer" somewhere on the label that may be available in retail outlets may contain the active ingredients acephate, carbaryl cyfluthrin, esfenvalerate or spinosad. When choosing a product for borer control, always consult the label to make certain that both the host plant and the insect are listed on the label. Unfortunately, labels on pesticide containers in retail stores are often sealed to the container making it impossible to see the entire label. However, company websites often display complete labels for their pesticide products. Any product sold in a retail outlet listing borers and ornamentals should be legal to use if label directions are followed carefully. However, given that there are hundreds of species of stem boring insects, there is no single product available to homeowners that will control all borers.

Cultural controls for borers should be standard practice in landscape plantings. Water newly planted trees carefully to keep the soil ball uniformly moist (not saturated) during the first two seasons. Avoid mechanical damage to the trunks of new trees and wrap over winter to prevent trunk scald. Fertilize moderately and keep broadleaf herbicides away from tree roots. Over the next several years, it is likely that our understanding of the efficacy of some of the insecticides mentioned above for control of different borer species will increase.

Chris Starbuck, Woody Ornamental Horticulture, UMC (573) 882-9630