Gardeners, prepare

Battling borers         

By John Fulton

[MAY 15, 2006]  Few things strike fear in the hearts of tree owners like the mention of borers. Borers are most often the larvae of beetles or moths, and they do their damage where you can't see it. The eggs are laid on or under the bark, and the small hatchling chews through to the part of the tree they feed on. They spend almost their entire lives inside the tree, where they can't be seen or controlled.

Peachtree borer

Most borers are named for their primary target, but they also affect other species of trees and shrubs. As an example, the flatheaded apple borer can also affect pin oaks and about 15 other species. Many borers leave a telltale hole in the trunk, but some, such as the ash borer, are weak and have to enter through a pruning or mechanical injury.

Here is a listing of common borers and their control times: ash borers, early June and early July; bronze birch borer, mid-May and repeat two times at two-week intervals; dogwood borer, mid-May and mid-June; flatheaded apple borer, late May and repeat in three weeks; lilac borer, early June and early July; locust borer, late August and mid-September; mountain ash borer, early June and mid-July; peach tree borer, mid-June and mid-July; viburnum borer, early June and early July; and Zimmerman pine moth, April or August.

Flatheaded apple borer

You can see borer control isn't an exact science. Treatment times and reapplications are hopefully timed to catch the hatching eggs on the outside of the trunk or discourage the adults from laying eggs. Some new research shows that trees under stress give off a certain pheromone that attracts more borers to the tree. So, keeping your trees in good growing shape will go a long way to helping the situation. Fertilizing with the same rate of broadcast fertilizer used for lawns will help (apply it early May and early September) and watering during extreme dry periods as well, with an inch of broadcast water per week.

Trees actually get their food and moisture from small roots located in the top 8 inches or so of soil. That big taproot is actually an anchor and doesn't take up much in the way of food or nutrients. The root mass generally goes out about 1.5 times as far as the branches do. The shallow roots are part of the reason it is hard to grow grass under trees. Sure the shade has something to do with it, but the tree roots are more efficient at getting water and nutrients.

Back to the borers. The product of choice for many borers is now permethrin, since Dursban is off the market. Fruit trees generally are treated differently, with Sevin or just using the regular spray program, due to the possibility of residue in fruit. Bronze birch borers can be controlled with imidacloprid. Treatment of the trunk and main limbs is the goal of borer treatments.

Zimmerman pine moth generally affects only severely weakened trees and goes just under the bark to girdle the cambium layer. It seems like older Scotch, red and Austrian pines are favorites when they begin to decline.

Good luck in your borer control attempts. If you miss the timing, you can try to run a stiff wire in the hole. At least it may make you feel better than doing nothing at all.

[John Fulton, unit leader, University of Illinois Extension, Logan County Unit]