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Borers and their control

By John Fulton

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[May 12, 2008]  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.

May 20-26 has been selected as Emerald Ash Borer Awareness Week because it coincides with the time of year that the adult beetles will begin to emerge. These borers have gotten a lot of press this past year. They are metallic green, about a half-inch long, and make "D"-shaped exit holes (unlike the ash borers we have had around here for many years, which make "O"-shaped holes).

Most borers are named for their primary target, but they also affect other species of trees and shrubs. As an example, the flat-headed 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.

Painted hickory borer has been a common nuisance pest the past week. These borers attack dead and dying trees and firewood. They are not usually a concern in healthy trees. They look very similar to the lilac borer, but the lilac borer adults are out in the fall.

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; flat-headed 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. For the emerald ash borer, although not confirmed in our area at this time, control time in Michigan begins in mid-May and runs through mid-July.

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 that are 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.

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The product of choice for many borers is now permethrin, since Dursban is off the market. Imidacloprid is fairly new on the market. One trade name is Merit, sold for homeowners as Bayer Advanced Garden Tree and Shrub Insect Care. The use rate for this product is an ounce per inch of circumference of the tree trunk. You then mix it with three gallons of water and pour around the base of the tree. It may take a few months for it to translocate though the tree. A good time to apply it is in early spring when the sap rises. These treatments need to be completed by late May to have a chance of getting the current season borers. Each treatment lasts about a year. Fruit trees generally are treated differently, with Sevin or just using the regular spray program, due to the possibility of residue in fruit.

Zimmerman pine moth is one of "those kinds of borers." It 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.

Bird damage from yellow-bellied sapsuckers on trunks and main limbs also looks like borer damage to many. This bird damage is easily recognized by the evenly spaced holes in a straight line.

[By JOHN FULTON, University of Illinois Extension, Logan County]


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