Archive for Sunday, May 6, 2012

Garden Calendar: Borers gaining attention in stressed trees

May 6, 2012


Borers have probably lived in native trees and shrubs in our area for as long as the trees and shrubs have grown here, but awareness of them seems to be increasing.

The cottonwood borer is one of the largest and most recognizable borers in Kansas. This one was collected last summer in Douglas County.

The cottonwood borer is one of the largest and most recognizable borers in Kansas. This one was collected last summer in Douglas County.

A borer exit hole, possibly from a Red oak borer, is seen on a northern red oak in northwest Lawrence.

A borer exit hole, possibly from a Red oak borer, is seen on a northern red oak in northwest Lawrence.

Rather than a testament to the insects’ personality, borers are so named for their ability to bore into and through wood. Typically an adult beetle or moth lays eggs in bark crevices or wounded areas on the tree (weed-eater wounds are favored by many). The larvae that hatch from those eggs burrow into the wood and feed in either the cambium or the wood itself. Borer tunnels in the cambium layer affect a tree’s ability to move water and nutrients to the leaves. Borer damage in the wood weakens the interior of the tree and increases the risk for tree failure.

Why are people paying attention to them now? Consider the last few years’ weather conditions — the ongoing drought and extreme high and low temperatures in 2011 have caused stress for trees and other plants in our area. Stressed trees give off a chemical signal that attracts certain insects, including many of those classified as borers.

Stressed trees attract borers that cause damage and stress the tree more. Seeing the borers’ exit holes in the trunk alerts us to their presence, though, and it is sometimes easier to pin the blame on them over Mother Nature.

Taking steps to ensure tree health will do more for the tree than treating the secondary problem. Water trees deeply and infrequently over extended dry periods. Avoid damaging roots or the trunk of the tree. Remove dead and damaged branches from trees as they occur. Select tree species and varieties that are adapted to the area and climate. Purchase and transplant only healthy trees into the landscape.

Common borers in our area include cottonwood borer, redheaded ash borer, bronze birch borer, flatheaded appletree borer, locust borer, ash/privet borer, ash/lilac borer, elm borers, and less discriminate carpenterworm moths, leopard moths and shothole borers. The red oak borer is also present in this area, although this is the first year I have observed oaks with what I believe are red oak borer exit holes.

Despite the lengthy list of borer insect pests, many of the tree species that commonly have borer problems live a substantial life. Cottonwood trees, for example, live about 70 years, according to most forestry resources. Ash may be less lucky, especially when planted in a tough urban environment, but may still outlast other less-adapted species.

Occasionally borers are substantial pests. The emerald ash borer is an exotic pest that was first found in Michigan and has now spread into several neighboring states. The emerald ash borer is devastating northern forests because our native ash species cannot withstand the stress of the insects’ feeding. The ash borers we currently have in Kansas are much different than the emerald ash borer.

If you choose to use insecticides to manage borer infestations, identification of the borer species is very important for timing and product selection. Actual insects are better for identification purposes than just looking at exit holes, so try to find the larvae. Adults are easier to identify but harder to catch. The Extension Master Gardener garden hotline (843-7058) can help identify borer species and management options.

Insecticides cannot take away the damage that is done, but they can help prevent additional damage if applied properly.

There are two options: treating the bark to reduce or eliminate egg-laying, or using a soil injection or drench that the tree will uptake into its vascular system. Systemic treatments still have to applied at the right time, require adequate soil moisture to thoroughly move through the tree, and are only effective on certain tree and borer species.

More information about borers is available through the K-State Research and Extension factsheets “Borers: Common Kansas Species” and “Borers: Management and Prevention.” Factsheets are available through K-State Research and Extension — Douglas County or online through

—­ Jennifer Smith is the Horticulture Extension Agent for K-State Research and Extension in Douglas County. Contact her or an Extension Master Gardener with your gardening questions at 843-7058.


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