Flatheaded Borers

Description of flatheaded borers

Flatheaded borers are beetle larvae that tunnel just under the bark of tree trunks, branches and roots.  They excavate shallow, winding tunnels through the tree phloem and outer sapwood.  Evidence of flatheaded borer infestation is a series of sawdust-filled galleries on the inside of loose bark pulled from injured, dying or dead trees.  The most common flatheaded borer is the flatheaded appletree borer.  Flatheaded borers are up to 1 inch in length, white, deeply segmented and legless.  There is a prominent flattened enlargement of the otherwise-slender body just behind the head.

Life cycle of flatheaded borers

Larvae live inside the tree for 1 year under most circumstances but the life cycle may take longer under certain conditions.  The adults are called metallic wood-boring beetles, referring to the attractive, metallic or iridescent color of many common species.  The adult beetles emerge from infested trees by chewing out through the bark.  They typically begin in May and continue to emerge for a period of several weeks.  Adults are attracted to weakened, injured, dead or dying trees and stumps.  Females lay their eggs on the bark, in bark crevices or at the edges of wounds.  Eggs hatch in about a week and each tiny borer chews into the tree to feed on living tissue in and surrounding the cambium layer.  Larvae remain inside the tree through the winter then pupate the following spring before emerging as adults to repeat the cycle 

Damage caused by flatheaded borers

Typical flatheaded borer damage
Winding tunnels right under the bark is typical  damage of flatheaded borer larvae.

Trees infested with flatheaded borers look unthrifty because of scant foliage, dead branches, and dead areas of the main trunk or larger branches where loose bark can be easily pulled off.  Unfortunately, by the time infestation symptoms are obvious the tree has been severely weakened and recovery is unlikely.

Management of flatheaded borers

The best management for flatheaded borers is prevention.  Avoid borers by maintaining trees in vigorous condition.  Actions that promote tree health include site and species selection, proper planting, mulching, watering as needed, fertilizer to correct nutrient deficiencies and active defense against injury (lawn mowers, trimmers, root disruption, etc.).  Eliminate sources of beetles by pruning and removing dead and dying limbs and trees.  Tree wraps are of little or no benefit in preventing borers.  Use wraps with care; periodically check under wraps for injury.

Once borers have infested a tree they are difficult to control.  External sprays are only effective if applied to the tree when the adult beetles are active and laying eggs.  Because emergence and egg laying can occur over a long period of time, monthly or more frequent insecticide applications may be needed over the course of a summer.  Homeowner use of insecticides is not practical.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

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Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.