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Past Articles Library | Organic Pest Control | Control Flathead Borer



Flatheaded borers have more than 150 species and varieties, all of which are very destructive to a wide and diverse set of tree species, both softwoods and hardwoods. The greatest damage results from the larvae boring into the sapwood.



Flatheaded borers are beautifully marked, metallic-colored beetles, varying greatly in size but usually somewhat flattened and boat-shaped. The wood- boring larvae are characterized by a hammer-headed shape produced by a flattened enlargement of the body region behind the head. They are whitish-yellow and legless.



Adult females lay eggs singly or in groups on the bark or in crevices in the bark or wood. The young borers (larvae) mine the inner bark or wood. The tunnels of the larvae are flattened, usually oval in cross section, and winding, gradually enlarging as the larva increases in size. These larval mines are always tightly packed with fine sawdust arranged in arc-like layers. The adult beetles on emerging through the bark or the wood leave a characteristic oval or elliptical exit hole. The life cycle may be completed in part of one season or extend to two years. Some forms complete their development in the summer, transform to adults, and do not emerge until the following spring.




A wide variety of softwood and hardwood trees are susceptible to flathead borer including, but not limited to: pine trees, sycamore, soft maple, boxelder, walnut, white and black oaks, yellow poplar, elm, beech, chestnut, hickory, hackberry, mountain-ash, serviceberry, hawthorn, redbud, basswood, buckeye, persimmon, apple, pear, peach, cherry, and willow.




Young, thin-barked trees are most susceptible to attack as well as newly planted or moved trees, trees newly exposed to full sunlight, trees highly stressed by changes in the environment. Typical flatheaded borer damage shows medium sized 1/4 to 1/2 inch (.625 to 1.25 cm), oval holes in the wood or bark; flattened tunnels in the sapwood of softwoods and hardwoods; powdery, pale-colored sawdust in tunnels. White infected liquids can be seen oozing from localized areas on the bark surface over newly damaged locations. Bark eventually darkens, appears wet, and shiny. Little exterior frass or wood dust is evident except in bark cracks. Hot, sunny portions of the main stem and branches are favored for attack. Overall decline of the tree and entire dead branches and sections of the tree will be evident. Larvae prefer fresh (moist) wood rather than seasoned wood, they do not feed in the heartwood.



Borers attack trees which have been recently transplanted, environmentally stressed, or have sustained bark damage. Localized damage from heating and drying of bark (sun-scald or soil heat reflectance) are opportune sites for attack. Drought-stressed trees are especially vulnerable.


1. Keep trees healthy, well fed and watered. Trees that have healthy vigor don't attract borers. Borers prefer stressed trees.

2. To minimize borer attacks, use tree wraps, tree shade screens, white trunk paint, or any type of tree growth shelter for newly planted trees.

3. Pruning or other types of bark injuries should be avoided when the adults are present and active.

4. Sanitation of fallen and standing deadwood, and removal of any pruned materials is critical.

5. Clearwing borer pheromone lures can be used to determine the specific time adults are flying in late spring and early summer. Preventative insecticides, such as Kaolin Clay, should be sprayed on the trunks one week after the first moths are detected, and again one month after the first application, if many moths continue to be detected.

6. An alkaline mixture of insecticidal soap plus caustic potash (lye) mixed to the consistency of thick paint is also recommended. This should be applied every 2 to 4 weeks, depending on rainfall, from April through August to deter egg-laying. Caution: Proper protective clothing and eye wear should be worn when working with lye.


1. There are several natural parasites and predators that keep this insect under control. Usually it is more effective to manage the tree's health rather than control pest populations, especially where wooded sites and forests are abundant.

2. Once the larvae have tunneled into the wood, insecticide sprays will not be effective.

3. Insecticidal control for preventing further attacks is possible and should be applied over the bark early enough in the year to prevent successful adult egg-laying and new larvae penetration and to kill newly emerging larvae. A good organic spray is Kaolin Clay, marketed under the trademark 'Surround'. Surround is sprayed on as a liquid, which evaporates, leaving a protective powdery film on the surfaces of leaves, stems, and fruit.

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