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Archive for Thursday, September 21, 2006

Twig girdler’ beetles likely culprit for littered yard

September 21, 2006

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Visiting the garden on these cool mornings, I see branches littering the ground, usually under my favorite tree. Picking them up, I see the end of each limb is manicured like the end of a good pool stick, sans chalk, with a matchstick-like break in the center. So maybe, all that noise from the cicadas was a cover-up for squirrels with miniature chain saws or portable routers. The cuts letting the limb drop are just too circular and uniform to be anything else. Well, not so. The squirrels did not suddenly get that sophisticated, and there is no defoliation conspiracy at work.

In the fall of the year, a grayish brown beetle (oncideres cingulata) is most likely the culprit. These "twig girdlers" are distributed through the eastern United States from New England to Florida and as far west as Kansas and Arizona. The beetle is 3/4- to 1-inch long, stout and cylindrical with long antennae. They are a long-horned beetle, and although the damage they do is noticeable, it is usually not detrimental to the tree.

Completing their development in late summer, the twig girdler emerges from beneath the bark of last year's branch. They mate, and in August the female moves from the ground up to a live branch to deposit her eggs. She uses her mandibles to cut a small slit, usually at the base of a leaf bud, and then deposits eggs beneath the bark, one egg per slit. The slit is then sealed with a secreted substance, giving the area a glossy appearance. Three to six eggs are set in each cut branch. The beetle now moves down the limb, toward the trunk, and with her mandibles begin to girdle the branch. She apparently does this because the sap impedes the development of the larvae. With the branch's vascular system destroyed, it will die and fall to the ground - hence our littered yards. The larvae develop in the fallen branches. This development is in the spring and early summer, and then they start again.

They pick on elms in particular but seem to like hickory, oak, linden, hack berry, apple, pecan, persimmon, poplar, sour gum, honey locust, dogwood and some flowering fruit trees. They do little harm to any tree, as the clipped ends in the tree will continue to grow with new buds next spring. Insecticide application is both impractical and inefficient. The only control is to break the life cycle of this insect with the removal and disposal of the fallen branches. Home composting may not be effective. Often natural mortality is high, with dry conditions or too many larvae per branch.

Conspiracy theory aside, squirrels do cause damage. A squirrel cut branch will be tattered and cut at an angle on the severed end. It will be an obvious difference. The squirrels may be sharpening or cleaning their teeth as they do not feed on the branches.

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