Why Are There Tiny Holes in Bark of Lace Leaf Maple?

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Lace leaf maple (Acer palmatum var. dissectum), also known as threadleaf maple for its elegant, deeply divided leaves, is a star in the yard, showing off its delicate form in spring and summer, and its bright gold foliage in winter. Lace leaf maples grow in U.S. Department of Agriculture plant hardiness zones 5 through 9, depending on variety. When small holes appear in a lace leaf maple's bark, boring insects are the likely culprits.

Boring Insect Damage

  • Flatheaded wood borers prefer to infest maples weakened or stressed by drought, injury, disease or insects. More than 100 species of flatheaded wood borers exist, but the usual suspects on maple trees are the flatheaded apple tree borer (Chrysobothris femorata) and the Pacific flatheaded borer (Chrysobothris mali.) The latter species is only found in the western part of the country.

    Adult beetles have coppery wing covers over dull bodies and are between 1/4 and 1/2-inch long. By the time the adults emerge, usually from spring to autumn, it is too late to mitigate the damage done by larvae. Between May and August, adult females lay yellowish eggs in crevices in the bark of unhealthy trees, or in areas of the tree that is bruised or injured. The moment the larvae hatch, they begin burrowing into the bark.

Damage

  • Larvae of both borer species do more damage than adults, which feed on foliage. The larvae chew their way into the area between the bark and inner wood of the maple tree, creating galleries, which are tunnels that can kill a tree if they completely girdle the trunk, branches or limbs. Frass, which is excrement mixed with sawdust, may pile up in the entrances to the holes, or drift into little piles on the ground at the base of the tree.

Treatment

  • Eliminating larvae inside the bark of the maple tree is virtually impossible, since they are protected. Thrusting a thin metal wire into each hole and jiggling it can kill larvae close enough to reach. Insecticides are best against the adult beetles and may help reduce the number of eggs. You can also try searching for the yellow, wrinkled egg sacs in the bark and scrape those out to kill the larvae before they hatch. Woodpeckers and parasitic wasps may help keep the borer population low without insecticides.

    Ready-to-use permethrin-based sprays may be applied when beetles are most active on the maple tree, according to the University of Minnesota's IPM of Midwest Landscapes. A general schedule for spraying is to make the initial spray during the third week of May, second spray during the second week of June and the last spray during the first week of July.

Prevention

  • Before planting lace leaf maples, you should make sure the plant isn't already infested with boring insects. Look for larvae, bits of sawdust or tiny holes in the bark. Keeping the maple as healthy as possible with proper irrigation and avoiding injuring the tree with garden equipment can also help avoid infestations. If the tree suffers breakage from wind, snow or ice, or is injured by fire, use protective tree paint to coat the injured areas.

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