If you have recently planted young hardwood trees in your landscape, you may want to protect them from appletree borers. Flatheaded appletree borers are serious pests of newly planted landscape and fruit trees. Maple and apple trees are especially favored, but these beetles damage many other deciduous trees. Damage is caused by the larvae, which tunnel beneath the bark, creating winding, frass-filled galleries and resulting in sunken lesions that scar and weaken trunks, retard tree growth and can even kill trees.
Fully-grown larvae are about one inch long and have a creamy-white, segmented body. The segments just behind the head are wider than the others, giving the grub a “flatheaded” or cobra-like appearance. These are big larvae that produce large feeding galleries, which means even a single larva can cause significant damage to a tree that is only a few inches in diameter. Trees are most susceptible during the first few years after planting, especially if injured or stressed. Healthy, well-established trees are much less likely to be damaged.
Adults are metallic gray, peg-shaped beetles, approximately 1/2 inch long. There is only one generation per year, with adults beginning to emerge in mid-spring and continuing to be active into late summer. Although these are day-flying insects, they are rarely seen. Female beetles can sometimes be seen crawling about on the sunny sides of tree trunks as they search for egg-laying sites. Egg-laying is focused on the sunny sides of trunks because these areas are most likely to suffer sunscald, a.k.a. southwest injury, which can occur during winter, but eggs are also laid around other points of injury, such as pruning scars, splits in bark or other points of physical damage.
Control: Cultural practices that prevent sunscald and bark damage and encourage vigor make trees much less susceptible and help trees grow out of the susceptible stage faster. This can include things like proper planting depth, proper mulching, and proper use of trunk wraps or trunk shading during the first winter or two. Keep trees adequately watered and use mulch, but not too deep, and not too high around the trunk.
Soil-applied systemic insecticides, such as imidacloprid (e.g. Merit 2F or BioAdvanced Tree & Shrub Insect Control) or dinotefuran (e.g. Zylam), can provide effective preventive control, but such treatments must be applied in early spring, shortly after budbreak, or in the previous fall, to have time to move into the tree before eggs hatch. Read labels carefully; treatment rates are given as amount of product per inch of trunk diameter or circumference. These systemic treatments work by killing young larvae after they have bored through the bark and have begun to feed. Trunk sprays of insecticides such as bifenthrin (e.g. Onyx Pro) or permethrin (e.g. Hy-Yield 38 Plus) are also effective but must be applied shortly before egg-laying begins. These treatments work by killing eggs and newly hatched larvae before they bore into the tree.
See pages 32-33 of MSU Extension Publication 2369, Insect Pests of Ornamental Plants in the Home Landscape, for more information.
Also see Bug's Eye View Vol. 6, No. 3, for information on granulate ambrosia beetle, another beetle that threatens newly planted trees.
Blake Layton, Extension Entomology Specialist, Mississippi State University Extension Service.
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