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Don't be alarmed by summer defoliators

What pests are chewing on your trees this summer?

August 14, 2016

4 Min Read

Editor’s note: From Fredric Miller’s monthly natural resources column, Tree Talk.

Take a look at your trees. Have you noticed branches with silk webs? Or maybe skeletonized leaves?

Generally, there is no cause for alarm. Most trees can tolerate some defoliation and by this time in the late-summer, most of their food making is winding down. Besides, there will be fewer leaves to rake in the fall.

So, who are the culprits? Most likely, they are the adult Japanese beetle, fall webworm, or maybe the walnut caterpillar. All three of these insects feed on the leaves and if populations are heavy, they can defoliate a good portion of the tree canopy.

Nemesis: Japanese beetles

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The Japanese beetle is an invasive exotic insect that arrived in the U.S. in the early 1900s from Japan and has spread across the U.S. It feeds on more than 300 hosts including many members of the rose family. Adult beetles commonly feed on linden, crabapple, peach, plum, cherry, birch and even raspberries and blackberries. The adult beetle is a metallic green color. They are common from early July into early September, feeding on leaf tissue between the leaf veins resulting in a skeletonized leaf. For reasons unknown, Japanese beetles like to start feeding in the top of the tree and work their way down. That means with light infestations, you need to look up. In heavy years, 2/3 to ¾ of the tree canopy may be defoliated.

A second and very common insect is the walnut caterpillar. This insect is a moth as an adult, but the larvae are responsible for feeding damage. Preferred hosts include black walnut and pecan, but they may also feed on birch, oak, willow, honey locust, and apple. Adult moths emerge in June and July, lay eggs and the young larvae begin feeding by skeletonizing the leaves. As they grow, they consume the entire leaf. There is usually one generation per year.

Next up: fall webworm

The fall webworm makes a web incorporating the foliage that it feeds on. In contrast to the eastern tent caterpillar, which appears in early summer, the fall webworm is present mid- to late-summer and extends its web as it feeds. In southern regions, there may be two generations per year while in the north there is only one generation. Like Japanese beetles, the fall webworm feeds on hundreds of hosts, but prefers hickory, walnut, birch, cherry, crabapple and persimmon. The larvae look much different from the walnut caterpillar, being pale yellow or straw color with white hairs. When mature, they are about an inch in length, compared to 2 inches for the walnut caterpillar. Fall webworms build webs on branch tips – another dead giveaway.

For all three of these insects, control is usually not warranted unless there is history of heavy feeding damage. For Japanese beetles, hand removal of the adults can be effective; otherwise insecticide treatments may be warranted.

Manage walnut caterpillars and fall webworm by removing larvae from the plant and/or pruning out the webs. Don’t burn the web while in the tree because the web is made of silk and may catch the tree on fire. There are parasitoids and predators that help keep fall webworm and walnut caterpillar check. Bacillus thuringiensis can also be applied for both walnut caterpillar and fall webworm, but thorough coverage of the foliage is essential as the larvae must consume the bacteria for it to work. Other insecticides can also be effective. Most trees can tolerate low to moderate levels of defoliation in a given year; however, successive years of defoliation can lead to plant stress and provide opportunity for pathogens and more lethal insects to invade the plant. Newly planted trees or trees already under stress should probably be protected.

When using pesticides, always follow the label directions (more is not better), be aware of honeybees, and wear all recommended personal protective equipment. Follow up with good plant health care practices including watering, fertilizing and mulching.

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