As we enter the dog days of summer, you may notice tree branches with silk webs or leaves that have been skeletonized. Generally, there is no cause for alarm. Most trees can tolerate some defoliation at this time of the year, because most of their food-making is winding down. Besides, there will be fewer leaves to rake in the fall, right?
So, who are the culprits behind these conditions? Most likely, they are the adult Japanese beetle, fall webworm or walnut caterpillar — or a combination. All three of these insects feed on leaves, and if populations are heavy, they can defoliate a good portion of a tree canopy. Here’s a look at these three pests:
Japanese beetles. This pest is an invasive exotic insect that feeds on over 300 hosts, including many members of the rose family. Adult beetles commonly feed on linden, crabapple, peach, plum, cherry, birch, grapes, and even raspberries and blackberries — and as farmers well know, soybeans.
Adult beetles are a metallic-green color with a row of white tufts or spots along their sides. They are common from early July into late August, 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, so with light infestations, you need to look up. In heavy years, two-thirds to three-quarters of the tree canopy may be defoliated.
Walnut caterpillars. A second and very common insect is the walnut caterpillar. This insect is a moth as an adult, but the larvae (i.e., caterpillars) are responsible for the 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 and lay eggs, and the young larvae begin feeding by skeletonizing leaves. As they grow, they consume the entire leaf. Young walnut caterpillars are reddish and turn maroon when mature, with thin white stripes and long white hairs. There is usually one generation per year.
Fall webworm. The fall webworm makes a web engulfing the foliage. In contrast to the eastern tent caterpillar, which appears in early summer, the fall webworm is present in mid- to late summer. In southern regions, there may be two generations per year, while in the north there is only one generation.
Webworm larvae feed on hundreds of hosts but prefer hickory, walnut, birch, cherry, crabapple and persimmon. The larvae are pale yellow or straw-colored with white hairs. When mature, they are about an inch in length.
For all three of these insects, control is usually not warranted unless there is history of heavy feeding damage. For Japanese beetles, removal of the adults can be effective; otherwise, insecticide treatments may be warranted.
Walnut caterpillars and webworm larvae can be removed from the tree or by pruning out the webs. Bt can also be applied for both walnut caterpillars and webworms, 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, but 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 be protected.
Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at email@example.com.