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Don’t let insects hit your trees early

Xurzon/Getty Images sun shining through green forest
Tree Talk: Guess what’s back in the spring? Insects! Here’s what to watch for and how to protect your trees.

Now that spring is in full swing, you will want to keep an eye out for a few leaf-feeding caterpillars and beetles. By themselves, there are not lethal to woody plants, but repeated early annual defoliations interfere with plants’ ability to make food. And that’s what they use for growth, reproduction, flower and fruit production, protection against insect pests and pathogens, and maintenance.

Some of the more common leaf-feeding caterpillars include eastern tent and forest tent caterpillars, spring and fall cankerworms, gypsy moth, viburnum leaf beetle and European pine sawflies. All of these insects appear in spring and feed for a few weeks; and when populations are high, they are capable of completely defoliating trees and shrubs. Let’s take a look.

Eastern tent caterpillar. Larvae hatch from overwintering egg masses about the time their host plants are leafing out. Good egg hatch indicator plants include silver maple, with leaf blades 1 to 2 inches long, and blooming Norway maples. Young caterpillars will appear black; but as they mature, they become hairy with a white stripe down the back, brown and yellow lines along the sides, as well as a row of oval blue spots on the sides. They grow to be 2 to 2½ inches long. Initially, they’ll stay close to the old egg mass, but then they’ll begin to disperse — forming silken tents in the branch crotches of host trees including peach, plum, cherry, apple, and other common forest and landscape trees.

Forest tent caterpillar. This kind feeds on a much wider variety of hosts including sugar maple, oaks, cottonwood, elm, birch, cherry, basswood and ash. They’re distinguished from their eastern counterparts by their brownish body with pale bluish lines along the sides, and a row of white spots. They are about 2 inches long when full grown. Forest tent caterpillars don’t form a tent, but instead form a silken mat on the trunk or branch. Both caterpillars consume the entire leaf. Feeding lasts about five to six weeks, at which time they leave the host to pupate. There is one generation per year.

Gypsy moth. This moth arrived in the 1860s from Europe. It has now spread to the Midwest and favors oak. Eggs hatch in spring, just as oak leaves are unfolding. Good indicator plants for GM egg hatch are when redbud is beginning to bloom, plum is blooming, and during the bloom period for Spiraea. Gypsy moth caterpillar are hairy and black, and when older, will have six pairs of red dots and four pairs of blue dots on their back. They do not form a web or tent, but will consume the entire leaf.

Inchworm. Technically known as spring and fall cankerworm, both prefer elm, hackberry and apple trees.

European pine sawfly. This insect differs because it prefers pines, and it typically breaks out in Christmas tree plantations, windbreaks and privacy plantings. It’s not a fly, but a nonstinging, wasp-like insect. Young larvae feed on the margins of last year’s needles and eventually consume the entire needle. Defoliation by EPS is not fatal to conifers, but repeated defoliation can weaken individual plants.

Viburnum leaf beetle. This exotic pest from Europe feeds exclusively on viburnum shrubs. Larvae appear in spring and feed on the underside of the leaf, resulting in a windowpane effect. The larvae pupate, and then adults emerge and skeletonize the leaf by consuming the leaf tissue between the main veins. There is one generation per year.

If warranted, you can apply insecticides to prevent feeding damage. Try to use the least toxic chemicals with the shortest residual to help conserve natural enemies. Keep your plants healthy by providing good growing conditions so they can tolerate occasional defoliation. Consult your local Extension office for insecticide recommendations.

Miller is a horticulture professor at Joliet Junior College and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at

The opinions of this writer are not necessarily those of Farm Progress/Informa.



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