Spongy moth caterpillars a serious threat to oaks, other trees

There will be some very confused moths this summer in some parts of Cook, DuPage, Kane, Kendall and other Illinois counties. And that’s good news for trees.

The insects are spongy moths (Lymantria dispar dispar, formerly known as gypsy moths), whose caterpillars are a serious threat to oaks and other trees because they are “feeding machines,” according to Stephanie Adams, a plant pathologist in plant health care at The Morton Arboretum in Lisle. To combat their spread, the Illinois Department of Agriculture is using small planes to apply a treatment called SPLAT that keeps the adult moths from reproducing by disrupting their mating.

The treatment uses sex pheromones to lure male moths away from the females. It is specifically targeted to spongy moths, so it doesn’t affect other species, and it is biodegradable and nontoxic to humans and other animals. The Arboretum is one of the sites where SPLAT will be applied.

Spongy moth caterpillars kill trees by removing too many of the leaves the trees need to live. “The caterpillars eat at just an incredible rate,” Adams said. “A large caterpillar can eat a square foot of foliage a day.” Some 300 species of trees are on the menu, including oak, maple, apple, crabapple, hickory, birch, pine, spruce and willow. The moths’ populations tend to rise and fall in cycles, but when populations surge in an area, they can do widespread damage.

A tree may not die right away, according to Sharon Yiesla, plant knowledge specialist in the Arboretum’s Plant Clinic. “But if a tree loses most of its leaves two or three times, it’s not going to be able to recover,” she said. “And if an evergreen tree loses more than 50 percent of its needles, it can’t come back.”

The moths have been spreading steadily through North America since the 1860s, when they were imported as part of a failed effort to begin a silkworm industry. Their voracious caterpillars have caused widespread damage to forests, parks and trees in streets and yards.

The caterpillars, the larval stage of the moth’s life cycle, start eating at the top of a tree’s leafy canopy when they are small. “We may not even know they’re there,” Yiesla said. If you notice that your tree is getting thin on top, use binoculars to look for the caterpillars. They are dark colored and covered with spikey hairs, with five pairs of blue bumps and six pairs of red bumps along their bodies and are about 2½ inches long at their largest stage.

If you think you have spongy moth caterpillars in a tree, Adams suggested tying a strip of fabric around the trunk as a trap. “The caterpillars come down out of the canopy at night,” she said. “And they will go underneath the fabric band. In the morning, you can squish them.”

Over the winter and into early spring, you may be able to see egg masses closer to the ground. They are tan or light brown and 1 to 2 inches across, with a spongy, fuzzy texture. The egg masses may be on tree bark or any flat surface, such as lawn furniture, sheds or sculptures. “You can remove them and drop them into a bucket of soapy water to kill the eggs,” Yiesla said.

Spongy moths are spread geographically when they lay eggs on movable surfaces such as vehicles, firewood or camping equipment that people carry to new sites. Vacationers can help slow the spread by examining their cargo carefully for the tawny-colored egg masses before leaving home, and by buying and burning firewood near the campsite. Bringing wood from home can spread eggs of spongy moths or other pests.

Consult with an arborist about a heavy infestation of spongy moth caterpillars. There are insecticides that may help if the right chemical is used, if it is applied by a professional at the right time in the insect’s life cycle and if the tree is not already too badly stressed or damaged. Spraying a large, mature tree is difficult and likely to be expensive.

To address adult moths, which emerge in June and July, pheromone traps are available that lure in male moths. However, Adams pointed out that the traps may not reduce your tree’s infestation because they attract moths from far beyond your yard. “You may end up with more moths,” she said.

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.

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