As a young child, I remember my grandmother telling me it was time to gather the eggs. Obviously, she was talking about chicken eggs. But as it turns out, winter is a good time to remove insect eggs before they hatch in spring. And gathering those eggs now will help reduce defoliation of your trees.
Four common leaf-feeding forest insects that overwinter as eggs include the gypsy moth, forest tent caterpillar, eastern tent caterpillar and bagworm.
1. Gypsy moth. They lay egg masses in mid- to late summer, which hatch the following spring. The larvae prefer oaks but will also feed on a wide variety of other forest tree species. Since the female moth cannot fly, she must crawl and lay egg masses near where she “grew up” during the summer. The eggs masses can be anywhere on the trunk or main branches. They will be about the size of a quarter to half-dollar and have a tan to light brown appearance. There may be hundreds of eggs in one mass.
2. Forest tent caterpillar. Don’t confuse this one with the fall webworm. Forest tent caterpillers do not form a web, but will spin silken mats on tree trunks or branches where they congregate when not feeding. They prefer aspen trees if available but will also feed on alder, basswood, birch, cherry, cottonwood, elm, oak, poplar and willow. Outbreaks occur periodically in some of our forests. They lay eggs in late summer and then overwinter. Look for tent caterpillar egg masses in the upper portions of the tree. The mass will be dark in color, have a shiny appearance and encircle the twig, resembling a sewing thimble.
3. Eastern tent caterpillar. This is also a common annual tree defoliator, but unlike the forest tent caterpillar, it does form “tents” or webs in main branch crotches. The egg masses are very similar in appearance and size to the forest tent caterpillar, and prefer peach, plum, cherry, crabapple and apple trees.
4. Bagworms. Their eggs are not found in a mass but overwinter in a silken bag. During the course of feeding in the summer, the female bagworm constructs a bag of silk and foliage from the host plant, lays her eggs when mature, and then dies. Bagworms particularly like evergreens like arborvitae, bald cypress, pines and spruces, but will also feed on apple, crabapple, linden, maple, honey locust and other deciduous trees. Bagworms are easy to see during the winter months. We crazy entomologists call them Christmas ornaments.
All four of these insects are serious defoliators of trees, and bagworms can be very destructive of evergreens, as they do not refoliate like deciduous trees. By physically removing the eggs mass, you can help reduce population levels for the following growing season, mitigate tree stress and reduce the need for chemical sprays.
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. The opinions of this writer are not necessarily those of Farm Progress/Informa.