The gypsy moth caterpillar is the destructive life stage of this insect pest. It can defoliate entire trees and forests if left unchecked. Most trees will recover, pushing a second set of leaves by July. However, defoliation does weaken the tree, leaving it more susceptible to other pests and diseases.
Adult moths lay eggs in August, with each egg mass holding 1,000 or more eggs. In early spring -- generally about mid-May, as tree leaves are expanding -- the eggs will hatch and young caterpillars will climb up into the canopy of the tree and begin feeding. They use silken threads to move from one tree to the next, feeding for approximately 5-6 weeks.
They are reported to feed on more than 600 species of trees and shrubs, with a preference for willow, aspe, birch, crabapple, hawthorn, linden, mountain ash and oak. Trees and shrubs showing some resistance are dogwood, green and white ash, honeylocust, nut trees, silver and red maple and most evergreens, with the exception of blue spruce, tamarack and white pine.
Gypsy moth caterpillars are very easy to distinguish from others. They are very hairy and have five pair of blue dots, followed by six pair of red dots on their backs. Similar caterpillars active at the same time of year are the Eastern tent caterpillar and Spiny Elm caterpillar.
Eastern tent caterpillar is only slightly hairy and will be found in and around the web-like tents they build in trees. Spiny Elm caterpillar has a black body with eight red spots on its back.
To control gypsy moth in the caterpillar stage, wrap bands of duct tape, covered with sticky material such as Tanglefoot, around tree trunks to entangle climbing caterpillars in early-to-mid-May, when they are hatching and climbing into the tree canopy.
In June and July, burlap skirts can be wrapped around tree trunks. Cut a strip of burlap 12-18 inches wide, fold it in half and suspend the skirt with rope or twine tucked through the flap and secured around the trunk about five feet above ground level.
In the afternoon, gypsy moth caterpillars will come down the tree seeking a safe place to hide until evening, when they will climb back up to feed. Lift the burlap flap from mid-afternoon to around 6 p.m. and put caterpillars found underneath in a bucket of soapy water to kill them.
Since their hairs can cause skin irritation, it is recommended you use gloves or a knife, tweezers or similar tool to drop the caterpillars into the bucket. Spraying the caterpillars with insecticidal soap has also proved effective.
In heavy gypsy moth outbreaks, trees can be sprayed with Bt, a biologically-derived insecticide that is harmless to animals, birds, fish, humans and pets.