June 12, 2020 – During the past week, forestry staff at the Upper Thames River Conservation Authority (UTRCA) have received many inquiries about population explosions of European Gypsy Moth and Euonymus Webworm. Both species of caterpillars can quickly defoliate trees and shrubs. Defoliation by these invasive species in successive years can be very stressful on trees, making them susceptible to other stressors.

“We want to alert people to these non-native threats to trees and shrubs,” says John Enright, UTRCA Forester. “We hope that landowners will take measures to protect the trees and shrubs on their property.”

European Gypsy Moth
A mature Gypsy Moth caterpillar is approximately 6 cm long, and can be identified by the pairs of dots along its back – five pairs of blue dots, followed by six pairs of red dots. The caterpillars feed mainly on deciduous trees and shrubs. Their favourite foods include oak, maple, birch and serviceberry. The young caterpillars will hang from trees on silk threads and be blown long distances by the wind.

Young caterpillars feed throughout the day, but as they mature they come down from the crown of the tree during the heat of the day to seek shade. At this stage, homeowners can trap the caterpillars on the tree trunk. To make a trap, wrap a strip of burlap, approximately 45 cm (17″) wide, around the tree trunk at chest height. Tie a string around the centre of the burlap and fold the upper portion down to form a skirt, with the string acting as a belt. The caterpillars will crawl under the burlap to escape the sun and become trapped. Later in the day, lift the burlap, pick off the caterpillars, and dispose of them.

Euonymus Webworm　
This year there appears to be a localized high population of Euonymus Webworm in the Ingersoll and Thamesford areas. This pest is not considered as serious, as it feeds mainly on small shrubs, in particular non-native euonymus species. While not as serious, the webworm creates mass webbing that completely envelops shrubs, making them look like something out of a horror movie.

The larvae that feed within the webbing are yellow, with a black head and a series of black dots down their back. The larvae mature at about 2.5 cm in length. Hundreds of larvae within the webbing will strip the foliage completely.

Photos of Gypsy Moth caterpillar and Euonymus Webworm  

Contact: John Enright, Forester