Residents and people traveling through Bethel, VT have been witness to a spectacular display of insect webbing. Entire trees, chain link fences and large patches of ground cover plants are covered with silken webbing – as if someone went overboard with decorations for Halloween. The scene has prompted calls to the Department of Forests, Parks and Recreation.
A field inspection revealed the culprit to be Yponomeuta cagnagella, the Euonymus ( you-on-i-mus) caterpillar. A pest of euonymus trees and bushes of Europe, the Middle East and some parts of Asia, this caterpillar was first discovered in North America in Ontario in 1967. Its primary hosts are the European, spreading, winged, and Japanese euonymus. The winged euonymus is also known as the “burning bush”. In Wisconsin, the euonymus caterpillar has also been reported on common buckthorn; a non-native plant found in many parts of Vermont.
The euonymus caterpillar is a gregarious feeder. Young larvae feed in groups after enveloping a branch in webbing. They can quickly cover and defoliate a tree. Since the defoliation occurs early in the season, trees have time to refoliate. Generally, trees are not killed, though repeated, severe defoliations can lead to mortality or predispose the tree to secondary pests or pathogens.
Caterpillars are creamy green/gray, with black spots and a black head. The adult moths are white with three rows of black spots on the forewings. Euonymus caterpillars have one generation each year. Females lay eggs on twigs, branches and in bud axils from mid to late July. The female covers the eggs with a sticky substance that hardens and protects the eggs. In mid-August, the eggs hatch and larvae may feed briefly, but shortly after hatching, larvae prepare to overwinter by hiding under their egg shell. The following spring the young larvae resume feeding, often starting at the outside of branches and working their way inwards. As they feed and grow, they continue to cover branches in webbing. By late June, the larvae form cocoons. They mature a few weeks later and emerge as adult moths.
Monitoring is critical for control. Small infestations can be controlled by hand pruning to remove affected branches. Large or problematic infestations might be controlled with insecticides. Larvae are more efficiently controlled if applications are made while webs are small and larvae are young. One ingredient that is sometimes used is Bacillus thuringinesis, a bacteria. Be sure that any product used is labeled for control of euonymus caterpillars and remember to follow all label instructions.
Article and photo credit: Jim Esden, Vermont Department of Forests, Parks and Recreation