An interesting email last week from a curious reader prompted me to review a topic I last wrote about in 2014: bagworms, insects we all despair of finding on our trees and shrubs. The reader mentioned that a black walnut (Juglans nigra) tree suffered from bagworm infestation for about 15 years until several years ago, when he eradicated the caterpillars by tying a piece of nylon mesh soaked in malathion to the trunk. The worms died and he hasn’t had a problem since.

Bagworm infestation is a serious problem that primarily affects evergreens like juniper, arborvitae, spruce, Leland cypress, and pine, causing extensive defoliation and, in some cases, tree death.

Deciduous trees can be attacked by bagworms, but far less often than evergreens, and the common name “bagworm” frequently is used to describe the Eastern tent caterpillar or the fall webworm. This is understandable. No one is going to say, “I have Thyrodopteryx ephaemeraeformis (bagworm) on my cedar tree, not Malacosoma ame...