A stealthy pest of evergreen trees and shrubs is just now getting noticed by some alarmed homeowners. Parts of their evergreens look dead; and what’s with all those little pine cones hanging on the branches? Take a closer look – you probably have bagworms.
Bagworm infested trees or shrubs have been probably been infested since early June, but unsuspecting homeowners may not have noticed them until now. That is because they were small, and because they use plant parts to disguise the silken bags they travel around in. From a distance, some may say they look like little hanging pine cones, or like Christmas ornaments dangling from a tree limb.
Before we get to control options, we need to understand the life cycle of this pest.
Dr. Timothy Gibb, Purdue Extension entomologist, said, “During July and August, bagworms may defoliate arborvitae, junipers and other trees and shrubs.” He said that bagworms are caterpillars that live inside bags made of silken threads and plant material. “They look like regular plant parts, so they are somewhat disguised from homeowners,” he said. “The caterpillars crawl part way out of the bag to feed, but retreat like a turtle into his shell if disturbed.”
Gibb said bagworms mature in late August or early September. “At this time the bags are about two inches long and the caterpillars can no longer be killed by pesticides,” he said. The caterpillars pupate, then turn into mature adults. The winged males leave their bags, mate with females, then the females lay eggs inside the bag. The females never leave their bag. He said each female bag can produce over 1000 bagworms. The eggs overwinter in the bag, and then next spring young worms hatch. They disburse and begin spinning their own bag, and they start to feed.
Cliff Sadof, Purdue Extension entomologist, added that with such reproductive potential, one would wonder why there are any trees left at all. Fortunately, not all bagworm caterpillars survive. He said, “Many are killed by cold weather, birds, rodents and small parasitic wasps.”
Sadof said the first step to protect young trees and shrubs is to thoroughly examine them in early spring for the presence of overwintering bagworms. “If you find less than 10 bags per plant, immediately pick them off and kill them by dropping them in a bucket of soapy water,” he said. If more than 10 are found, you might want to spray an insecticide.
You may remember that I wrote about this pest earlier this year (about the time that chemical controls could be effective). We are past that time now. Put scouting for the pest on your to-do list for around early- to mid-June next year.
In a conversation I had with Dr. Sadof in recent years, he said, “The question to be asking this time of year is, ‘Are they still feeding?’ If they are not feeding, pesticides will have no effect on this pest.” That is why in late season, hand-picking is the preferred method of control. Pick them off and drop them in a bucket of soapy water. If your trees are heavily infested, this can be a daunting task.
For more information, search for publication E-27-W, “Bagworms,” at Purdue Extension’s Education Store at mdc.itap.purdue.edu. Or, stop by the local Purdue Extension office at your convenience to discuss the pest and receive a free copy of the publication.