Our adult daughter lives on the 23rd floor of a high-rise in Miami so she welcomes a chance to garden when she visits our acreage. The outdoor projects we had planned came to a screeching halt when I happened to notice brown patches on our 20-foot arborvitae. Oh no! A huge infestation of bagworms! And they were large ones, past the spraying stage (see below). We spent the entire morning picking them off into buckets of soapy water. Drat. I have put a note on my June 2019 calendar: CHECK FOR BAGWORMS!
During June, July and August, bagworms chomp away on their favorite treats, pine, spruce, arborvitae and junipers. If left unchecked, they can easily kill a bush in a few weeks. If evergreens are not handy, the bagworms are not picky; they’ll also munch on 128 plant species, including deciduous trees and shrubs, but since those leaves re-grow, damage is usually not so serious.
Bagworms are caterpillars that live inside spindle-shaped bags which they construct to protect themselves. These bags, composed of silken threads and bits of foliage, look so much like a part of the tree that you may not notice them until they’ve done extensive damage. You have to get up close and personal to see the li’l fellers.
Early in June, the insects hatch from eggs which over-wintered in the old bags and start to spin their own bags. Each female can produce up to1,000 babies! Bagworms mature in late August or early September. At this time, the bags are about 2 inches long and pesticides are no longer effective. The winged male, a small, furry black moth with clear wings, fertilizes the wingless, maggot-like, yellowish-white female, who never leaves the bag. She lays eggs in the bag where they pass the winter. There is only one generation each year.
When you find bags in the tree, pick them off and drop in a bucket of soapy water. This method is only effective if you catch them before eggs hatch out of the bags in June. If the thousands of baby caterpillars have already flown the coop, you need to proceed to Plan B.
Plan B -- Spraying the bags with insecticides. For the most environmentally-friendly attack, use a biorational pesticide, such as Bacillus thuringiensis, sold as Biotrol, Dipel and others. The biorational materials will only kill the caterpillars, not beneficial insects such as bees, praying mantis and butterflies. Biorational pesticides are most effective when directed against small bags.
Two weeks after application, check again for live bagworms (they wiggle as they chow down) to determine if you need to spray again. As bags approach their full size, pesticides become ineffective. Hand-picking is the best control measure at this point.
Here are some helpful websites, or you can just type in bagworms in the search box, which calls up 377,031 articles in .66 seconds.:
If you have other questions about your garden or landscape, feel free to contact a master gardener at the University of Illinois Extension office in Charleston at 217-345-7034. You can also check out the many horticulture webpages at the U of I Extension’s website by visiting http://web.extension.illinois.edu/ccdms/ . And be sure to like the Master Gardeners’ Facebook page, at www.facebook.com/ColesCountyMasterGardeners.