Question: I have a 21-year-old arborvitae tree that has what looks like little pine cones all over the shrub. There are black worms inside of each “pod.” I never had them before and I don’t like them! How do I get rid of these “bag worms” and get my shrub back to its healthy self?
Answer: Your arborvitae is suffering from an infestation of bagworms. These pests are common on certain plant species from the late summer through the fall. In addition to arborvitae, gardeners can also find these pests on cedars, junipers, pines, cypress and spruce.
Bagworms are actually fascinating creatures. Though the word “worm” is in their common name, they aren’t worms at all, but rather they’re caterpillars. As adults, male bagworms are dark-colored, night-flying moths that are not generally noticed by humans at all. But, unlike nearly all other moth species, the females don’t ever turn into a flying moth. Instead, the females of the species remain a caterpillar their entire lives. They spend their whole life inside the little “pods” or “bags” hanging on your arborvitae.
The caterpillars build the protective bags around themselves, using silk and plant debris to construct them. The bags are very distinctive and are most often noticed from mid-July through autumn. The caterpillars housed inside the bags come out and feed on the plant’s foliage during the night-time hours, returning to the bag before sunrise. Severe infestations can cause significant defoliation on affected plants, so you should spend time removing as many of the bags as possible.
This is especially important this time of year because by the end of the season, each bag could contain up to 1,000 eggs. These eggs overwinter and the hatching young go on to attack more plants the following spring.
To remove the bags, use a small pair of scissors to cut the small band of silk that attaches the bag to the branch. Do not pull the bags from the plant as it will lead the band of silk intact. This could cause branch death and browning several years after the bag was removed.
Though there are several natural enemies that attack and eat bagworms, including spiders, parasitic wasps and some birds, their bags afford them a high level of protection. Still, plant plenty of flowering annuals and herbs around your arborvitae to naturally keep the population of these pests at a lower level.
A perfectly timed application of the organic pesticide Bt (Bacillus thuringiensis) sprayed during the last week of June is effective in controlling the young caterpillars before they have a chance to build their bag shelters. However, the timing is very critical, so be sure to mark your calendar for next June so you don’t forget. If you wait just a week or two too long, the Bt will be ineffective.
I recommend you hand-remove as many bags as you can as soon as possible, tossing them into a sealed container and then into the garbage. Then, in late June apply the Bt for continued control.
Horticulturist Jessica Walliser co-hosts “The Organic Gardeners” at 7 a.m. Sundays on KDKA Radio with Doug Oster. She is the author of several gardening books, including “Attracting Beneficial Bugs to Your Garden: A Natural Approach to Pest Control” and “Good Bug, Bad Bug.” Her website is jessicawalliser.com. Send your gardening or landscaping questions to [email protected] or The Good Earth, 622 Cabin Hill Drive, Greensburg, PA 15601.