Q: I noticed lots of thinning to some of my arborvitaes, and upon closer inspection, it looks like they have bagworms. What can I do to stop the damage?
A: By this time of year (late-June or later), the only viable way to manage bagworms is to hand pick them, as insecticides will not be effective.
Bags may be pulled or trimmed from affected plants and dropped into a bucket of soapy water. Continue to monitor for bags and remove those you find in this manner throughout the remainder of the growing season and into the fall. Because bagworms use plant materials from the plants they inhabit to create their bags, they are usually well camouflaged and are easy to miss.
Additionally, bagworm eggs overwinter inside bags created during the growing season, so thorough removal before winter can help insure you don’t have a big problem next year. If your arborvitaes are tall and you aren’t able to hand remove all of the bags, start monitoring your plants for newly hatched bagworms in early May next year. They are tiny (only about one eighth of an inch long) when they first emerge, so look at plants closely.
While bagworms typically hatch in late May or early June in the greater St. Louis area, which generally coincides with when catalpa (Catalpa speciosa) trees are in full bloom, timing will vary based on the weather (this year, bagworms began hatching about two weeks earlier than usual).
Be sure to monitor other landscape plants as well, as bagworms will feed on a variety of plants in addition to evergreens. If you find newly hatched bagworms next year, they can be successfully managed with the use of low-risk insecticides like those containing Bacillus thuringiensis (Bt) or Spinosad according to product instructions.
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