There has been considerable interest and worry in the southern half of Iowa this spring as homeowners and property managers start thinking ahead to whether the bagworm caterpillars will defoliate their spruce, cedar or arborvitae trees again this summer as badly as they did last. It's a logical concern, but please; wait a little while longer.
Bagworm insecticides such as Bacillus thuringiensis, spinosad, Sevin, permethrin or bifenthrin will be ineffective and a waste of time if they are applied too early (just like they were a complete waste of effort when they were applied too late last summer!). Insecticide control becomes less effective as the season progresses and the larvae increase in size. So we want to spray just at the time the eggs are hatching and the small larvae appear.
According to various references, the bagworm eggs that spent the winter inside the silk pods hanging on the trees from last year's infestation should start to hatch in late May or early to mid-June. That's quite a range in time that gives us plenty of room for error (too early or too late).
One method for finding the best time to treat is to frequently and thoroughly inspect trees that were infested last year. This will take time and diligence as the newly emerged caterpillars and their bags are quite small. Also, bags are made of silk and bits of plant foliage. Newly-constructed bags with fresh plant bits are effectively camouflaged in among the needles and are hard to find.
A second method for predicting when new caterpillars will be on the trees is to use phenology. Phenology is the study of recurring biological events, especially natural plant and animal rhythms that occur earlier or later in the year in response to seasonal and climatic changes in the environment.
Phenology has been closely studied by many keen observers over the years. One outcome of their records is the emergence of patterns of coincidence in the landscape. Researchers watching bagworm eggs hatch and the caterpillars appear on infested trees noticed that the catalpa trees and Japanese tree lilacs in the area happened to be in bloom at that same time. These showy, common landscape plants became known as phenological indicator plants and by watching the indicator plants we can accurately predict specific insect activities. Note that the indicator plants are not infested with the pest in question. They are merely an indicator of what might be happening elsewhere in the landscape. For more on phenology and its use for timing insect control actions, see University of Kentucky Extension pamphlet ENT-66.
Bottom line: don't spray too early. Wait for catalpa trees and Japanese tree lilacs to bloom in your area, and then go inspect conifer trees for the presence of caterpillars and tiny bags. If they are present, then is the time to spray.