It is, however, a problem for evergreens, especially since evergreens don’t regenerate new needles. These caterpillars can strip an evergreen branch, leaving it permanently bare.
I have never really seen a bagworm infestation kill a large tree, but I have seen it totally defoliate a small evergreen shrub making it really difficult for the plant to survive.
On deciduous plants, since they lose their leaves, bagworms aren’t generally a big problem. The caution is to make sure they don’t move on to nearby preferred evergreens.
Most gardeners don’t recognize bagworms until later in the season, usually around September. This is about the time when their “bags” turn brown and are noticeable.
However, if you find them now, they are easily controlled by Bacillus thuringiensis (Bt), a very safe pesticide that eliminates the small caterpillars.
Once caterpillars are bigger, Bt is not effective, and you have to switch to another pesticide.
Bagworms feed on foliage, and in the early stages, use pieces of the foliage to make a protective house or cocoon-like structure. This protective house really does a good job of keeping them safe.
The silk used to build this cocoon structure is really strong. If you have ever picked one off of a plant and tried to pull it apart, you know what I mean.
These cocoons are what you see in September. Right now, they are about one-eighth to one-quarter of an inch long. They have really tiny pieces of foliage covering them. This covering resembles an upside-down dunce-cap protruding from the foliage.
As I mentioned, they are really small, and you have to look for them. Start looking on those plants you might have had bagworms feeding in the past.
If you don’t want to use pesticides, you can hand-pick and destroy the bags. This is usually a little easier to do when they are a bit larger.
I don’t normally worry about using pesticides on trees and shrubs unless the pest has a potential to really damage a plant.
In this case, I recommend using pesticides to eliminate as many of these bagworms as possible, especially on your evergreens.
Stripping of the foliage is stressful to a plant and should be avoided. Look for newly-hatched bagworms now.