Spray to bag young pests
If you have had bagworms on your evergreen shrubs in the past, it’s time to start monitoring them for the newly hatched caterpillars.
I would suspect that you might find the tiny caterpillars feeding on the needles and foliage of arborvitae, spruce, junipers and other evergreens. You also might find them feeding on deciduous plants that are nearby the evergreens, as this is quite common.
You have to really look for them at this stage. Check the needles closely and look for tiny brown patches along the needles or for movement. When they are first hatched, they don’t consume the entire needle, just tiny little fragments of the needle. If you watch close enough, it looks like the needles are moving. You will see the tiny green caterpillar with the black head capsule.
The reason I am alerting you to their hatch and young stage in life is you can use a caterpillar spray that is effective on young ones and is safe to use.
The pesticide I am referring to is a naturally occurring biological insecticide called Bacillus thuringiensis var. kurstaki (Btk). This is found in products such as Dipel, Thuricide and others.
The catch is this product only works on very young and tiny caterpillars or those less than a half inch in size.
The other good thing about Btk is that it won’t kill the good guys such as the predators and parasitoids.
Egg hatch also occurs over an extended period of time. Since Btk is a stomach poison (the caterpillar has to ingest it as it eats the foliage), it’s recommended you continue to monitor shrubs and trees, and repeat sprays if necessary.
Bagworms, if left unchecked, can kill an evergreen. Continual feeding over several years will result in a plant that is completely stripped of its foliage. A high population of bagworms can either destroy an evergreen or leave it looking pretty ugly. I have seen spruces with complete sections of the plant defoliated by bagworm.
In most cases, homeowners don’t recognize bagworms on their plants until they begin to form the brown bags that look like little pine cones. This is usually in August.
In August, the caterpillars are big enough that they “tie” their bags (with silk strings) to a branch and hunker down for the rest of the season. Once those bags are formed, they are protected from pesticides.
When you see the brown bags attached to branches, the best control is to hand pick them or wait until next spring and catch them at hatch.