Every year, around Labor Day, we start receiving calls of damage to many of our evergreen trees. There are numerous small, two inch long brown to gray football shaped bags on the plants, devouring the needles. These are called bagworms. Feeding damage is heavy enough on some plants; spruces, junipers and arborvitae in particular, to have a significant impact on the overall health of the infested plants. Portions of these plants are thinning and/or turning brown and approaching the point of not being able to recover from the damage. In some cases, the bagworms are still very small with their bags being less than 1/2″ long.
The good news is young bagworms can be effectively controlled using a biological insecticide that will not hurt beneficial insects. The efficacy of these biological insecticides declines once bags reach 3/4″, so a standard insecticide will need to be used after bags exceed this length. However, if the bag gets too big and the worm has too much protection from the bag, no insecticide will work. If possible and if the infestation is not too bad, you can simply pick them off and drop them in a bucket of soapy water (picture source: University of Kentucky ENTFACT 440).