Many homeowners mistake bagworms for “pinecones” on their evergreens. A closer inspection will reveal an insect on their plants. This insect is easy to recognize by the case or bag that the caterpillar forms from the plants on which it is feeding. The caterpillar partially emerges from the bag to feed. When it finishes all the foliage on one branch, it moves to the next, dragging its bag along. They can defoliate and kill the plant they are feeding on.
In the fall, the male moth emerges and flies to a bag containing a female, mates and dies. The female will lay 500 -1000 eggs in her bag before she crawls out of her bag, drops to the ground and dies. The eggs will spend the winter in her bag and in late May through early June will begin hatching and the cycle begins again.
Bagworms are a constant pest of arborvitaes, junipers, spruce, pine and other evergreens (their favorite foods). It can also infest honeylocust and sycamores or any host plant of opportunity. That means it may have started on your arborvitaes but moved to your lilacs right beside them. Lilacs are not their first choice for dinner, but since it’s so close, why not?
A number of natural enemies help to keep bagworm populations in check and bagworms can be controlled by handpicking (yes, you can just pluck them right off the plant - you may wear gloves if you like). This can be done in late fall, winter or spring before the eggs begin to hatch. Handpicking will greatly reduce the number of eggs in the spring. After picking, dispose of them, don’t just drop them onto the ground or they may reenter your plants.
When infestations are heavy an insecticide may be necessary. Bt (Bacillus thuringiensis) is effective and the least toxic. Sevin and systemic controls such as acephate are also effective. Use chemical controls from late May to early July while the larvae are small. To avoid damaging valuable plants, be sure to apply the material to plants specified on the label and always follow the label directions.