Question
We have bag worms on spruces  that we just noticed and they are over 1 inch long. we have constant debris falling on the concrete right underneath .How do we treat these at such a late stage and will the trees eventually die?

Answer
Bagworm females cannot fly and local populations can build rapidly when established on preferred hosts, especially arborvitae, cedar and juniper. Crowded larvae may eat the buds on these conifers causing branch dieback and open, dead areas. Excessive defoliation of these conifers may cause entire plant death during the following season. Moderate defoliation is unsightly.

As the larva feeds and grows it enlarges the bag and begins to incorporate bits and pieces of plant material. By mid-August, the larvae are mature and they often move to a sturdy branch or other structure where they attach the bag firmly with a strong band of silk. The larvae orient themselves with their heads down and pupate. The female pupa looks much like the larva while the male forms characteristic wing pads and other adult-like structures. Within four weeks the males emerge and actively fly about in search of females. Mature females call by releasing a sex attractant pheromone. After mating the female literally mummifies around the egg mass ,which remains in place until the next spring.

If the bags are few in number and easily reached they may be picked off the plant and squashed. This can be done easily in the late fall when deciduous foliage has been dropped or the bits of plant material on the bags turn brown and can be easily located on evergreens. Be sure to cut the attachment silk band so that the branch will not be girdled in the future.

Stomach insecticides are very useful for control of bagworms. Remember that the plant foliage is to be thoroughly covered because the larvae are protected from contact by being in the silk bag. Again, early sprays against young larvae are more effective than later applications. Products registered for bagworm control are: acephate (Orthene), bifenthrin (Talstar), carbaryl (Sevin), chlorpyrifos (Dursban),  diazinon, dimethoate (Cygon),  malathion, nicotine sulfate, pyrethrum, and rotenone.