Thyridopteryx ephemeraeformis Identification Bagworms form brown spindle-shaped bags one to two inches long made of plant material and silk. These bags are obvious during the wintertime on their evergreen and deciduous hosts. In summer, similar but smaller bags made of fresh plant material may be seen moving over the foliage of the infested plant materials. Newly hatched caterpillars may be easily overlooked. The caterpillars feed on foliage near the old bag and construct a new bag with silken threads and bits of foliage taken from the host plant.
Bags differ in appearance because of the plant material on which the bagworms are feeding.
The bags made out of maple leaves look very different from those made out of arborvitae. The caterpillars add plant material to the bag as they feed. Accordingly, the bags grow as the caterpillars grow. Young bags tend to point upwards whereas older bags hang downward. Since the caterpillars are the mobile stage of bagworms, populations may be concentrated on one plant or adjacent plants.