From Dennis Patton:
It is one of the most easily identified insects found in the area; the bagworm. Bagworms will be making their annual appearance and start munching their way through many landscape plants. They form a silken bag mixed with plant parts up to 2 inches in length.
Bagworms will feed on many different plant materials, but their favorite tends to be evergreens such as junipers, spruce and arborvitae. Because of their wide host range, give all landscape plants a quick glance to prevent damage. The young worm-like insects eat the foliage resulting in an initial browning of the area followed by death of the plant under severe feeding. Understanding the bagworms life cycle is important for control.
Bagworms spend most of their lives attached to a branch or stem eating, never leaving the comfort of the spin bag. Only the males leave the bags to mate with the female, then they die and the female lays eggs for next year’s hatch.
Bagworms over-winter in the egg stage in bags attached to the plant. The eggs hatch in late May through mid-June. The hatchlings are very small at first. They start out about the size of a sharpened pencil lead. They grow quickly, spinning a larger bag until mid to late summer. The bag is thick, about 2 to 3 inches in length. At that point the bagworms mate, eggs are laid and the whole process is ready to start over for next year.
Bagworm populations build up to damaging levels very quickly as each bag of eggs can produce more than one thousand hungry little worms. Control of bagworms is best done just after they hatch in the late spring or very early summer. Just about any insecticide will kill the worms while they are small and the silken bag not highly formed. The larger the bag becomes the less effective the control. By late summer chemical applications are worthless. At this stage handpicking and destruction of the bags is recommended. This is a slow task that most people would prefer not to do.
There are a number of insecticides that are effective on bagworms at this time of year. Products to apply include spinosad, acephate, cyfluthrin or permethrin. An organic product called Bacillus thuringiensis is also effective.
Now is a good time to give your evergreen plants a close inspection for bagworms. I just love the calls when people are attempting to identity this pest and they say something to the effect, “they just appeared overnight.” Even though they seem to appear almost overnight, there are clues to their arrival. Keep in mind they can do a lot of damage in a short time so be on the lookout.