Bagworms
Bagworm caterpillars (Thyridopteryx ephemeraeformis), are pests that devour the foliage of several species of fruit, shade and ornamental 
trees, as well as evergreen shrubs. Some common hosts include red cedar, arborvitae, juniper, white pine, crabapple, and honeylocust. These pests can cause complete defoliation, which may result in a severely weakened or dead host.
In June, newly emerged larvae begin to form a bag (made of silk and small pieces of host foliage) around their bodies. The bag is enlarged as the caterpillar feeds and grows. By late August or early September, the caterpillar has matured and it anchors its bag to a host twig. The caterpillar now transforms into the pupal stage. The adult male bagworm is a small moth which emerges from the bag in September and mates with the female bagworms (still in their bags). After mating, the female deposits 800-1000 eggs and then dies. The eggs stay protected within the bag through the winter, then emerge in June, repeating the cycle.
By the time you see the bags in the fall, it is too late to chemically control the infestation that year. The best option is to hand-pick or cut the bags from the host. This process can be done anytime you see a bag - from Winter through June (when the larvae emerge). To dispose of the bags and kill the eggs, you can either burn the bags or soak them in a bucket of soapy water.
To chemically control bagworms, you MUST spray as soon as the larvae begin to emerge, which usually occurs in June. One product that can be used is called BT (be sure to read the label for application instructions). Stop in to Iowa Outdoor Products to pick up a bottle today, and start fighting bagworms before they damage your trees and shrubs!
For more information on bagworms, please visit the following websites:
*http://www.extension.iastate.edu/news/2009/feb/060602.htm
*http://www.extension.iastate.edu/newsrel/2002/sep02/sep0207.html
*http://woodypests.cas.psu.edu/FactSheets/InsectFactSheets/html/