While checking on your evergreen trees and shrubs this time of year, you may notice small pinecone like objects dangling from the branches. These are actually insect pests known as bagworms, which are caterpillars that live inside these bags which they construct to protect themselves against birds and other enemies. These bags, composed of silken threads and bits of foliage, look so much like a part of the tree that they may go unnoticed until extensive damage has occurred.
Damage from bagworms can be serious, as it begins around the first of June, and continues through much of the summer. Early in June, the insects hatch from eggs which wintered in the old bags attached to tree branches. As soon as the young worms appear, they start to spin bags and continue to enlarge these as they feed and grow. The caterpillars crawl part way out of the bags to feed. If disturbed, they retreat safely inside, and it is almost impossible to pull them out. Each female bag can produce over 1,000 bagworms. During July and August, bagworms may defoliate arborvitae, junipers and other trees and shrubs. While they seem to thrive on the evergreens, they can also defoliate walnut, fruit trees and crabapple trees.
Bagworms mature in late August or early September. At this time the bags are about 2 inches long and can no longer be killed by pesticides. The worms then attach the bags firmly to branches or other objects and change into the adult stage. The wingless female never leaves the bag and is fertilized by the winged male. The eggs are laid in the bag where they pass the winter.
Bagworms tend to be a problem on trees that are isolated or in urban settings. When bags are found in the tree, simply pick the bagworms off and drown them in a bucket of soapy water. This method is most effective before eggs hatch out of the bags in June.
Bagworms can be controlled by spraying the foliage with insecticides after eggs have hatched and small bags are seen on the trees. Caterpillars must consume the foliage for the insecticide to kill them. The earlier in the season the critters are detected, the more foliage you will save. There are two groups insecticides for use on bagworms. Biorational products are considered more environmentally friendly, and include products like azadirachtin, bacillus thuringiensis (BT), chlorantraniliprol, indoxacarb, and spinosad. More traditional insecticides for bagworms include acephate, aceamprid, beta cyflurin, bifenthrin, carbaryl, cyfluthrin, deltamethrin, dinoflurin, lamba-cyhalothrin, malathion and permethrin. Like the biorational products, these insecticides need to be in place as early as possible be the most effective.