Adult female bagworms are wingless moths. A female lays 500 to 1,000 eggs in the bag she creates around herself from leaves of the plant she was born on. They hatch in May or June. The caterpillars will eat lots of leaves, and as they grow, they will enlarge their bag. They haul the bag around with them like a sleeping bag, and whenever they are disturbed, they duck back into the bag for protection.
The caterpillars keep eating and growing until August or September. The wingless females stay in their bags, and the males develop wings and fly around. After mating, the males die, and the females fill their sacks with eggs and then die. Since the females don’t move very far, the infestations spread slowly from plant to plant. Caterpillars move from plant to plant by crawling and by sticking to animals that move them to a nearby plant. The tiny caterpillars can also release a strand of webbing that allows them to float in the wind to other plants.
The bags prevent the caterpillars, females, and eggs from being killed by most insecticides. The best, safest, and most pesticide-free time to kill off an entire population of bagworms is from fall through winter until early spring, while the eggs are still in the bag. The 1- to 2-inch-long bags remain hanging on the tree. Many of them may be empty bags from previous years or empty male bags. Pull them all off and stomp on them, burn them, or put them in a bucket filled with soapy water.
If nothing is done until the caterpillars hatch and are crawling on the tree, then the best and safest pesticide product to use is one that includes the bacteria Bacillus thuringiensis. This will kill young caterpillars, but as they mature and create large protective bags during the summer, handpicking again becomes the best way to kill them. A systemic insecticide applied to the tree early in the season will kill the bagworms as they eat the leaves.
For the first year or two, the bagworm population may not be high enough to cause enough damage to be noticed. But at some point, there will be enough of them on the tree to devour all of the leaves. If there are not too many this year, be sure to pull off all the bags so that there won’t be any next year.
