Bagworms are caterpillars. They begin life in late spring as a tiny worm emerging from last year’s bags. They will use tiny strands of silk to fly to nearby landscapes on a gust of wind. One of the first things they do while feeding is to roll up a piece of leaf with a strand of silk and stick it onto their back. This quick camouflage works better than you would think as it is hard to pick out the little chunk of moving leaf from the rest of the plant.
As the bagworm gets bigger it eats more and more foliage every day. Large groups of bagworms can strip part of a tree of leaves. They feed on almost any ornamental tree or shrub including crabapple, arborvitae, and spruce. As they feed they grow larger and modify their bag so their whole body fits inside, only the head and front legs stick out the bag opening while moving or eating.
Late in the season they go through the normal change from caterpillar to pupa to adult moth. But, this is where things get a bit strange. Only the male moth completes the process and emerges as a fully developed moth.
The female moths do not emerge as a moth, but rather remain in the bag as a partially developed female that is sexually mature. The males find the female bags and mate with the females still in their bag.
The females then produce a large amount of eggs and die in their bag full of eggs.
This is the time of year that they become very apparent in the landscape. The bags are as large as they will get and as the remaining leaves drop from trees the bags really stick out. Large numbers of bags look like brown Christmas ornaments hanging from the trees—especially if they are on spruce trees.
Large numbers of bags mean that there will be even more caterpillars and more damage next year. To control small populations of these caterpillars bags are easily pulled from branches and crushed.
You can dissect the bag if you are curious. An empty bag was probably from a male. A female bag will have the dried mummified body of the female full of, and surrounded by, eggs.
The bags are tougher than they look and it usually takes scissors or a sharp knife to cut open the woven silk and leaf bag.
If you have a large number of bags or bags you can’t reach all is not lost. Since these are caterpillars any pesticide that kills a caterpillar will kill a bagworm. The trick is to apply it at the right time.
Do not do anything but pull bags in the fall and winter. In the spring, look for the tiny caterpillars every week in late spring. When you see them walking around with the tiny leaf tied to their back then that is the time to treat.