Is That a Bagworm?

— Written By Bill Hanlin and last updated by JoAnne Gryder

Certain pests seem to show up the same time every year, and it is only a matter of time before we start getting calls on bagworms. These caterpillars can build up into large numbers very quickly. If bagworm populations are left unchecked, they can kill trees like Leyland cypress.



Bagworms are those half to 2-inch caterpillars that carry their home with them while they proceed to consume your tree. Bagworms make their bags out of silk and plant material from the tree they are feeding on. These bags often look like pine cones, only these pine cones move.

Most homeowners don’t want to know about the life cycle of pests like bagworms, but in this case, it is important to know how the insect develops in order to control it. Bagworms overwinter as eggs with the caterpillars emerging in May or June, forming their bag as they feed. They will feed all summer and about August they will attach the bag to trees, fences or just about anything else they can crawl on to . Once they permanently attach themselves, they form a cocoon and become adults a short time later. These cocoons are very difficult to remove and will, most likely, have to be cut off.

Along with Leyland cypress, juniper and spruce seem to be favored hosts for bagworms, but no tree is completely immune. Defoliation by bagworms will, at least, weaken the tree and, at worst, kill the tree. Homeowners have a couple of different options available to them when it comes to protecting their trees from bagworm damage.

Start looking for the tiny bags that will start forming this time of year. Picking them off the tree can be done very easily. The caterpillars inside the bag should be destroyed; otherwise they will crawl back to the tree if you just throw them down on the ground. If they are too numerous or they are out of reach, then insecticides like Sevin and Malathion are effective. Dipel or thuricide can be used for those gardeners who want an organic spray option.

The key to insecticide effectiveness is applying it while the worms are still feeding. Insecticides are also more effective on smaller caterpillars. Insecticides are no longer effective once the bag is closed to form the cocoon; then the only method left is the difficult task of cutting them off the tree.