How to kill bagworms
Bagworms (also known as case moths) are the nemesis of evergreens and can also attack deciduous trees. The bagworm encases itself in a protective coating made from the leaves of the tree and will lay its eggs inside a distinctive sack. This makes bagworms difficult to kill. In order to save your trees from the bagworm threat, you will need to understand enough about the bagworm lifecycle to time your attack.
Make a preemptive strike in the winter.
During the winter, bagworm eggs remain dormant inside their bags, which look a bit like pine cones and are approximately 1-2 inches long. Spraying the bags with insecticide will not kill the bagworms, however this is your chance to do significant damage to the bagworm population before the bagworms begin damaging your trees in the spring. Each bag contains literally hundreds of bagworm eggs, so every bag you pick off and throw away will help your trees siginificantly.
Using gardening shears, snip off as many of the bags as possible, collecting them in a trash can or bucket as you work. Do not simply throw the bags on the ground, as the eggs will still be alive. You may need a stepladder to reach the higher branches. Make sure you have someone to spot you as you are climbing on the ladder.
When you do not see any more bags hanging on the tree, dispose of the bags by sealing them in a plastic bag and placing them with the rest of your garbage. It is important that they be taken away and incinerated or buried in a landfill.
Meet them head-on in the spring.
In the spring, any remaining bagworm larvae will begin to hatch. Once they have hatched, they will begin their attack, eating the leaves on your trees and using them to construct their own protective bags. The bagworm larvae can even use silk lines to travel to other trees in search of food. The tiny larvae are not easy to see in their early stages, which is why a bagworm infestation usually goes unnoticed until late summer. By then, it is too late to undo the damage. In order to prevent irreversible damage to the trees, be proactive and spray the larvae in May or June.
Some good insecticides for bagworms include Seven and Pyrethrin. Be sure to follow the manufacturer’s instructions when applying insecticide and take all proper safety precautions.
Engage in biological warfare in the summer.
Bagworms will continue to grow and feed throughout the summer. Even if you have rid your trees of bagworms, new bagworms can spread to your trees from outside sources. The insecticide BTK consists of live bacteria called bacillus thuringiensis. These bacteria are harmless to pets, humans, and plants, but deadly to bagworms. Unlike other insecticides, bacillus thuringiensis will not harm beneficial insects such as bees. It is therefore ideal for spraying as part of a maintenance regimen.
Bagworms are almost impossible to obliterate. Once you have had a bagworm infestation, you are likely to have the same problem again. Get in the habit of inspecting your trees for bags each winter, spraying them with a strong insecticide each spring, and spraying with BT at least twice each summer.