As I remember, I had a large quantity of bagworms last year. How do I prevent that this year? AP

You are correct, last year we had what some called a “bumper crop” of bagworms. It seems like they were everywhere and on everything. Some people were even finding bagworms hanging from the eaves of their homes or other outdoor structures.

Because of the bumper crop last year, logic would suggest that means we will have a repeat performance this year, but we’ll just have to wait and see. Nature is hard to predict. So, let’s talk about the bagworm lifecycle and what you can do to limit any damage bagworms might do to your plants this year.

Bagworms have a preference for arborvitae, eastern red cedar, and other junipers. However, they will also take up residence on true cedars, pine, spruce, bald cypress, maple, boxelder, sycamore, willow, black locust, and oak. Last year we were even finding bagworms on roses.

Bagworm eggs overwinter in the bags most of us are familiar with and begin to hatch in late April or early May. After hatching, they exit the bag and begin to feed. Typically, we become aware of their presence when we start to see those familiar bags hanging from branches. Bags can look different depending on their host since larvae construct the bags from what they find on the host plant. So, a different host plant…a different looking bag.

The bags begin about 1/4 of an inch in length but the larvae increase the bag’s size as they grow. Fully grown larvae usually have bags about 1.5 to 2 inches in length. The larvae move around to feed with their head poking out of the bag. When they mature, they attach themselves to the host plant with a kind of silk produced by their salivary glands.

The fully grown larvae then pupate inside the bag with adult males emerging in late summer to early fall. These adult males are small, black, hairy moths with a wingspan of about an inch. As soon as the male moths emerge, they begin looking for females.

Females lead a much more solitary life as they spend their mature life inside the bag. While the males become moths, the female are wingless, without functional legs, eyes, or antennae. They continue to look like larvae.

When the male bagworms find a female, they enter the bag to mate. A fertilized female can lay between 500 and 1000 eggs inside of the bag. These eggs will overwinter inside of the bag to hatch the following spring and the cycle begins again.

Bagworms are typically not a huge problem unless you have a rather large concentration on your plant. Deciduous plants will re-grow the leaves that are lost due to the bagworms fairly quickly. However, weakened plants are more susceptible to disease or wood-boring insects. Evergreen plants do no produce new foliage every year so their recovery from bagworm damage may take several years.

The bags make controlling bagworms a little more problematic than some other insects because those bags shield them from any type of insecticide you might consider using. An easy and very basic way to deal with bagworms is to physically remover them and by that, I mean, put on your garden gloves and go harvest some bagworms. You can just pull them off and deposit them in the trash. This strategy does get a bit more challenging when the bagworm infestation is on a large plant, but it is still very effective.

Controlling bagworms with insecticides is problematic because they live most of their lives very well protected inside of their bags which will not allow pesticides to penetrate. Your best option in this case is to use an organic pesticide called bacillus thuringiensis. Bacillus thuringiensis is a natural compound found in soil that is toxic to caterpillars. For this strategy to be effective you will need to spray your plant while the bagworms are young and feeding. They will eat the bacillus thuringiensis, become sick, quit eating, and expire.

To utilize this strategy, keep an eye out for the small bags in early June when they are actively feeding. Spray the area around the bags with bacillus thuringiensis and wait. Spinosad is another organic pesticide that will control bagworm populations but again, it will only be effective while the larvae are feeding.

There are several parasitic and predatory wasps that adversely affect bagworm populations as well as a few fungal pathogens. It’s possible this explains the difference in bagworm populations we experience from year to year.

While timing for the pesticide treatment is critical, you can pick the bags off any time of year. Just be sure to throw them away and don’t put them in your compost pile because you will just have relocated them rather than gotten rid of them. See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo: William Fountain, University of Kentucky, Bugwood.org