As the growing season comes to an end and the leaves begin to fall from the trees, you may notice leftover leaf material and debris hanging from your trees and shrubs that look like miniature pine cones. Upon closer inspection, you will see that silk holds this bag-like object together.
What you see is the cocoon of the bagworm, Thyridopteryx ephemeraeformisand if you step back to look at the whole tree, you might find yourself in a nightmarish scene: a tree covered in sac caterpillar structures.
These bugs have an insatiable appetite and can strip a mature cedar tree of its leaves. You may have seen the work of these insects in the parking lot of your local bank or department store. You may see a lot of brown, dead leaf material under a clump of exposed branches with no green on top. We see these pests every year in the metro, where they feed on various trees, shrubs, and other ornamental plants.
The most devastating damage is found on evergreens such as junipers and arborvitae. This is because the growing points of these plants are at the ends of the awl or scale-like leaves. If a branch no longer has green growing points, that branch will not grow back.
Fortunately, there are two times of the year when bagworms are exposed and can be controlled. In late spring, the larvae hatch from the eggs and fall out of a cocoon that the female made the following fall. They begin to crawl to the nearest food and start chewing. At first, they are hard to spot because of their size (1mm), but as they grow, they begin to do what their name suggests. They start to make a bag of silk around their body and place bits of leaves on the silk to help hide them. This combination of dead plant material and silk is recognizable to anyone with juniper, spruce, or arborvitae.
Late spring is the best time to control these insects, as the eggs are hatching. Applying a foliar pesticide with a long residual potency during their feeding phase will significantly reduce the number of bagworms. By September, however, the damage of the season has already been done. No chemical at this stage of life will have the effect you are looking for. The method of control this late in the season is to do what seems logical: pull off those suckers. Gently.
Pulling out the bags of worms and destroying them immediately is an effective method of control for the following spring. Destroying them by smashing them or drowning them in soapy water is guaranteed to keep their numbers down in the spring. It may seem daunting at first, especially if you have a significant infestation on a large tree or shrub, but this method does not have to be done all at once. The ones you don’t get today will be there tomorrow. Pick a few here and there until the eggs hatch in late May.
When pulling the bags from twigs on juniper and abortifacients, be careful to avoid damaging the plant. The silk that these worms use to cover themselves and attach to the host plant is very strong. Weak twigs and branches can break when you try to remove the bagworms.
If your tree or shrub looks like there isn’t much left, a spring insecticide treatment may be necessary. Females can lay 500 to 1,000 eggs, which overwinter in each sac. Reducing the number of sacs you see is very important to limiting future damage to your tree or shrub.
Markis Hill is a horticulture agent with Kansas State University Research and Extension. Need help? Contact the Johnson County Extension gardening hotline at 913-715-7050 or email garden.help@jocogov.org.
