Around this time of year, I frequently get phone calls from clients describing a bagworm infestation but referring to them as web worms, and vice versa.
It is easy to see why these pests are often confused with each other given their names. However, the insects and their treatment is quite different, so a distinction should be made before any action is taken.
Webworms often attack deciduous trees, appearing in suburban areas, roadsides and forest edges. Their appearance resembles literal webbing, as if giant spiders are working their way through the tree canopy. If you look inside the webbing, you will see yellowish-green caterpillars with black spots.
It is extremely rare for webworms to damage a tree to the point of killing it, but their appearance is unsightly.
By this time of year, treatment is unwarranted as insecticides do not penetrate the webbing very well, rather note which trees are most affected and make plans for next year. Small webs can simply be pruned off and destroyed if you can reach them and only a small portion of the tree is affected.
Pesticides should be used when the extent of webbing is too large to make pruning practical. For a list of recommended products and other tips on webworms, please refer to Purdue Extension Entomology publication E-255-W: https://extension.entm.purdue.edu/publications/E-255/E-255.pdf.
Switching gears, bagworms are usually found on evergreen trees and shrubs. They are easy to miss as they actually resemble small pinecones dangling from the branches.
When dissecting one of these, you will find a silky web-like substance and sometimes a caterpillar inside. Damage from bagworms can be serious, and if not controlled, can kill a tree over the course of a few years.
Bagworm damage begins in early June as the young worms hatch from the bags and begin feeding on foliage.
By this time of year, bagworms will be mature and very difficult to control with pesticides. If you can reach them, it is best to just pick them off the tree and throw them in a bucket of soapy water. If that is not feasible, spraying the tree with an insecticide is encouraged, but should be done when the bagworms are most susceptible, which is typically right after hatching in that June time frame.
For a list of recommended products and further information, please refer to Purdue Extension Entomology publication E-27-W: https://extension.entm.purdue.edu/publications/E-27/E-27.pdf.
These publications will also be helpful in providing pictures that will help you identify the differences between webworms and bagworms.
For more information on insects, both pests and of the beneficial kind, check out the Purdue Extension Entomology website at https://extension.entm.purdue.edu.