Fall Webworms Are Back

July 16, 2019

Have you been seeing these nests in trees around town? We have received a few calls about them. Our Parks staff want you to know that these are not tent caterpillars; these are actually fall webworms.

Fall webworms are a native moth in this region. They make a similar nest as tent caterpillars but with a couple of differences: the nests appear later the season, and the nests expand so that the caterpillars can feed inside the protective nest. Usually they cause little lasting damage to the trees. The webworms are consuming leaves, and while this is somewhat stressful to the tree, the growing season is nearly finished for the year, and the leaves will grow back next year.

There are a couple of tools to get rid of fall webworms from trees.

One method that is used for tent caterpillars is to spray a biological insecticide - actually a bacteria - called BTK. This bacteria is sprayed onto the leaves on which the caterpillars would then feed. However, this solution can be less effective on fall webworms because the leaves they are consuming are inside the protective webbing which inhibits some of the BTK solution reaching the leaves.

Another potential solution for removing the webworms is pruning them out of the tree. Our Parks staff advise that, unless the webworms are pruned out at the beginning of the season when the nests are still small, this option is generally more stressful for the tree than having the leaves consumed. The tree will grow new leaves again next year, but pruning material out of the tree may remove branch tissues that have taken the tree several years to grow, and would take the same amount of time to replace.

So, although the webs in the trees might look like they are very stressful for the trees, they generally don’t cause lasting damage and are more of an aesthetic issue than a tree health issue. Furthermore, they are a natural phenomenon and are subject to many natural enemies including predatory ants, beetles and wasps as well as parasitic flies and wasps.

Fall webworms on City trees will be left in place to be taken care of by Mother Nature.

Find out more about fall webworms from Natural Resources Canada:
https://tidcf.nrcan.gc.ca/en/insects/factsheet/8125