Break out the torches and pitchforks! Get out the axes! Let’s attack!

Such has been the rallying cry of angry villagers since the Middle Ages. Everyone in town picked up a weapon and the mob gathered to attack their foe. A similar cry is used today by some gardeners when they see webworms in their trees.

To them I say:  “Bring the pitchforks, but be careful with those torches. Leave the axes; we won’t need them. On the other hand, maybe let’s go back to bed and forget about it.”

The common foe, fall webworm, is active now. You can find the caterpillars and their nests in the canopies of chokecherry, cherry, birch and elm trees.

These nests look dramatic, but the damage is actually minimal. The caterpillars eat the leaves but cause little harm since the leaves are mostly done for the year. In most cases, people who see caterpillars in trees are suffering from more stress than the trees are.

If you can’t sleep at night while worrying about your tree, you can use a pitchfork or other tool to pry the nests out of the tree. Then you can stomp on the nests, or perhaps even torch them on the ground.

Or, you can spray the caterpillars with insecticides. The caterpillars are an easy target inside their nests. Carbaryl and pyrethrin are widely available and kill caterpillars immediately. This approach appeals to many of us who seek revenge against bugs and enjoy watching them suffer and die. These are the “Clint Eastwood a.k.a. Dirty Harry” gardeners, which includes me.

Other gardeners are more sophisticated and enjoy having the pests suffer a slow and painful death. To these gardeners I recommend a group of safer insecticides such as Bacillus thuringiensis, neem or spinosad. These are most effective when the nests first appear and take a couple days or longer before the pests die.

Protecting a young tree from webworms is more justified than protecting a large, mature tree. Young trees don’t have a lot of leaves to begin with. They need these leaves to overcome transplanting shock and to prepare for winter. On the other hand, it’s hard to justify spraying a big tree. It is under minimal stress, and you may put yourself at risk of taking a shower of toxic insecticide when you spray the tree.

Some gardeners attack the nests with propane torches or pruners. These strategies cause more harm to the tree than the caterpillars. The pests were only eating a few leaves. Torching or pruning trees will kill or remove entire branches. 

Written by Tom Kalb, Extension Horticulturist, North Dakota State University. Published in the NDSU Yard & Garden Report, August 25, 2014. Photos were made available under Creative Commons licenses specified by the photographers: kimberlysteinmann and Dendroica cerulea.