It’s not exactly a case of pestilence and plague but if you look around your neighbourhood and on the slopes above the highway and other roads, anywhere there are trees, you will no doubt see webbing wrapped around upper branches. If you look closer it appears that the branch is moving as dozens of one inch long caterpillars twist and jerk about inside the webs. It’s fall webworm season!
We can always count on a few webworms every year beginning in late July but the numbers seem to be higher this year. Outbreaks seem to peak every four to seven years and then natural controls take over and reduce numbers. Our friendly yellowjacket wasps are very efficient predators but have you noticed that there are fewer of those around this summer than last?
The fall webworm is a classic example of adult, egg, larvae and pupa insect metamorphosis. Adults start emerging in late May through July and lay hundreds of eggs on the undersides of leaves. The eggs hatch in a week and the feeding frenzy begins. Caterpillars (larvae) start skeletonizing leaves and creating webs. Where large populations are present the webbing can cover multiple branches. After six weeks the teenagers’ stomachs are full; they drop out of the tree and pupate in the ground for winter.
Webworms are also a classic example of “it’s not as bad as it looks.” Webbing and skeletonized leaves is not a good look for a tree but the damage is entirely superficial. The caterpillars can only skeletonize the leaves, not completely destroy them and the percentage of damaged leaves is very small when you look at the entire tree.
Writhing masses of caterpillars are not for everyone however. If you feel like you need to do something to rid your trees of this infestation here are some solutions. Physical controls include pruning out the affected branches (or having a child or grandchild do it for you if you’re squeamish). Torching the webs does more damage to the tree than the caterpillars will ever do but it’s a lot of fun for young and old alike. I wouldn’t recommend it during the hot and dry weather; you don’t want to start a fire somewhere else. A very strong blast of water will dislodge the culprits in mid-chew.
The bacterial insecticide Bt is very effective against all chewing caterpillars and will stop the feeding of the fall webworm in short order while being harmless to humans. Chemical insecticides like Malathion or Sevin can also be applied.
As I mentioned earlier wasps are an effective predator of fall webworms, along with birds, stink bugs and parasitic flies. Webworms are really low hanging fruit for predators so anything you can do to encourage their presence will help. If you can re-think your control strategy for wasps you’ll likely find fewer webworms in your garden as we enter late summer.

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