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Fall webworms have arrived

Bob Dluzen
Special to The Detroit News

The summer solstice happened a few days ago, marking the beginning of summer, astronomically speaking. In a practical sense, most of us consider early June as the beginning of summer.

I got a little behind with some of my garden projects and have been playing catch-up for a couple of weeks now. One job I finally got to work on was tying up my grapevines. They were pretty unruly with vines that were supposed to grow to the left, but grew to the right. Some went up instead of down and vice-versa. It took some time, but I got it mostly sorted out.

While doing all that, I found an infestation of fall webworms.

There are months to go before fall arrives, so why are fall webworms making their appearance now, during the early days of summer?

It’s not really that early for fall webworms. They’re called fall webworms because their web nests get bigger as the caterpillars grow and move to eat more leaves. By fall, the webs are very big and unsightly giving the insects their name.

Fall webworm eggs hatch and begin eating when catalpa trees are blooming making it a reliable sign that the tiny caterpillars are out and about.

They feed on more than 100 different species of trees and shrubs. In my case, a momma webworm moth decided her babies deserved grape leaves this year so that’s where she laid her eggs.

The first thing the newly hatched larvae do is spin a protective web under which they chew on the tender parts of leaves, leaving the tougher veins behind. Later, as the worms grow and their mouth parts get stronger, they’ll eat the entire leaf, veins and all, while continually expanding their web. The webs by themselves wouldn’t be so bad but the worms also defecate into the webs, making them even more unappealing.

Tender leaf tissue is eaten by dozens of small webworms causing the tougher parts to dry up and turn brown. These tiny larvae have already formed their protective web.

All of that leaf eating and defecating doesn’t do any lasting physical damage to the plant despite the way it looks.

A spray of B.T. or insecticidal soap will kill very young larvae.  However, once their web thickens it will protect the worms making insecticides ineffective. Indiscriminate spraying of insecticides will do more harm than good by killing the webworms’ natural predators and parasites.

Mechanical methods such as tearing down the nests and cruising them work best with small infestations like you would find in a backyard.

A jet of water strong enough to tear a hole into the web will allow natural predators to get to the worms through the opening. It will not eliminate the nest but may reduce the number of caterpillars.

I don’t spray my grapevines with insecticides; most of the pests, so far, are kept under control by naturally occurring beneficial insects. I’m sure the lack of sprays is one reason why the webworms were able to get a start in my grapes in the first place.

I’m interested in how well the predators and parasites will keep the webworms in check. I’ll leave them alone and watch them for the time being. In the future, if they get too bad, I’ll remove them at that time.