How come webworms prefer an Adina apple tree over a Mutsu apple tree? I don’t know either, but I’ve got side-by-side proof they do. There’s likely some fine-tuned answer about differing levels of some acid or sugar or whatever in the leaves. But the simple fact is one variety gets infested with webworms some years and the other never does.
There are two common types of moths with several species each that cause silk webs in trees. They are webworms and tent caterpillars. With each, the webbing is to protect the caterpillar stage from birds and predatory insects.
Webworm moths lay eggs on leaves near the outer edge of limbs. The eggs hatch and baby caterpillars go to work spinning the web around them and adjoining leaves. As they grow, they expand the web, taking in more leaves to eat. Webworms do not leave the web until it is time to go pupate. Tent caterpillar moths, on the other hand, lay their eggs on bark in limb forks. Their caterpillars spin tighter, thicker webs than webworms. And they don’t encompass leaves in the tent. They leave the tent at night, go eat, and come home before daylight.
For the most part, webworms and tent caterpillars don’t threaten long-term tree health. Trees put on more leaves than they need to feed themselves via photosynthesis. They can sacrifice about twenty percent of total leaf surface to bugs or blights and not feel it. Full-grown trees could even survive one growing season with total leaf loss. It’s mostly the unsightliness we object to, especially in landscape trees and shrubs. Only young yard trees might be badly hurt by a big dose of leaf-eaters. Pecan trees are often hit by webworms. But commercial pecan orchards deal with much worse insect threats and keep webworms in check when spraying for the other bugs.
Getting rid of tree caterpillars and their webs is best done by physically removing the web/tent. If you can’t reach them with a long pole, you are not likely going to reach them with the three-gallon pump-up home sprayer. The hose-end applicators set on “jet stream” will reach much higher. Organic Bt insecticide as well as Sevin and malathion are all labeled to control the worms. The goal is to spray the leaves twice, a week apart, so the worms ingest the product as they eat. I like the idea of nailing two or three nails in the end of a pole or narrow board. That way you can stick the end into the webbing and do a vertical spaghetti roll to tear it down. And just ripping webs apart in place betters the odds of birds getting to them.
Country folks know about torching webworms with a burning rag wrapped on a long stick. I read a torch remedy from one guy testifying on an online garden blog. He duct-taped a hand-held butane burner onto a long pole and ascended a ladder to get really way on up there.
Laugh, but please don’t.
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Terry Rector writes for the Warren County Soil and Water Conservation District, 601-636-7679 ext. 3.