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Insect Control

Fall Webworm

Photo by Katja Schulz under license CC BY 2.0

What are fall webworms?

The fall webworm (Hyphantria cunea) is known for it's webbed nests among hardwood trees in the late summer. Although the webs may look unsightly, the fall webworm will cause no harm to a healthy tree. The fall webworm is an insect native to North America and can be found from Canada south to Mexico. In Canada there is one generation per season, with larvae appearing in late summer through early fall.

Photo by Ben Sale under license CC BY 2.0

What does the fall webworm look like?

The adult moth lays her eggs on the underside of leaves. The eggs are deposited in hairy clusters of a few hundred and take a week to hatch. The caterpillars vay in colour from a pale yellow to grey with hairs. They can grow to a length of 35mm and create nests at the end of branches.

The adult moth is white in colour with brown spots, very hairy and may have orange markings on it's legs. It's wingspan is approximately 40mm.

What does fall webworm damage look like?

The fall webworm caterpillar, upon hatching, will create a webbed nest at the tips of branches. They will generally consume the leaves within their nest. If your tree is relativley healthy, the nest will cause no harm to the tree.

How can I control fall webworms?

A tree is normally very tolerant of some defoliation and control efforts are unnecessary when only a few caterpillars are present. If a citizen is concerned they can control them with the use of biological products or chemical pesticides. Biological products like Safer's BTK™ Biological Larvicide contain the active ingredient Bacillus thuringiensis var. kurstaki (Btk) to control caterpillars. Btk is a selective biological insecticide which controls lepidopterous larvae (caterpillars). Btk crystals release a toxic protein when dissolved in the alkaline digestive system of the insect. Btk has to be applied when caterpillars are actively feeding, and applied so that all foliage is thoroughly covered. The caterpillar stops feeding soon after, and dies within five days. Other insects, mammals, birds and fish are not affected by Btk. These products can be found at most local hardware stores. All Pest Control products purchased and used must be registered with Health Canada and contain a Pest Control Product (P.C.P. or PCP) Number on their label.

Last update: May 16, 2019
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