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Insect Control

Elm Spanworm

elm span worm adult

What are elm spanworms?

The elm spanworm is a serious defoliator of shade and forest trees in the eastern Canada and the United States. Generally, this native pest feeds on elm, hickory, oak, red and sugar maple, American beech, and ash. During July females lay eggs in irregular, single-layered, compact masses on the underside of twigs, large branches, or on tree trunks. Each female lays an average of 250 eggs. The eggs are oblong-elliptical, with a white serrated ring around the end. Elm span-worm spends winter in the egg stage.

Egg hatch may begin in mid- to late May. After hatching, the larval stage passes through five or six instars, and then it pupates. To prepare for pupation, mature larvae spin coarse, netlike cocoons of silken threads, often on partially defoliated leaves. In severely defoliated stands, cocoons may be spun on exposed branch tips, in bark crevices, or stumps in the undergrowth. The pupal period varies from 9 to 17 days. Adults emerge in late June through July. They are on the wing at night. If an infestation is close to a urban area, male moths may fly to lights in large numbers frequently described as resembling a snowstorm in the summer.

elm span worm adult

Photos by Andy Reago & Chrissy McClarren under license CC BY 2.0

What does the elm spanworm look like?

The elm spanworm adult is a powdery white moth with a wingspread of 30 to 37 mm (1 to 1 1/2 inches). Its body is fairly stout and hairy. Females lay the eggs incompact masses on the underside of twigs, large branches, or on tree trunks. The eggs are bright yellow green when first laid in summer but darken to dull olive gray or brown in winter. Mature larvae, some-times referred to as "loopers" or "inchworms," are about 50 mm (2 inches) long. The body of the larval stage maybe dull or slate black and the head rust-colored. Some larvae may be light green with yellow head capsules. When population levels are low, there is a higher proportion of lighter colored larvae. The pupal stage is light brown, sometimes patterned with dark brown spots.

What does elm spanworm damage look like?

All damage is caused by the larval stage of the elm spanworm. As soon as egg hatch occurs, larvae begin to feed on the underside of leaves, causing a shot hole effect. As larvae mature, they eat all leaf material between the major veins. Larvae are capable of completely defoliating shade trees and large areas of mixed hardwood forest during outbreaks.

How can I control elm spanworms?

Trees should be monitored from mid-May through early June for signs of elm spanworm larvae. If necessary, a registered insecticide should be applied when larvae are small. Where possible, prune small twigs that are infested with masses of eggs. Two egg parasitoids, Telenomus droozi and Ooencyrtus entomophagus are known to keep this pest at low population levels. These small natural predators can destroy more than 80 percent of eggs during an outbreak.

Bacillus thuringiensis var. kurstaki (Btk) can also be used to control Elm Spanworm. 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. The caterpillar stops feeding soon after, and dies within five days. Other insects, mammals, birds and fish are not affected by Btk. Btk is most effective when larvae are in their first instars. Btk has to be applied when larvae are actively feeding, and applied so that all foliage is thoroughly covered. Rain washes Btk off the leaves and sunlight breaks it down within a short period of time, approximately one to two days. 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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