Description. 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 (Image 2) in compact 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 (Image 3), sometimes referred to as “loopers” or “inchworms,” are about 50 mm (2 inches) long. The body of the larval stage may be 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.
Life History. 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 spanworm 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.
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