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Oak Pests - A Guide to Major Insects, Diseases, Air Pollution and Chemical Injury

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Ennomos subsignaria

Importance. -- Elm spanworms attack red and white oaks, and other species especially Carya, (hickory, pecan and related trees) throughout the East. This is a destructive forest pest, particularly in the southern Appalachians where widespread, severe outbreaks have occurred. Repeated defoliation can cause growth loss, dieback, reduction in mast crops, and mortality.

Identifying the Insect (figure 1a). -- Larvae are slate grey to brownish black with yellowish body markings (yellow or green at low population densities) and 1.6 to 2 inches (40 to 50 mm) long. The adults are snow-white moths. The olive green eggs are laid in masses on the underside of small branches (figure 1b).

Identifying the Injury (figure 1c). -- Young larvae feed on the edge and undersides of leaves, causing a shot hole appearance. Later, they consume the entire leaf except the main veins, leaving a feathered appearance to the tree.

Biology. -- Overwintering eggs hatch in early spring when the buds break, usually April in the South. The larvae feed for 4 to 6 weeks, and then pupate in net-like cocoons on the host tree or understory. Six to 10 days later, in late June to mid-July, the moths emerge and deposit their eggs. There is one generation per year.

Control. -- Insect parasites attack the eggs of the elm spanworm. Other natural enemies also are important in keeping infestations in check. Chemical controls are often needed to protect high-value trees.

Figure 1a.
Figure 1b.
Figure 1c.

Figure 1. -- (a) Elm spanworm larva; (b) elm spanworm egg mass on branch; (c) defoliation by elm spanworm.

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