Fall cankerworm—Alsophila pometaria
Fall cankerworm (a looper, family Geometridae) is an occasional pest of pear and stone fruits.
Identification
Larvae of fall cankerworm have three pairs of prolegs (fleshy, leglike appendages) near the rear of the abdomen. These in combination with the three pairs of true legs on the thorax allow them to move in a looping manner by pulling their rear forward as they arch up their back. Larvae commonly rest with much of the body held upwards twiglike.
Larvae are commonly green with longitudinal, light-colored stripes along the sides of the body. Larvae can also be brown or otherwise dark colored. They grow up to 1 inch long.
Adults (moths) are mottled brown, gray, and silverish. They are about 1/2 inch long with a wingspan on males of about 1 inch. Adult females are wingless, but the body is covered with hairlike scales that resemble the coloration of males.
Eggs are barrel shaped and laid in a tight cluster of several dozen eggs. Eggs generally occur on bark where they commonly encircle a stem or twig. The oblong pupae are initially green to yellowish then become brownish before the adult emerges. Pupae are about 1/2 inch long.
Life cycle
Fall cankerworm develops through 4 life stages: egg, larva, pupa, and adult. They overwinter as eggs on bark. Larvae hatch and feed on leaves in spring and early summer. They develop through 5 increasingly larger instars as they feed for about 5 or 6 weeks. They can move between hosts by dropping on a silk strand that allows them to be carried by the wind to nearby plants. Mature larvae (prepupae) drop on a silk strand to pupate on soil and in organic debris on the ground. The adults emerge in the fall, mate, and the females lay eggs. There is 1 generation per year.
Damage
Larvae primarily chew and skeletonize leaves and commonly leave behind only leaf veins. Occasionally they chew and feed on young fruit, sometimes creating relatively deep holes. Fruit wounds commonly leave large, scarred depressions similar to the injury caused by larvae of green fruitworms.
Larvae can chew and feed on many species of broadleaved plants. But in California they are rarely or never abundant enough to be a pest problem on most of their hosts. For a complete list of plants on which they are known to sometimes feed see HOSTS - a Database of the World’s Lepidopteran Hostplants.
Solutions
On small trees, infested twigs can be pruned off and disposed of if this does not remove a substantial portion of the plant’s foliage and wood. If larvae and their chewing damage were abundant early in the growing season, the next generation can be controlled by applying a sticky barrier (e.g., Tanglefoot) on trunks around midsummer or whenever mature larvae are observed dropping to pupate on the ground. This prevents the wingless females from climbing up trunks in the fall to lay eggs on plants.
First wrap (encircle) the trunk with a 6-inch-wide band of heavy paper, landscape fabric, or other flexible material. Apply the sticky material in a several-inch-wide band on top the wrap. This makes it easy to remove the material and prevents potential bark damage from the product. Bands must remain sticky and clear of excessive debris. This can be done by periodically stirring the sticky material with a stick or removing the old band and applying a new one.
If larvae are abundant and causing unacceptable defoliation, infested foliage can be thoroughly sprayed with Bacillus thuringiensis (Bt) or spinosad. Avoid applying spinosad to plants in bloom because it is toxic to honey bees and some natural enemies. If applying Bt, make a second application about 7 to 10 days after the first because Bt is not persistent, and not all the fall cankerworms may be in stages susceptible to Bt at the same time.
Overwintering eggs can be controlled during the dormant season by thoroughly spraying egg-infested bark with horticultural oil. Oil during the dormant season can also control aphid eggs and scale insects. Because completely dormant plum trees may be damaged by oil, wait until late winter when buds have begun to swell but before they open, and make the application then during the delayed-dormant season.
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Leaf skeletonized by larvae of fall cankerworm.
Larva of fall cankerworm, which moves in a looping manner.
Adult male fall cankerworm.
Adult female fall cankerworm, which is wingless.
Eggs of fall cankerworm are barrel shaped and laid adjacent in a mass.
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