FS-0876-GO
Revised 1994
To Order

Spring and Fall Cankerworms

Mark E. Ascerno and Robert P. Wawrzynski

Cankerworms, or inchworms, are common shade tree defoliators in Minnesota. Hosts include elm, apple, hackberry, basswood, oak, boxelder, maple, and ash trees as well as shrubs growing beneath heavily infested trees. This publication deals with the two most common pests of the Cankerworm group: the spring and fall cankerworm. Cankerworms go through natural cycles with periods of abundance ranging from two to seven years (with an average of four years) followed by periods of low populations lasting 13 to 18 years. At the time of this writing (1994), the spring and fall cankerworms have not had a significant population outbreak in nearly 15 years. One explanation for this may be the reduced number of elm trees in Minnesota. As the population of elm (the preferred host for cankerworm in Minnesota) continues to decrease due to Dutch elm disease, the number of remaining hosts may not be suitable for supporting large populations of cankerworm. Thus, it's difficult to determine when the next population outbreak may occur.

Identification and Life History

Although similar in the kind of damage they cause, the spring and fall cankerworm have different egg-laying habits and slightly different body shapes. Both feed at the same time of year, often on the same tree, and spend the summer as nonfeeding pupae in the soil.

Fall Cankerworm — This species emerges from the soil as an adult moth, generally in late October after a hard freeze. The gray-brown male moths are winged, about 1/2 inch long, and emerge a few days before the wingless, spider-like females. Mating takes place as the females crawl up tree trunks in search of small twigs on which to glue neatly ordered egg clusters. The males and females die shortly after the eggs are deposited.

Eggs stay on trees throughout the winter and hatch in the spring. Egg-hatch varies from late April to mid-May depending on weather conditions. It normally coincides with the opening of elm buds. Newly hatched fall cankerworm larvae are less than 1/16 inch long, vary from green to reddish brown, and spin silk threads that allow them to readily blow from tree to tree. They reach one inch in length when fully grown, which takes about four weeks. Larvae then drop to the ground to pupate in the soil before emerging as adults later in the season. Fall cankerworm larvae have three pairs of false legs called prolegs on the back half of the body.

Spring Cankerworm — Spring cankerworms differ from fall cankerworms in several ways (Table 1). First, the pupae in the soil do not emerge as adult moths until early spring. Emergence of the winged male and wingless female (Fig. 1) typically occurs during the third week in March in the Twin Cities area.

Figure 1. Female, spring cankerworm.

Figure 2. Mature, spring cankerworm larva.

Adult spring cankerworm males and females are similar to fall cankerworm males and females in color, size, and appearance. Another difference is that eggs are deposited in loose clusters in bark cracks and under bark scales and are not easily seen. Spring and fall cankerworm eggs hatch about the same time in the spring and the larvae have similar life histories. Finally, spring cankerworms have only two pairs of prolegs instead of three (Fig. 2).

Damage

A mature, vigorous tree is able to withstand a single season of complete defoliation with little effect on tree health. Even two seasons of defoliation produce only a slowing of the growth. However, limb dieback and loss of vigor can result if the same tree is completely defoliated for three or more consecutive years. Young, newly transplanted, or weakened trees are more susceptible to the detrimental effects of defoliation. Therefore, a tree's age, size, vigor, and previous history of defoliation should be considered before choosing a control measure.

Many people are not as concerned about the health of their tree as they are about the nuisance that cankerworms cause. Large numbers of worms spinning to the ground on silken threads, crawling or falling on picnic tables, doors, walks, and/or house siding, can create an unpleasant environment. Fortunately, this behavior usually lasts only about one week because control measures are not effective at this stage of the worm's life cycle. Cankerworms may also create a nuisance during the spring and fall as moths crawl up exterior walls of homes. Again, control measures are not effective nor recommended at this time.

Control

Chemical — Whether the purpose is to protect trees or to eliminate the nuisance of cankerworms, control measures need to be taken when the worms are relatively small and inconspicuous. Cankerworm larvae begin to feed on buds or expanding leaves immediately after egg-hatch. Normally, feeding during the first two weeks is not noticed by the casual observer. However, a close look will reveal small BB-size holes in the leaves. The best time for chemical control is in the second week of feeding when the damage is minor, the worms are still small (less than 1/2"), and leaves have fully expanded. If leaves are still growing and pesticides are applied, pesticide coverage will be reduced as the leaves continue to grow, reducing control efficacy.

The worms rapidly increase in size during the third and fourth week after egg-hatch. During this period, damage becomes extensive and very noticeable. In many cases large portions of the leaves have been eaten and only the veins remain. Unfortunately, this is the worst time for control because the damage to the tree is already done, and the insecticide irritates the worms causing them to drop from trees in larger numbers.

Several insecticides are effective for controlling cankerworms when applied at the correct time including: Bacillus thuringiensis (Dipel, Thuricide); carbaryl (Sevin); acephate (Orthene); and malathion (Malathion). However, the bacterial insecticide Bacillus thuringiensis should be used when possible. This product has the advantage of being specific for the caterpillars of moths and butterflies and will not harm beneficial insects, wildlife, or humans. Bacillus thuringiensis must be applied while the cankerworms are still small (less than 1/2") to achieve good control. Other conventional insecticides should be used as a last resort when cankerworms are larger and/or feeding damage is extensive. The following steps are suggested to achieve good cankerworm control.

1. Determine egg-hatch date by either:
   a. Finding fall cankerworm egg masses and observing them during early spring for signs of hatching (hatched eggs will have a hole in the center).
   b. Examining foliage of several trees for the first signs of young cankerworms or their feeding.
2. Add 10 days to the date of hatch as determined in step one. This will be the best date for spraying.
3. Examine representative trees during the 10-day post-hatch period to determine if development has slowed or quickened because of weather conditions.
4. Modify the spray date as necessary, keeping in mind that the worms should be less than 1/2" in length and that at least 50 percent of the leaf surface should be intact.
5. Spray with one of the suggested materials.

Trunk Banding — One widely-known method of controlling cankerworms is the use of sticky barriers applied to tree trunks. As the wingless females crawl up the trunk to lay eggs, they are trapped in these sticky bands. Despite the publicity, there is little evidence that this method is worthwhile even in areas where most trees have been banded. The possible exception would be on a tree that is isolated from other cankerworm-susceptible trees.

Although the practice is fine in theory, it generally fails because of one or more of the following reasons: (1) people do not clean and renew the sticky material as often as the bands become crowded with moths; (2) people fail to recognize and band for the two species (one moving up the tree in the fall, the other in the spring); (3) unbanded trees in an area may produce young cankerworms that can readily blow to banded trees on silk threads. Furthermore, the sticky material is expensive, messy to work with, and tends to mar the appearance of the trunk for a long time. Thus, banding is not recommended.

Mark E. Ascerno Head of Extension Entomology

Robert P. Wawrzynski Research Fellow, Entomology

Information source: University of Minnesota Extension Service.

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