Cankerworms in North Dakota

■ Identification

Adult moth:

• Male cankerworm moths are about 1 inch long and brownish gray, and have two light, irregular bands on their forewings (Figure 1).
• The wingless females are also brownish gray and approximately ¾ inch long (Figure 2).

Caterpillar (larva) (Figure 3):

• Caterpillar size varies from less than 1/16 of an inch long (young) to 1 inch long (mature).
• They are light green to brownish green, with white lines running along the sides and a dark band down the back.
• Each caterpillar has three pairs of fleshy legs called prolegs (Figure 4) on the back half of the body; one of the pairs is relatively small and rudimentary.

■ Life Cycle (Figure 5)

The fall cankerworm overwinters as an egg (Figure 6). Eggs hatch from April through May, usually when elm buds open. Newly hatched caterpillars readily feed on newly formed leaves. Feeding continues through June into early July.

Caterpillars are about 1 inch long when fully grown and complete their development in approximately four weeks. Mature caterpillars drop from their tree host in early July and construct silk pupal cocoons in the soil (Figure 7).

Adults emerge from cocoons from late September through October. Adults mate, and the wingless females climb up the trunks of trees, where they lay compact egg masses high up on the trunk and branches (Figure 6). The female fall cankerworm dies after egg-laying.

■ Identification

Adult moth:

• Males are brownish gray, with three dark bands visible on the forewings (Figure 8).
• Females are wingless and brown to black, with a dark stripe down the middle of the back (Figure 9).

Caterpillar (larvae) (Figure 10):

• Caterpillars of spring cankerworms are similar to fall cankerworms in size.
• Caterpillars vary from green to reddish brown or black, with faint dark lines or a yellow stripe on the sides.
• Spring cankerworms have only two pairs of abdominal prolegs (Figure 4).

■ Life Cycle (Figure 11):

The spring cankerworm passes the winter as a mature larva in the soil. It pupates in late winter, and cankerworm moths emerge in early spring, typically during the last week of March in North Dakota, or as soon as the ground thaws.

Wingless females mate and climb up the trees, where they deposit their eggs in loose clusters under bark in crevices and other protected places on the trunk and lower branches (Figure 12). Eggs hatch in early May, and larvae begin feeding on the new foliage. Caterpillars complete feeding in mid-June through early July and descend to the ground on silken strands, where they pupate in the soil and remain dormant until the following spring.

Caterpillars of both species often will spin down from the tree on silk threads when disturbed. These threads also serve as a means of transport by wind, and small caterpillars may be blown a considerable distance where they infest new trees.

Early detection of cankerworms and defoliation is important. Monitor upper tree branches for young caterpillars and early defoliation using binoculars during spring, from early to late May.

Early larval feeding injury is often overlooked until large holes appear in leaves and caterpillars are mature. Insecticide treatments are usually not warranted this late because extensive defoliation has already occurred, and mature caterpillars will be dropping to the soil to pupate.

Tree banding is not an effective control of cankerworms, although it can be used for early detection. For early cankerworm detection, place a 3- to 4-inch-wide band of sticky adhesive compound completely around the trunk, about 5 feet above the ground (Figure 15). Because female moths are wingless and must crawl up the trunk of their host tree to lay eggs, this band traps females and prevents them from depositing eggs on the upper trunk and branches.

Tree banding should be done during early to mid-September to trap fall cankerworms and again in mid-March to trap spring cankerworm females. The canopies of treated trees should be isolated from the canopies of neighboring trees. If the canopies of several trees intermingle, the trunks of all of the trees must be banded.

When banding is used as an early detection technique in shelterbelts, the adhesive should be applied as soon as the snow melts. The bands should be placed selectively on three or four trees along the length of the shelterbelt and monitored weekly for the presence of trapped moths. If large numbers are present, serious defoliation is likely, and management may be warranted when cankerworm larvae begin feeding.

Adhesive compounds are available as a paste and also as a pressurized aerosol spray. Because the paste can be applied easily with a putty knife, it is often the best formulation to use for the first application of a new band.

Old bands can be easily rejuvenated by spraying with the aerosol formulation. Adhesive compounds are generally nontoxic and present no potential hazards to the applicator or environment.

Several species of parasitic wasps are known to attack eggs or caterpillars of fall and spring cankerworms (Figure 16). However, the parasitism rate is typically too low (about 15% to 20%) for effective management. A nuclear polyhedrosis virus can also infect and kill caterpillars (Figure 17), especially during outbreak years and moist conditions.

Insecticidal control measures are most effective when caterpillars are young (about ½ inch long), and defoliation is low. Spray timing is critical, and if done correctly, only a single application will be needed.

Insecticides can be applied by ground-based or aerial equipment and should be directed at the trunk, branches and newly expanding leaves. Aerial application is fast and convenient, and most often is used to apply insecticides in shelterbelts.

Ground-based equipment is effective and may give better vertical coverage of foliage in the tree canopy. Good coverage of the infested trees is critical for acceptable control.

While timely insecticide treatments targeting the early larval stage of cankerworm provide effective control, many trees in residential areas are too large to be thoroughly treated with the spray equipment available to the average homeowner. Also, the homeowner often is unaware that a cankerworm infestation exists until after extensive damage has already occurred to the tree. Insecticides listed for the management of cankerworms include biopesticides and conventional insecticides.

Biorational pesticides (Table 1) are natural pesticides derived from materials such as minerals, bacteria, plants, animals or insect growth regulators.

Bacillus thuringiensis var. kurstaki (Btk) is a selective bacterial pathogen that kills the caterpillars of cankerworms and other moths and butterflies. These bacteria do not have a detrimental effect on beneficial insects, wildlife or humans.

Bacillus thuringiensis var. kurstaki is highly recommended for management of cankerworms where possible. For optimal control, apply Btk when caterpillars are less than ½ inch long, about 10 days after egg hatch.

Highly refined petroleum-based dormant oils or “Superior” horticultural oils contain mineral oil that can be used to coat the cankerworm eggs, causing suffocation. Applied oil in the winter or early spring before bud break. Eggs should be completely covered in oil for effective control.

Synthetic insecticides (Table 2) are recommended when caterpillars are greater than ½ inch long and extensive defoliation has already occurred.

Table 1. Biorational pesticide products for cankerworm control.

Table 2. Synthetic insecticide products for cankerworm control.

“Always read, understand and follow the pesticide label. The label is the law.”