In recent years, an increased population of fall cankerworms (commonly called inchworms) in our area has led to bothersome webs hanging from trees along with loss of leaves in the spring.
The best defense is to band trees in the fall to prevent cankerworms from traveling to the top to lay eggs. Depending on the types of trees and shrubs on a property and the property owner’s level of tolerance, there are several tips for cankerworm prevention and removal.
First, consider these important points:
- Cankerworms won’t last forever. Our experience in Cabarrus County indicates the larva stage will be present only about 4 to 5 weeks from the time they hatch out in early spring.
- Fall cankerworms will not kill a healthy tree. If all the leaves are removed, the tree will wait a few weeks and then put out new leaves. After new leaves form, there still will be enough growing season for a healthy tree to recover.
- The best control for reducing future infestations is tree banding (instructions below). Place bands on trees in November and monitor them until the larva disappear in April.
- Spraying is less effective. Since the larva stage is short term, the cankerworms won’t kill the tree and spraying a mature tree is difficult or expensive. Most homeowners will ignore the spring infestation on mature shade trees.
- If you wish to control fall cankerworms on understory shrubbery during the spring larva stage, any pesticide that controls caterpillars will work on fall cankerworms. Purchase a pesticide containing synthetic pyrethroids (aka the “thrins”). These can be identified by the last five letters in the active ingredient which is required by law on the pesticide label. Common examples are cyfluthrin, bifenthrin, permethrin, and lamda-cyhalothrin but there are dozens more. Look for a product that can be applied to outdoor shrubbery.
Tree banding in November and December will reduce cankerworm numbers. By applying a glue barrier to the tree trunk, the wingless female cankerworm moths are trapped as they climb the tree.
For best results, trees should be banded around the last week of November. It is important to wait until most leaves have fallen from the trees so the leaves don't get stuck to the tree bands. Focus on trees that are higher than 20 feet.
Cankerworms have a preference for maples, oaks and elms, so band those and other trees that have had previous damage. Pines, yellow poplar, and Carolina poplar are very resistant, so they don’t need banding. There are local contractors who will band trees for a fee or you can do it yourself.
How to band a tree:
To band a tree, the following materials are necessary and can be purchased at most hardware and garden stores:
- Staples or electrical tape *
- Staple gun
- Disposable elbow length gloves
- Roofing felt
- Cotton batting or fiber glass
- Tanglefoot™ (glue)
- Putty knife
* For small trees, substitute electrical tape for the staples. Do not use nails.
- Install a strip of cotton or insulation around the tree at least three feet from the ground and below the lowest limb. This is critical, so that no insects can crawl underneath the band.
- Position a band of roofing felt over the strip and attach it to the trees with the staple gun. Avoid using staples on small, young or thin barked trees. Instead use electrical tape to hold the bands.
- Wearing disposable gloves and using a putty knife, put a film of Tanglefoot™ (glue) directly on the band, approximately 1/8” thick. Start with a 2-inch wide strip of Tanglefoot. Monitor the trap and add additional Tanglefoot as necessary.
- Keep the band in place until the larvae stage disappears in late April. If the cankerworms run out of food, they will start crawling around and may become trapped in the band during the larva stage.
Don’t expect banding to totally control every cankerworm in your landscape. Even if a trap is 100% effective, cankerworms may still blow in from other places, particularly on understory plants.
Chemical control may be used on the larva stage. There are local companies that will spray for cankerworms in mature trees. Homeowners seldom have the equipment necessary to spray mature trees, but may decide to spray understory trees and plants.
Numerous pesticides will work. Anything that kills a caterpillar kills the cankerworm.
Safest pesticides to use:
One of the safest and more benign pesticides is Bacillus thurigensis or Bt. This product is sold under various brand names including Dipel or Thuricide. This bacterium causes a bacterial disease specific to moths and butterflies. This pesticide is legal for organic food production and has been used for decades with no known adverse effects on humans, wildlife, pets, fish, or honey bees. This pesticide is best used when insects are small to give them time to get sick and die. Don’t expect immediate results.
Pesticides with quickest results:
For quicker results, look for pesticide made of synthetic pyrethroids (aka the “thrins”.) These can be identified by the last five letters in the active ingredient which is required by law on the pesticide label. Common examples are cyfluthrin, bifenthrin, permethrin, and lamda-cyhalothrin but there are dozens more that give equivalent results. Look for a product that can be applied to outdoor shrubbery. The use of Sevin is not recommended. This is a pesticide that the insect has to consume before it works. Spraying Sevin on insects that are not feeding will not lead to results.
What are fall cankerworms?
Fall cankerworms (Alsophila pometaria) are insects in the inchworm family within the moth and butterfly order. In other places inchworms may be called loopers or spanworms. The fall cankerworm is native to our area. It is typically found from Georgia to Nova Scotia and west to Texas.
History of cankerworms
The biblical cankerworm refers to a particular life stage of the locust (an unrelated insect); however, fall cankerworms have been a resident of our forests since those days. Far northern areas of the cankerworm range report cyclic populations, but regular boom and bust cycles haven’t been documented in our area. The City of Charlotte has had severe infestations for more than 20 years. At least one Concord resident recorded a minor infestation in the 1970’s. Numbers then stayed low until 2009. In 2009, a major infestation was noticed south of Harrisburg. This spread in 2010 to include parts of Concord and Kannapolis. The Cabarrus County cankerworm populations continued to increase in 2011 and 2012.
Life Cycle of cankerworms
Fall cankerworms emerge from the ground as adults after cold snaps in November, December or January. The nearly wingless females are a dull gray color and crawl up on tree trunks to await a winged male. The males are about 25 mm (1 inch) long, dull gray in color and often have two light, wavy stripes on the forewings.
After mating, the female lays a cluster of barrel-shaped eggs, often encircling small branches. The eggs overwinter and hatch in late March to mid-April.
Upon hatching in March and April, the young larvae rapidly feed on the fresh tender spring leaves of various trees.
By late April or May larvae have matured and descended to the ground on silk threads.
The larvae then burrow into the ground to a depth of 2.5 to 10 cm (1 to 4 inches), spin a silken cocoon and pupate. The pupae remain in the soil until the late fall.
Damage caused by cankerwoms
Young larvae chew small irregular holes in young leaves, skeletonizing the leaves. As they mature, the larvae begin eating larger irregular holes and finally entire leaves, leaving only the major leaf veins. Although low populations do not damage healthy trees, high populations can defoliate whole trees, forcing them to expend considerable resources to re-foliate.
Many of the cankerworms and loopers descend from the trees on a strand of silk when they are ready to pupate. These larvae drop onto people, cars and picnic tables and are sometimes considered nuisance pests, although they cause no harm to mammals.
Why an increase in cankerworms?
While it is interesting to speculate why cankerworm populations have been so large recently, entomologists just don’t know. Charlotte’s original infestation was in a large monoculture of old willow oaks. Some people theorize this promoted the infestation, but the cankerworms have since spread into areas with no willow oaks.
Another theory is that the 90% decrease in neo-tropical migratory birds over the last few decades has reduced cankerworm predation from birds.
Another predator of cankerworms is a ground beetle. We are seeing some increase in this predator, but it may not thrive in the closely mowed turf of residential areas.
Higher temperatures during spring emergence favor the cankerworms. This allows them to get bigger rapidly. As a result, birds wind up eating less total numbers to get the same volume of food, so more cankerworms survive.
On a smaller scale, defoliations do seem to be more severe in wetter areas. Finally it is typical of all living things for the population in one area to be higher than the population in another area.
More Information about cankerworms:
NC State Entomology Note
VIDEO: Successful Gardener with Jack McNeary, local cankerworm expert