Inchworms are a type of caterpillar that crawls by drawing the rear part of its body up to its head to form a loop, then moving its front legs forward. Inchworms are also known as cankerworms, spanworms and loopers. There are many species of inchworm, each with its preferred tree host. Several species feed on willow leaves.
Fall cankerworms are apple green or brownish green caterpillars that grow 3/4 to 1 inch long. They have a dark stripe down their middle and narrow white lines on each side. Omniverous looper caterpillars can be yellow, light green or pink with a gold-coloured head, and grow to be 2 to 2 and 1/2 inches long. Older caterpillars have brown, black, green or orange stripes down their sides. The Bruce spanworm starts out with a pale yellowish-green colour that becomes dark green or brown as it matures. Full-grown spanworms have light yellow stripes along their sides and grow to be about 1 inch long.
Young omnivorous loopers only eat the surfaces of leaves while young fall cankerworms chew BB-sized holes in them. Older caterpillars of all three species eat all the way through the leaves, skeletonising them. Inchworms also feed on tree buds. Healthy trees can survive one or two years of defoliation, although growth may be slowed. However, limb dieback occurs after three or more years of infestation. Inchworms are also a nuisance to people when they spin to the ground on silk threads, landing on cars, walkways and picnic tables. This obnoxious behaviour only lasts a week as the caterpillars get ready to pupate.
The best time to control most inchworms is when they're less than ½-inch long. By the time they're skeletonising leaves, treatment is ineffective. Several insecticides are available to treat inchworms, but the University of Minnesota Extension recommends a bacterial insecticide called Bacillus thuringiensis because it doesn't harm beneficial insects, wildlife or people. Control of Bruce spanworms isn't usually necessary.
A common method for controlling inchworms is to band the trunks with a sticky barrier that captures females that are crawling up the trunk to lay eggs. However, according to the University of Minnesota Extension, there are several reasons why banding is ineffective. People often fail to renew bands once the original one is full of moths. In addition, inchworms can hatch on unbanded trees that are located in the same area and then travel to banded ones on silk threads they produce. Sticky barriers are also messy to work with and expensive.
Inchworms have several natural enemies that help keep their populations in check. These include parasitical wasps and flies, spiders, lacewings and birds. Inchworms are also susceptible to the granulosis virus.
- University of Minnesota Extension; Spring and Fall Cankerworms; Mark E. Ascerno, et. al.; 2003
- Ohio State University Factsheet; Spring and Fall Cankerworms; David J. Shetlar
- BugGuide: Fall Cankerworm Moth
- Integrated Pest Management Center; Crop Profiles For Avocado in California; July 1999; pp 13-14
- University of California Statewide Integrated Pest Management Program: Omniverous Looper
- State of Maine: Bruce Spanworm