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What is An Inchworm?

By L. Whitaker
Updated May 21, 2024
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Inchworms, which get their name from their peculiar form of movement, are the caterpillar or larvae form of several species of geometer moth. This type of moth gets its formal name Geometridae from two Greek roots that refer to the visual spectacle of inchworms "measuring the earth" as they move forward. They are also variously known as loopers, spanners, or measuring worms. There are 1,400 species of geometer moths in North America, with a total of 35,000 different types worldwide.

The peculiar movement style of inchworms is the result of its lack of legs in the middle portion of the body. Inchworks have two or three paired sets of legs on each end of the body, but none in the middle. As a result, inchworms propel themselves forward by drawing the back end upward to form a loop shape, then reaching forward with the front portion of the body. When disturbed by a potential predator, inchworms often freeze in place so they resemble a still twig.

Inchworms are generally hairless and have smooth bodies. The color can be brownish, green, gray, or black depending on the type of moth that the inchworm larvae will become. Some species have black spots on the head. Inchworms tend to be camouflaged from predators by blending into the surrounding environment.

Most species of inchworms primarily eat leaves of deciduous or coniferous trees, but at least one species is know to be carnivorous. Many species of inchworm, such as the cankerworm, are considered pests by farmers and gardeners. Farmers sometimes use the pathogen Bacillus thuringiensis to treat inchworm infestations. Natural predators include some types of birds and wasps.

The adult geometer moth can vary in size but typically is about 0.39 inches (1 cm) long, with some species ranging up to 2 inches (5 cm). Unlike many other types of moths, which tend to fold their wings on top of the abdomen when at rest, adult geometer moths keep their wings spread erect after landing, similar to a butterfly. The adult moth is generally noctural, but some species do operate during the day.

In each species of geometer moth, one generation is born annually after adult moths lay large egg clusters during the winter season. The eggs, which are gray and shaped like cylinders, can typically be found on tree limbs. After the eggs hatch in the spring, the inchworm larvae eat leaves for up to five weeks before being ready to pupate. They then create silken cocoons in a shallow layer of soil.

What Do Inchworms Eat?

Inchworms are relatively social insects that are known to gather in groups to feed on leaves, buds, berries and foliage. Inchworms are also often found feeding alone in the same places they congregate together. These insects primarily feast on the leaves of coniferous and deciduous trees, such as maples, elms, pines, firs, oaks, lindens and various fruit trees.

Inchworms as a whole have a fairly varied herbivorous diet, but at least one known inchworm species makes a meal of fellow insects and creatures such as spiders, flies and crickets.

After a female inchworm lays and hides her eggs under leaves or in the bark of a tree, the larvae hatch and can become fairly destructive to the tree, bush or plant structure they have chosen as their feeding grounds. For this reason, inchworms are considered unwelcome pests by both farmers and gardeners alike. If left unchecked, their masses can destroy the leaves of an entire tree before moving to their next target.

What Do Inchworms Turn Into?

Inchworms are actually moth larvae that eventually become geometer moths in their adult lives. Though most moth larvae or caterpillars exhibit a fuzzy or hairy appearance, inchworms are typically smooth and free from hairs. These insects graze on leaves and foliage for approximately five weeks before they move toward the ground, where they form a cocoon and undergo metamorphosis for a period of several weeks or a few months.

When they emerge from their cocoon, the adult moths are hardly larger than an inch. In fact, most geometer moths measure less than half of an inch when fully matured. Their bodies are typically muted brown, with wings that may feature thin shapes, tan or brown colors or striped markings.

What Are the Common Types of Inchworms?

With over 35,000 species in the world, the name “inchworm” actually refers to a large number of different species of moth larvae. There are, however, two prominent kinds of inchworms: fall cankerworms and spring cankerworms.

Fall Cankerworms

These inchworms are green-bodied and typically hatch from their eggs in the late spring or early summer. They feed on fresh, newly-grown foliage during the warmer months of the year and then fall to the ground where they pupate until approximately October when they emerge as adult geometer moths.

Spring Cankerworms

More varied in color than fall cankerworms, the spring variety can be brown, black, yellow or green. They may also have a small white stripe that runs the length of their body along the side. These insects follow approximately the same larvae cycle as fall cankerworms, but they typically do not emerge from their cocoons until the beginning of the spring season in early March.

Other Common Inchworm Types

Other types of inchworms include elm spanworms and linden loopers. These varieties are often mistaken for spring or fall cankerworms, but a few differences set them apart from other common types.

  • Elm Spanworms: Generally found on trees with broad leaves, these inchworms are brown or gray and usually much larger than cankerworms. They can be identified and differentiated from spring cankerworms by their reddish-brown heads and rear parts.
  • Linden Loopers: A noticeable brown or black stripe on their backs, as well as a set of yellow stripes on each side, distinguishes linden loopers from cankerworms and other inchworm types. Another unique trait of these insects is that they leave holes behind in the leaves they eat, as they rarely consume an entire leaf before moving to a new location.

Where Do Inchworms Live?

Inchworms live out their days near a food source, so they are most often spotted on or near the leaves, branches, bark, stems and other plant parts of their chosen meal sites. Because inchworms are simply the larvae stage of the geometer moth’s life cycle, this entire phase of life is spent primarily eating to prepare for the pupal stage. Inchworms rarely travel elsewhere until it is time to spin silk and create a cocoon.

Inchworms can be found in a variety of habitats, as they are fairly hardy and adaptable insects. They are considered a problem for many farmers and gardeners when their populations reach significant levels. In large groups, inchworms are capable of defoliating entire groups of trees, shrubs and other crops.

These insects are most commonly found in regions with dense, lush foliage. They may congregate in orchards, forests, groves and other natural areas but then live an entirely different, increasingly mobile life after they emerge as fully matured moths.

Frequently Asked Questions

What exactly is an inchworm?

An inchworm is the larval stage of a moth from the family Geometridae. These caterpillars are known for their distinctive looping gait, which resembles the measure of an inch, hence their name. They have a sparse number of legs, with true legs at the front and prolegs at the rear, creating their characteristic "inch" movement.

How does an inchworm move?

An inchworm moves by drawing its hind end forward while holding on with its front legs, then extending its body forward to anchor with its prolegs. This looping motion is often referred to as "looping" or "measuring," and it's a unique adaptation that helps inchworms navigate through foliage with ease.

What do inchworms eat?

Inchworms primarily feed on leaves of a wide variety of plants, including trees, shrubs, and crops. Their diet can be quite diverse, but they are often found on the plants they favor for camouflage. Some species are considered pests due to their voracious appetite for economically important plants.

Are inchworms beneficial or harmful?

Inchworms play a dual role in ecosystems. While they serve as an important food source for birds and other predators, some species can become pests in gardens and agriculture. For instance, the cankerworm species have been known to defoliate trees when present in large numbers, according to agricultural extension services.

How do inchworms defend themselves from predators?

Inchworms use several defense mechanisms to evade predators. Their green or brown coloring provides excellent camouflage against foliage. Some can drop off leaves on a silk thread when disturbed, and others may play dead. Additionally, their erratic and unpredictable movement can make it difficult for predators to capture them.

What is the life cycle of an inchworm?

The life cycle of an inchworm consists of four stages: egg, larva (the inchworm), pupa, and adult moth. The female moth lays eggs on host plants, which hatch into inchworms. After feeding and growing, they pupate, often in the soil or in leaf litter. The adult moth emerges to start the cycle anew, typically living for only a few days to a week.

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Discussion Comments

By candyquilt — On May 02, 2013

@ankara-- There are bacterial sprays that you can buy for inchworms and other types of pests from nurseries. The bacteria kills the worms but doesn't cause any harm to the plants. You just have to make sure that you have the right type of bacteria. So ask specifically for the one that's effective against inchworms.

By bluedolphin — On May 01, 2013

@donasmrs-- They're not so cute when they're eating your trees.

There seems to be hundreds of them in my yard this year. They're happily munching away at my hickory trees. I know that eventually they will turn into moths but my tress will probably be destroyed by then. I need to get rid of these inchworms fast.

What's the best way to do that?

By donasmrs — On May 01, 2013

I have to admit that this is the cutest worm I have ever seen. And the way it moves is very interesting.

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