Inchworms

Inchworms fascinated us as children. We watched their measured steps with front legs, followed by back legs, giving them the characteristic arching motion. 

Inchworms, also known as loopers, span worms, and measuring worms, aren’t worms at all. They are the larval form of Geometer moths. The Latin name, Geometridae, means to measure the earth.

Inchworm (Keith Edkins) CC BY-SA 3.0

Inchworm description

​Inchworms are generally smooth, hairless, and about an inch long. (Big surprise, right?) Inchworms have 3 pairs of true legs in front, like other caterpillars, but only 2 or 3 of pseudo ‘prolegs’ in back. Depending on the parent moth, inchworms can be brown, green, or black, and some have vertical racing stripes. Some species have camouflaging projections that make them look like twigs. When disturbed, they hold themselves upright, reinforcing that image. Most adult moths have slender bodies and wide wings that are held open when resting, similar to most butterflies. They tend to be a little over an inch wide and have intricate patterns on brownish wings. The antenna of males are often feathered.

Inchworm on yarrow

The life of an inchworm

​Inchworms start out as the eggs of Geometer moths, laid on the underside of leaves. In spring, the eggs hatch and inchworms start feeding, usually at night. It is this feeding that can cause some conflict between inchworms and gardeners, but we’ll get to that in a minute. Inchworms eat so much that they outgrow their skin and have to molt several times before they’ve eaten their fill. Then, they build a hard shell around themselves where they can pupate into adult moths.

Types of inchworms

Some inchworms, called omnivorous loopers (Sabulodes aegrotata), are particularly fond of avocados. Others, called cankerworms, are particularly destructive to plums and prunes. There are two varieties of cankerworm: the spring cankerworm (Paleacrita vernata) and the fall cankerworm (Alsophila pometaria). Some claim that English sparrows were first introduced into North America to check spring cankerworm populations in Central park, back in the mid-1800’s. Whoops. (When will we learn???) Actually, the St. Johnswort inchworm (Aplocera plagiata) was introduced to California in 2014 to help stop the spread of Canary Island hypericum. We’ll have to wait and see how that one backfires.)

Host plants

There are over 35,000 different types of inchworms worldwide and 1,200 different kinds in North America. Each type of inchworm has a favorite food, or host plant. Common host plants include:

• blueberry spanworms
• Bougainvillea looper (Disclisioprocta stellata)
• cabbage loopers (Trichoplusia ni)
• California oakworm (Phryganidia californica)
• cranberry inchworms
• spruce spanworms

Inchworm damage

A single inchworm won’t cause noticeable damage in the garden, but a bunch of them can wipe out some of your favorite crops. Inchworms generally feed on tender new shoots, fruit, and the edges of leaves, creating a scalloped effect. When disturbed, some inchworms quickly spin a silken thread from their mouth and repel out of danger. Even if you do not see the culprits for yourself, you may see the dark fecal pellets (frass) they leave behind.

Tiny inchworm on raspberry (Beatrice Murch) CC BY-SA 2.0

Inchworm controls

Birds and other natural enemies provide some control. Bacillus thuringiensis (Bt) can be used on heavy infestations. Since moths are attracted to lights at night, you can reduce the chance of infestation by selecting bulb colors that do not attract moths in the first place. According to recent research, warm colored LEDs (yellow/orange) are the best at not attracting pests at night, followed by ‘bug lights’, cool colored LEDs (blue/green), halogen globes, and CFLs, respectively. Providing good air flow and sun exposure with pruning can also reduce inchworm populations.

There’s an Old Wives Tale that says if an inchworm is crawling on you, it is measuring your coffin, but it’s really just looking for food. When I find an inchworm, I feed it to my chickens.