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Pesky. What are these worms hanging from the trees?

09 Apr 2021

Pesky. They plagued tree-lined trails this time of the year—those nasty worms hang on silk threads and land on cyclists and those on foot. A short ride is all it takes to get plastered with these pests. Some of these worms can sting and cause welts; otherwise, it's the itch.

Hard to see the silk droplets from a tree. Without sunlight, they are almost invisible
Hard to see the silk droplets from a tree. Without sunlight, they are almost invisible

So—what are these pests? They are the spring cankerworm (Paleacrita vernata), a moth of the family Geometridae. Prevalent in Texas, California, and Alberta, these worms were discovered by William Dandridge Peck in 1795. These caterpillars are about 1 inch (25 mm) in length and come in various colors—predominantly yellow, brown, and green. The darker color allows them to blend with tree branches. The spring canker has two pairs of pro-legs.

The cankerworm before it makes its way to slide down the silk and presents itself as a pest to cyclists and those on foot
The cankerworm on a leaf—photo: icon0.com

What is the lifecycle of these worms?

In early spring, the females crawl from the ground up tree trunks and lay eggs on the branches. Larvae hatch from eggs as caterpillars at a time when new foliage is emerging. They feed for about three to four weeks before they begin producing silk threads as their route to the ground where they remain in a pupa stage until adulthood. They eventually become moths.

After the emergence of adults, the females crawl up to the branches and deposit eggs in clusters of 100s in bark crevices. The eggs remain there until spring.

Two moths born from the cankerworm caterpillars—photo: Smithsonian Environmental Research Center
Two moths born from the cankerworm caterpillars—photo: Smithsonian Environmental Research Center

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