The familiar “inchworm” is the common name for the larval stage of thousands of moth species belonging to the family Geometridae. This family is one of the largest in the order Lepidoptera, with nearly 24,000 species recognized globally. The caterpillar earns names like “looper” or “measuring worm” from its distinctive movement. Lacking prolegs in the middle section of its body, the larva must draw its rear end forward to meet its front, creating a characteristic loop as it measures its way across a surface.
Global Distribution
The geographic range of the Geometrid moth family is exceptionally broad, encompassing almost every landmass on Earth except for Antarctica. Inchworms can be encountered in nearly any environment that supports vegetation, from coastal scrublands to high-elevation forests. The greatest diversity, however, is often found in tropical and temperate regions. Within these zones, the moths and their larvae are highly concentrated in environments with dense, lush foliage, such as forests, groves, and orchards. The Neotropical region, particularly the wet tropical Andes, is considered a global hotspot for Geometrid species richness.
Preferred Habitats and Host Plants
An inchworm’s specific habitat is intrinsically linked to the particular plant species it feeds upon, as the larval stage is primarily non-mobile and must remain close to its food source. Inchworms are overwhelmingly herbivorous, exhibiting a strong preference for the foliage of woody plants. They are primarily arboreal, living directly on the branches, leaves, and stems of trees and shrubs.
Many species are generalists and will feed on a variety of deciduous trees, including common genera such as oak, maple, elm, and linden. Other inchworm species, like the spruce spanworm, are highly specialized and feed exclusively on coniferous trees. The fall cankerworm, for instance, is known to be destructive to apple and fruit trees, which dictates its presence in orchards and cultivated areas.
Female moths deposit their eggs directly onto their chosen host plants, such as under leaves or in the crevices of tree bark. The larval diet can also extend beyond woody plants to include ornamental shrubs, garden vegetables like cabbage and tomatoes, and fruit crops such as blueberry.
Seasonal Timing of Their Appearance
The presence of the inchworm stage is a temporal event, governed by the moth’s life cycle and the local climate. The inchworm, or larva, is the most visible stage, appearing during the warmer months of the year, spanning from early spring through the summer.
The life cycle begins when adult female moths lay eggs in late summer or fall, often in protected locations on their host plants. Depending on the species, the insect will overwinter in the egg stage or as a pupa buried in the soil or leaf litter beneath the host tree. The larvae typically hatch in early spring, often coinciding with the budding of their preferred deciduous trees.
After several weeks of feeding, the larvae will drop to the ground, often on a silk thread, to pupate in the soil or in a cocoon spun in the leaf litter. Spring-hatching larvae may emerge as adult moths in mid-summer to lay a new generation of eggs. Those that hatch later in the season usually remain in the pupal stage throughout the winter, delaying emergence until the following spring.