Cankerworms are a common sight, especially during spring and fall, often appearing in large numbers. Many wonder if these caterpillars pose any direct threat to humans. This article clarifies their characteristics and impact.
Understanding Cankerworms
Cankerworms are the larval stage of moths belonging to the Geometridae family. They are recognized by their “inchworm” or “looper” movement, arching their mid-body as they move. These caterpillars typically grow to about one inch in length and can vary in color, appearing green, brownish, blackish, or mottled green and brown, sometimes with stripes along their bodies.
There are two primary species in North America: spring cankerworms and fall cankerworms. While both species have a similar appearance and feeding habits, they differ in their life cycle timing. Fall cankerworm eggs overwinter and hatch in spring, whereas spring cankerworms overwinter as pupae, with adults emerging in early spring to lay eggs that hatch around the same time. Larvae emerge in early spring, typically when tree buds open, and feed for approximately three to six weeks before dropping to the ground to pupate in the soil.
Do Cankerworms Bite Humans?
Cankerworms do not bite or sting people. Their mouthparts are specifically adapted for chewing leaves, not for piercing skin. They are not venomous.
The concern about cankerworms biting often arises from their sheer numbers during outbreaks, when they may drop onto people or outdoor surfaces. While direct physical irritation from their hairs or droppings is not widely documented as a significant human health concern, their presence can be a nuisance. People might find them crawling on their clothes, cars, or outdoor furniture, which can be unsettling but does not involve biting or stinging.
Cankerworm Impact on Plants and Trees
While cankerworms do not pose a direct threat to humans, their primary impact is on plants and trees, particularly through defoliation. These caterpillars feed on the leaves of a wide variety of deciduous trees, including elm, apple, hackberry, oak, maple, ash, and cherry. Initially, young larvae chew small holes in developing leaves, and as they grow, they consume more tissue, often leaving behind only the leaf’s midrib and major veins.
Severe defoliation during outbreaks can stress trees. While healthy, mature trees can often withstand one to two seasons of complete defoliation, multiple consecutive years of heavy feeding can weaken them significantly. This weakening can lead to reduced growth, branch dieback, and increased susceptibility to other pests, diseases, or environmental stressors, potentially resulting in tree mortality. Beyond the damage to trees, large populations of cankerworms can create a nuisance with their silk threads, which allow them to drop from trees, and their frass (droppings) accumulating on surfaces below.