The spongy moth (Lymantria dispar) is an invasive insect that has become a significant forest pest in eastern North America. As larvae, these caterpillars are voracious defoliators, capable of stripping the foliage from hundreds of tree and shrub species, particularly oak. Severe defoliation weakens trees, making them susceptible to other pests and diseases, often leading to mortality. This raises the question of whether birds serve as effective natural predators to control this destructive insect population.
The Caterpillar’s Natural Defenses
The primary reason most common songbirds avoid the spongy moth caterpillar is the dense covering of tiny, barbed hairs, known as setae. These hairs act as a physical and chemical deterrent, making the larvae unpalatable and difficult to manage. When ingested, the setae cause significant irritation and inflammation to the sensitive lining of a bird’s digestive tract.
Most North American bird species prefer non-hairy caterpillars and consistently choose alternative prey. This widespread avoidance mechanism allows the caterpillars to thrive unchecked, even in areas with high bird populations. The defensive hairs can also irritate the thin skin around a bird’s eyes upon contact.
Avian Species That Consume Spongy Moths
While many birds avoid the mature larvae, a few specialized avian species have adapted methods to successfully consume the spongy moth caterpillar. The most notable exceptions are the Yellow-billed and Black-billed Cuckoos, which are known caterpillar specialists. Cuckoos possess a unique physiological adaptation: they can periodically shed and regrow the inner lining of their stomach, effectively eliminating the buildup of irritating setae.
Other birds have also been observed feeding on the caterpillars. These species often employ behavioral strategies to bypass the defenses, such as rubbing the caterpillar vigorously on a branch or substrate. This action helps remove the dense hair coating or ruptures the skin so they can consume only the soft internal tissues.
Specialized Consumers
Birds observed consuming the caterpillars include:
- Blue Jay
- Baltimore Oriole
- Common Grackle
- Downy Woodpecker
- Gray Catbird
Even with these specialized feeders, their impact is usually localized. They seldom consume enough caterpillars to significantly control an outbreak once populations reach high densities. Birds are most effective as part of a complex of natural enemies that help regulate the pest when numbers are low.
Timing and Vulnerability in the Life Cycle
The spongy moth is most vulnerable to avian predation during its less-defended life stages. Predation is common on the tan-colored, fuzzy egg masses, which are laid in the late summer and overwinter on tree bark. Birds like Black-capped Chickadees and nuthatches peck at these egg masses throughout the winter to extract the individual eggs.
The newly hatched larvae, or early instars, are also susceptible because they are very small and their defensive hairs are not yet fully developed. A broader range of generalist insectivorous birds readily consume these tiny caterpillars before they grow into larger, heavily armored stages. This early predation is most effective just after the eggs hatch in spring, before the larvae disperse.
Finally, the pupal stage, where the larva transforms into the adult moth, offers another opportunity for bird consumption. Although many pupae are hidden, species like Hairy Woodpeckers and Eastern Wood-Pewees occasionally feed on them. This vulnerability means that birds contribute to population control across the entire seasonal cycle, not just during the peak caterpillar feeding period.