The gypsy moth, Lymantria dispar dispar, is an invasive insect species in North America, originally introduced from Europe in the late 1800s. Its caterpillars are notorious for defoliating over 300 species of trees and shrubs, particularly in the eastern United States. This extensive feeding can cause significant damage to forests, sometimes leading to tree mortality if defoliation persists over multiple seasons. Despite their destructive potential, various natural predators and diseases play a role in regulating gypsy moth populations.
Avian and Mammalian Predators
Birds prey on gypsy moths at various life stages. Yellow-billed and black-billed cuckoos, blue jays, orioles, and rufous-sided towhees feed on caterpillars. Black-capped chickadees consume egg masses, reducing initial populations. Robins, nuthatches, and grackles also eat gypsy moths, especially during outbreaks.
Small mammals also contribute to gypsy moth control, especially by targeting larvae and pupae found on the ground or at the base of trees. White-footed mice, shrews, and voles feed on these stages. Mice prefer larger female pupae, impacting the overall population. Chipmunks, skunks, raccoons, and squirrels also consume gypsy moth larvae and pupae. These mammalian and avian predators help to reduce localized populations, especially when gypsy moth numbers are not at outbreak levels.
Insect Predators and Parasitoids
Insects and spiders are natural enemies of the gypsy moth. Predatory insects like ground beetles, ants, and various predatory bugs consume gypsy moth eggs, larvae, or pupae. The Calosoma sycophanta beetle, an iridescent green species introduced from Europe, is a specialized predator whose adults and larvae feed almost exclusively on gypsy moth caterpillars and pupae. Ants prey on young caterpillars, and many spiders will consume gypsy moth larvae or pupae.
Parasitoids, including certain wasps and flies, play a different but important role. They lay eggs inside or on the gypsy moth; the developing larvae consume and kill their host. For example, the tiny wasp Ooencyrtus kuvanae parasitizes gypsy moth eggs, often killing 20% to 30% of the eggs in a mass. Parasitic flies, such as Compsilura concinnata and Parasetigena silvestris, lay their eggs on or within gypsy moth larvae, with emerging fly larvae developing inside the caterpillar. Wasps like Glyptapanteles liparidis and Apanteles melanoscelus parasitize early larval stages, while others target pupae.
Microbial Control Agents
Pathogens and diseases regulate gypsy moth populations. The gypsy moth nucleopolyhedrovirus (LdNPV) is a naturally occurring virus specific to gypsy moth larvae. This virus causes a fatal disease, particularly during high-density outbreaks, leading infected caterpillars to die and hang in an inverted “V” shape from trees. Their bodies then liquefy, releasing viral particles that can infect other larvae, spreading the disease. LdNPV is always present and contributes to their decline when numbers are high.
The fungus Entomophaga maimaiga causes widespread mortality in larval populations. This fungus was introduced from Japan and is widely established in North America, particularly effective in wet conditions. Caterpillars infected with E. maimaiga typically die hanging head-down from tree trunks, and their stiff, dry bodies release spores that can overwinter. The fungus spreads through both resting spores and conidia, airborne, infecting other caterpillars, especially during frequent rainfall.
The Interplay of Natural Controls
The management of gypsy moth populations is not dependent on a single natural enemy but rather a complex web of interacting factors. Birds, mammals, predatory insects, parasitoids, and microbial agents collectively exert pressure on gypsy moth numbers across different life stages. While individual predators or pathogens may have localized impacts, their combined action contributes to broader population regulation. For instance, the presence of the Entomophaga maimaiga fungus can affect the dynamics of the LdNPV virus, with both diseases potentially occurring simultaneously during outbreaks.
This intricate network of natural controls often leads to boom-and-bust cycles in gypsy moth populations. Outbreaks, characterized by high densities of caterpillars and widespread defoliation, are eventually suppressed by the cumulative effects of these natural enemies. The increased availability of hosts during an outbreak allows predator and pathogen populations to increase, eventually causing a collapse in gypsy moth numbers. Maintaining biodiversity within forest ecosystems is important, as it supports a greater variety of these natural control mechanisms, fostering more resilient and balanced environments.