The organism commonly known as the gypsy moth is now often referred to as the spongy moth (Lymantria dispar). This insect is a prolific pest of hardwood forests, but the vast majority of its life cycle does not involve feeding. The only life stage responsible for extensive defoliation and damage to trees is the larva, or caterpillar. This voracious larval phase must consume enough foliage to fuel the entire, non-feeding adult stage.
The Larval Diet: Preferred Host Trees
The spongy moth larva is highly polyphagous, meaning it feeds on a wide array of plants, but it clearly prefers certain deciduous hardwood species. Oaks (Quercus species) are the most favored food source for the caterpillar throughout its development. This preference for oaks, such as white oak and red oak, is a major factor in forest susceptibility to outbreaks.
Other preferred hosts include apple, birch, willow, aspen, and basswood. The caterpillar’s feeding behavior changes as it matures through its developmental stages. Newly hatched, small larvae prefer the tender, nutrient-rich new leaves that emerge in the spring.
As the caterpillars grow larger, they consume tougher, mature foliage. Older, late-stage larvae cause the most significant defoliation, sometimes consuming up to one square foot of leaves per day. This consumption helps the larvae store the energy reserves necessary for pupation and the adult phase.
Secondary and Contingency Food Sources
While a core group of hardwoods is favored, the spongy moth can feed on over 500 different plant species when preferred food is exhausted. During high-density outbreaks, caterpillars readily switch to secondary hosts like maple, hickory, and cherry. These species are considered intermediate in susceptibility and generally sustain little injury unless the infestation is severe.
Conifers, such as pine, spruce, and hemlock, are also consumed, but primarily by older, larger larvae. Younger caterpillars often cannot survive on conifer foliage, possibly due to chemical defenses or nutritional deficiencies. Certain tree species, including ash, sycamore, and tulip poplar, are consistently avoided and remain green islands in a defoliated forest.
Why Adults Do Not Feed
The adult spongy moth has a short existence, lasting only one to two weeks, and does not feed. The adult stage possesses non-functional, or vestigial, mouthparts, meaning they cannot ingest solid food. Their sole biological purpose is reproduction, relying entirely on energy reserves accumulated during the larval stage.
Adult females are flightless; they emerge, mate, and lay their eggs nearby using the stored energy. Although they do not consume foliage, adult moths can imbibe moisture, but this provides no energy or nutrition.
Impact of Defoliation on Tree Health
The consequence of the caterpillar’s feeding is defoliation, which severely limits the tree’s ability to perform photosynthesis. This loss of foliage depletes the tree’s stored food reserves, resulting in reduced growth and physiological stress. Healthy hardwood trees, like oaks, can often withstand a single season of complete defoliation by producing a second flush of leaves later in the summer.
Refoliation further drains energy reserves, and repeated defoliation over two or more consecutive years increases the likelihood of mortality. Stressed trees become vulnerable to secondary threats, such as attack by bark beetles like the two-lined chestnut borer, and root diseases. Conifers are particularly susceptible because they are unable to produce new needles after severe defoliation, often leading to death after a single severe attack.