The spongy moth, Lymantria dispar, is an invasive insect whose caterpillar stage defoliates hundreds of different tree and shrub species across North America. This pest, formerly known as the gypsy moth, was introduced to the United States in 1869 and continues to cause widespread damage during periodic outbreaks. The severity of the damage depends heavily on the type of tree the caterpillars encounter, as they display distinct preferences for certain foliage. Understanding this hierarchy of host plants is important for managing infestations and protecting forest ecosystems.
Trees They Prefer Most
Spongy moth caterpillars exhibit a strong preference for specific hardwood species, which provide the optimal combination of nutrients and leaf texture for their development. The most frequently and severely damaged trees are those in the Quercus genus, making oaks the primary and most desirable food source. These caterpillars thrive on the foliage of species like white oak, black oak, and burr oak, often leading to complete defoliation during an outbreak.
The preference for oaks is partly due to the leaf chemistry, which supports rapid caterpillar growth and survival. While the caterpillars will consume a wide variety of leaves, they perform best on foliage that presents a favorable balance of nitrogen and carbohydrates. High-density infestations on these trees can result in defoliation within days, and two consecutive years of heavy feeding can cause significant stress or death, especially if the tree is already weakened.
Beyond oaks, other hardwoods are also classified as highly susceptible hosts. This group includes apple, birch (especially gray and white), willow, aspen, and linden. Fruit trees like apple and crabapple are particularly vulnerable and can be quickly stripped of their foliage. The presence of these highly preferred species is a significant factor in determining the severity and longevity of a local spongy moth infestation.
Secondary and Occasional Host Trees
When the most preferred hosts are depleted or during times of high population density, spongy moth caterpillars will expand their diet to include secondary and occasional host trees. These species are generally less palatable to the younger caterpillars but are consumed by the older, more mature larvae. This feeding behavior is often a sign that the local food supply from the primary hosts is running low.
This secondary group includes maples, such as red and sugar maple, cherry, beech, and elm. While these trees can be defoliated, they typically sustain less damage than oaks unless the infestation is overwhelming. The damage to these trees usually occurs later in the season as the caterpillars grow larger and become less selective about their food source. Hardwoods in this category, having been defoliated, will often produce a second flush of leaves, which uses up the tree’s energy reserves.
Conifers, or evergreens, are also included in the occasional host category and are especially vulnerable once the caterpillars start feeding on them. Species like pine, spruce, and hemlock are typically avoided by younger larvae, but older caterpillars will consume their needles. Unlike hardwoods, evergreens cannot easily regrow their needles after a complete defoliation. A single season of heavy feeding on conifers can be fatal because they cannot quickly produce a new crown of foliage.
Trees Spongy Moths Will Not Eat
A number of tree species are considered resistant or virtually immune to spongy moth feeding because their foliage is unpalatable or toxic. These trees contain natural chemical compounds or have leaf structures that repel the larvae. This resistance makes them safe choices for homeowners and land managers in areas prone to outbreaks.
Among the most reliably avoided species are ash, sycamore, tulip-poplar, and black locust. These trees possess chemical defenses that discourage feeding, meaning the caterpillars cannot sustain themselves on their leaves. The list of avoided trees also includes dogwood, catalpa, redbud, and tupelo.
Certain evergreens are also rarely fed upon, including arborvitae, juniper, and fir. These non-host species maintain their foliage and health during even the most severe spongy moth outbreaks. Maintaining a high proportion of these resistant trees in a landscape can help mitigate the overall ecological and aesthetic impact of a local infestation.