Tent caterpillars, such as the Eastern Tent Caterpillar (Malacosoma americanum), are a common sight in spring landscapes across North America. The most recognizable sign of their presence is the distinctive, white, silky tent they weave in the crotches of tree branches. These conspicuous communal nests, often appearing suddenly in May, lead to immediate public concern about the health and survival of the host tree. Understanding the specific feeding habits of the caterpillar is key to assessing the true level of risk to a tree’s long-term health.
How Defoliation Affects Tree Health
The primary mechanism of damage from tent caterpillars is defoliation, the consumption of a tree’s leaves. These caterpillars are early-season defoliators, beginning their feeding almost immediately after tree buds break in the spring, typically in late April or early May. They complete their larval feeding stage relatively quickly, usually by early to mid-June. The immediate harm to the tree is the temporary loss of its photosynthetic machinery.
A tree stripped of its leaves cannot produce the necessary sugars for energy and storage. However, because defoliation occurs early in the growing season, the tree has a long window of time to recover. The loss of foliage forces the tree to utilize energy reserves stored in its roots and branches from the previous season. This expenditure of stored energy is a significant physiological stress, but it rarely causes the death of a healthy, established tree.
Resilience and Long-Term Tree Recovery
Mature, healthy trees are highly resilient to a single, severe defoliation event. They cope with this early-season stress by producing a second flush of leaves, a process known as refoliation. This second set of leaves typically appears within two to four weeks after the caterpillars finish feeding. The energy required for this refoliation effort is drawn from the tree’s stored carbohydrate reserves.
The true long-term consequence of defoliation is not mortality, but a measurable reduction in vigor and growth. Utilizing stored energy to regrow leaves means the tree has less energy to invest in radial growth, root development, and defense mechanisms. A single defoliation event can significantly suppress a tree’s growth rate for that year and potentially the following season. Repeated, consecutive years of severe defoliation will deplete the tree’s reserves, making it progressively more susceptible to other stressors.
Which Trees Are Most at Risk
Tent caterpillars, particularly the Eastern species, exhibit a strong preference for trees in the Rosaceae family. Primary host species frequently targeted include cherry, apple, crabapple, and plum trees. The Forest Tent Caterpillar, another common species, feeds on a broader range of hardwoods, including sugar maple, oak, ash, and aspen.
Damage becomes severe only when the tree is already compromised or faces multiple concurrent threats. Trees that are newly planted, young, or suffering from existing stress are the most vulnerable to lasting damage. Factors such as drought, disease, poor soil conditions, or construction damage can drastically reduce a tree’s ability to muster the energy for refoliation. Trees suffering repeated, near-complete defoliation for three or more consecutive years may experience branch dieback or, in rare cases, mortality, as their energy reserves become exhausted.