Can caterpillars defoliate trees and shrubs?

"Caterpillars defoliate trees" is true of several very different insects with very different consequences, and the marketing that lumps them together is the actual problem. Eastern tent caterpillar builds obvious silk tents in branch forks in spring; fall webworm builds looser webs enclosing leaves later in summer. Both look alarming and both are, for an established healthy tree, a cosmetic, one-season event. Spongy moth (Lymantria dispar, formerly gypsy moth) and winter moth (Operophtera brumata) are invasive species that feed without building an obvious web, and their risk is real specifically because they return, year after year, on the same trees.

Per UNH Extension, fall webworm feeds late in the summer, after most of the tree's photosynthesis for the year is already complete — meaning the tree loses little practical benefit from the leaves it sheds to feeding. Eastern tent caterpillar feeds earlier but on a tree with a full season still ahead of it to refoliate and recover. In both cases, a ragged-looking tree in July is very often a perfectly healthy tree again the following spring, and the damage never touches the tree's survival, let alone a home's structure.

This is where the fear is genuinely justified. Per the USDA Forest Service Northern Research Station, tree mortality from spongy moth usually occurs following two or more consecutive years of defoliation, and can occur after just one year if other stressors — particularly drought — are already present. FIDL-162 documents oak-dominated stands reaching mortality rates above 90 percent under repeated defoliation, with death often delayed three to five years after the outbreak. Winter moth adds pressure on maple, birch, apple, and blueberry, and specifically reduces fruit and flower bud set. New Hampshire has lived through both: the 1981 statewide spongy-moth outbreak, and a serious winter-moth problem on the seacoast before the biocontrol fly Cyzenis albicans brought populations down. Notably, UNH Extension's own post-1981 survey found New Hampshire's mature red oaks lost only about 6 percent on average — a reminder that even the genuinely dangerous species mostly threaten already-stressed trees under sustained, multi-year pressure, not every oak in the yard.

Whichever species is involved, the fix lives in arboriculture and forestry, not structural pest control. Correct identification, monitoring consecutive-year defoliation, plant-health basics, and — where a valued tree faces real repeated risk — an ISA-certified arborist's assessment are the entire honest answer. None of this touches a home's structure, wiring, or systems, and dressing it up as a pest-control matter to sell a service would be exactly the kind of overreach this hub exists to call out.