Many species of caterpillars feed on trees and shrubs. Caterpillars are larvae - the immature stage of moths and butterflies that hatch from eggs throughout the growing season. The most serious of the defoliators occur in early spring when leaves are just emerging. These early spring caterpillars have the biggest impact on tree health.
This time of the year, bud-break and leaf development are fueled by stored carbohydrates that were produced from photosynthesis during the previous growing seasons. Those carbohydrate reserves are replenished when new leaves are produced and starts over the process of photosynthesis. When trees are defoliated early before carbohydrates can be replenished, this causes further depletion of reserves when another set of leaves must be produced in the same year.
Keep in mind that successive years of early defoliation or heavy defoliation in combination with other stresses such as drought, transplanting, or old age can result in decline and death. You should monitor your property as defoliated trees are much more susceptible to attack by secondary pests such as borers, bark beetles and canker and root disease pathogens.