The caterpillars emerge from their overwintering sites in April or May and begin feeding on newly-developing foliage. Young larvae mine within the sheaths surrounding new needle bundles. The larger larvae are more general feeders but also only on new foliage. The larvae continue to feed as the foliage develops, going through a succession of moults before reaching the pupal stage in late June. The feeding larvae produce a great deal of silk that surrounds the bases of the needles on which they feed.
Damage is caused by the larvae feeding on the needles.
If there are many larvae, they can eat much of the new foliage on a tree. If this happens several years in a row, serious tree damage can result. Severely affected trees have thin crowns with conspicuous bare stubs at the branch ends. Many trees severely stunted by ponderosa pine budworm can be found in the major outbreak areas. The ponderosa pine budworm normally is not considered a tree-killer.