Look for defoliation caused by larvae feeding gregariously on old and new foliage and tender bark. They tend to defoliate an entire branch before moving on another area of the tree. Fascicle sheaths or tufts of curled, strawlike needles may remain after larvae feed. There is one generation per year. Winter is spent as a prepupa in topsoil or duff.
Adults emerge in early spring and lay eggs in slits in current or previous years' needles.
Eggs hatch in 4 to 5 weeks. A range of larval age groups may be present throughout the summer. Young larvae feed on the edges of needles, leaving a central spine, but large larvae consume entire needles. Damage tends to be worse in years with warm, extended falls. From July through September, larvae reach maturity and drop to the soil to overwinter.