What starts an outbreak is poorly understood. Prior to most outbreaks, localized pockets of defoliation are observed. Within a year or two, these small areas blossom to include millions of acres. Several factors can terminate an outbreak or lessen its intensity. Freezing weather just prior to, during, or following egg hatch, can kill eggs and young caterpillars.
Starvation can occur when millions of caterpillars literally eat all of their food supply.
The resulting starvation leads to disease epidemics. FTC has natural enemies that include parasites, predators and pathogens. The maggots eat the developing pupa inside a cocoon. S. aldrichi becomes very numerous near the end of outbreaks. In many cases the flies become more of a nuisance than the caterpillars.