There was such a large moth flight last July, that streetlights, which attract the moths, and nearby trees were covered with eggs. Usually, the caterpillars emerge from their eggs about the time the first aspen begin to get leaves. Defoliation will be noticeable by the end of May, but the heaviest defoliation and the biggest nuisance will occur in June. The table gives a projected schedule of FTC life stages. The timing varies from year to year depending on weather and location.
Populations collapse due to starvation, predation and parasitism.
In the forest, FTC defoliation usually does not affect forest health because catterpillar populations usually collapse before tree damage occurs. Producing new leaves puts some stress on the tree because it must use more of its energy reserves. This can weaken the tree and make it more prone to other problems. But, in most cases, the tree will survive the defoliation with no other problems. For your trees, the best defense is to keep them in a healthy vigorous condition between and during outbreaks by keeping them well watered and avoiding damaging their roots or trunks.