Old-time farmers usually have plenty of planting advice, such as "When the daffodils bloom, it's time to plant peas," or "Plant corn when oak leaves are the size of a squirrel's ear." Experience has taught them that seasonal changes correlate with animal behavior and plant growth. This kind of folklore is based on phenology: the relationship between the annual cycles of plants and animals and how they respond to seasonal changes in the environment.
Phenology has been used for ages in gardening and agriculture to determine when to plant, when pest insects will become a problem, and when plants will bloom. It turns out there is scientific basis for these observations. Modern plant scientists have found that phenology corresponds to a measurement called growing degree days. Growing degree days are calculated by adding the average daily temperature to, or subtracting it from, 50°F. This information provides a way to estimate the timing of certain events, such as when controls for pest insects need to be used to maximize their benefit.
If you want to have some fun exploring the phenology of your region or elsewhere, visit the phenology Web site compiled by a Appropriate Technology Transfer for Rural Areas (ATTRA) - a USDA-sponsored project. You'll find links to sites that chart insect emergence, the bloom times of wildflowers in various regions, the timing of bird and butterfly sighting patterns throughout the country, and much more. The Web site is located at http://www.attra.org/attra-pub/phenology.html
By the way, if you see caterpillars later than usual this fall, you can expect a mild winter.