Peristenus digoneutis was discovered in one new state (Maine) and in 15 new counties in Maine, New Hampshire, New York, Pennsylvania, and Vermont during 1997–1999. These new records increased the number of counties where this parasite has been found by 42%, to a total of 51 counties in eight states. No permanent establishments of P. digoneutis have been found south of New York City, which suggests that high summer temperatures are limiting its spread southward. A good agreement was found between the parasite's southernmost establishment locations and a maximum summer temperature/duration of 30°C (86°F) for 14 to 30 days. This temperature limit suggests that P. digoneutis can disperse westward both north and south of Lake Erie, across southern Ontario and northern Ohio, to Michigan and Wisconsin. Establishment south of New York City (latitude 40.5°N) is unlikely, except possibly along the higher/cooler elevations of the Appalachian Mountains.