We examined the relationship between the accumulation of thermal energy and the onset of oviposition and eclosion of the northern deer tick, Ixodes scapularis, and explored the usefulness of comparing degree days (DD) required for larval emergence with area-wide National Weather Service (NWS) data to construct maps indicating where the establishment of this vector tick would be climatologically constrained. Initially, the validity of basal temperatures for egg and larval development was confirmed by prolonged incubations of gravid females and eggs at 6 degrees C and 10 degrees C respectively. Next, the number of DD accumulated in situ from the placement of gravid females to oviposition, and from oviposition to larval emergence, were measured using temperature data loggers placed next to fall- and spring-fed ticks held within individual vials under leaf litter in multiple enclosures located in diverse biophysical regions of Maine. Finally, when it was found that total DD to larval emergence, as measured in ambient air above the enclosures, compared favorably with DD accumulated simultaneously at nearby NWS stations, maps were constructed, based on archived NWS data, to demonstrate where temperatures were sufficient to allow the hatching of larvae both within one season and over the last three decades as I. scapularis has advanced into northern New England.