Really? I'm pretty sure I've been chigged in January before.
Ben,
The chigger adult winters over in soil and according to this fact sheet, they don't emerge until soil temps reach 60. Even on a warm day in the winter soil temps are pretty static.
In addition, the adult doesn't feed on humans, the larvae of the next generation do. The larvae would not hatch and survive in the winter temps.
http://ohioline.osu.edu/hyg-fact/2000/2100.html
Ticks winter over as adults too but can become active above 45 degrees. They, unlike our chigger friends, feed at every stage of development.
So above 45 they are on the prowl or "questing" as this fact sheet tells us.
http://www.cdc.gov/ticks/life_cycle_and_hosts.html