In the wild the juvenile larvae would burrow into the soil to find suitable warmth in order to over-winter. Temperatures at about 50F will cause the larvae to go completely dormant. If the temperature drops below about 35F for more than 4 hours unprotected juvenile larvae will typically perish. Likewise if the temperature of an active bed goes above 113F for more than a couple of hours juvenile larvae will die.
The juvenile stage will also be extended during cooler weather when the temperature falls below F. When the temperature drops below you will likely find your juvenile BSFL will slow down or stop eating and moving as they enter a near hibernation state.