that would depend what you call summer, I've had litters in june that have needed them because of wet cold conditions and it would also depend where you are keeping them, common sense really, get yourself a incubator thermometer,it has a digital read out and a remote sensor on the end of a wire that can be placed in the bed with the bitch, I keep mine around 19*C.
Corkman, I always used to let nature take its course until finding 3 "dead" pups on my kitchen floor at 6-30am one sat morning and another large bull headed pup half hanging out the bitch, I quickly put the 3 "dead" ones in my dre