Hi.
That depends on where the stock is from. More northern stocks support temps at low as almost 0°C, and unfortunately seem to require 0-12°C. Southern stocks (S-Spain, N-Africa) seem to do well (it's not quite sure) with about 12-18°. Stocks from other locations seem to be inbetween. There is still not known sufficiently how exactly the temps have to be regulated during diapause. Often the problems do not arise during winter, but later, when they are "reactivated" (molting problems, unknown deaths), if the larvae are kept too cold or too warm during diapause.
Regards,
Christian