Above are two hibernating larvae photographed in the wild on 2nd January. I was delighted to report that none of the wild larvae I'm following were lost during the cold December. Perhaps the snow and hoar frost had concealed them well, and also drove tits out of the woods and into nearby gardens in search of food. Although my sample size is much smaller this winter, there is a much higher percentage of the mottled grey form this time round (as in the upper picture), and fewer are hibernating alongside buds. They tend to match the substrate they're on, as above.
Wintry conditions, like below, may well be good for hibernating iris larvae -