Note also the comment below from a study by Jason Williams at Miami University (my italics):
This study illustrated that dormancy in overwintering insects that was primarily thought to be an adaptation to conserve metabolic fuels, also may be essential for water conservation. I also examined cold-tolerance and desiccation resistance in three widely separated populations of overwintering E. solidaginis larvae from Michigan, Ohio and Alabama. Larvae from the most northern population had higher concentrations of the cryoprotectant glycerol, were more cold-tolerant, and had lower rates of overall water loss after acclimation to 5 °C. In contrast, southern larvae had lower rates of metabolism and transpiratory rates of water loss after acclimation to 20 °C.
It would seem quite possible then, that anything that induced a larva to slip prematurely out of the dormant state (perhaps unseasonable weather, or unfortunate microclimate effects) would increase its susceptibility to dessication.
Some related studies have also shown that temperature-stressed ova give rise to larvae which are less tolerant of extreme climatic conditions. Again, not specifically in iris but it would seem a reasonable hypothesis.