Winter woollies
The Emperor spends the winter as a larva, hibernating on or close to the sallow plant on which it feeds. Many larvae are lost during hibernation, but the cause of these losses is not always clear. Successful monitoring of larva in the wild is also fraught with difficulty.

 

from Matthew Oates

Checked hibernating iris larvae in my Wiltshire wood yesterday. Although there was no St Valentine's Day massacre, I must report that two larvae were lost to apparent predation during the first half of February and another has shrivelled up and died. This is the second larva that has done that over the last month, and I anticipate more as some others are looking a bit small and shrunken. I should have measured them. Next time I will.
The problem of hibernating larvae shrivelling up and dying is familiar to those of us who breed iris. Does anyone have any idea what causes it? From my limited captive breeding experience the problem seems to be prevalent in some winters, perhaps wet winters; and it occurs mainly in late winter / early spring.
Possible explanations are:- 1) winter weather - is desiccation associated with particular types of winters? 2) Poor food plant quality during the autumn? 3) some form of virus, which may be weather triggered?

 

 

 

from Piers Vigus

In response to MO's interesting report, dessication certainly seems to be a problem with iris larvae during certain winters, certainly in captivity, and it would appear in the wild also. I imagine that the problem is complex; could this possibly linked to the significant desiccating effects of repeated frosts as well as a other factors affecting microclimate? Conversely in mild damp winters mould is also a significant threat to the over wintering larvae, although this phenomena may be greatly exaggerated in captivity due to the limited air circulation in a netting sleeve.


Camilla also suffers significant losses due to apparent dessication (larvae shrivel and harden) during some winters, both in the wild and in captivity, almost always towards the end of the winter, approaching the time at which one would be expecting to see the larvae preparing to become active again. I wonder if the larval mortality rate in iris during this phase is the factor that has the most influence upon whether or not we have a 'great' iris year.Matthew's study is certainly raising some interesting questions.

from Derek Longhurst

Dessication is certainly an issue for over-wintering insects. A number of studies have been made of the goldenrod gall fly larva, which appears to be able to survive extreme winter dessication scenarios by exuding a hydrocarbon coating.

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 possible.