One of my four captive larvae seems to have shrivelled up and died. This seems to be a familiar problem to those of us who breed iris, occurring in late winter / early spring. But what causes it? Is the bitter east to north-east wind which has plagued us for almost a month responsible?
I need to check the wild larvae again, to see if they're alright, but in three years' recording to date I have only once had a wild larva desiccate.
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I purchased 5 iris larvae last year from a well known breeder and 3 of them have perished in the last few weeks . They were sleeved on a potted sallow , shelterd from the wind but open to rain and snow .The last one fed till early Nov before entering hibernation . I think that the larvae have a critical time which they can stay asleep before their energy store becomes depleted , and a long winter like we have now will weed out the weak (suvival of the fittest ). Last year the larvae were waking a lot earlier and i have seen captive larvae nibbling at the catkins of male sallows ( mainly stamens and pollen ) while waiting for the leaves to appear .This year seems to be totally against them already as they have drying winds ,cold spring and very few sallows budding yet . The 2 larvae i have left look healthy and a slight green colour still , i assume they managed to feed on more nutritious parts of the tree .These are only my thoughts so feel free to comment . If we knew everything about HIM would the fascination be as great ?
Rather an extensive paper by Klok and Crown (1998) appear to believe that exposure (i.e. lack of effective protection) to high wind speed is the main causative effect in any observed high rates of larval desiccation. They report that larval ingestion prior to diapause appears have little effect on mortality rates:
http://www.sciencedirect.com/science/article/pii/S0022191098000523
Appropriate host plant and site selection, therefore appear to be critical factors when determining the rate of desiccation within diapause larvae.
There are four or five references to desiccation in 'Notes and Views' in particular Heslop notes that one of the factors limiting the range of Purple Emperor in the UK could well be dryness, and suggests that it was found most commonly in areas where the annual rainfall exceeds 25". Also that extreme cold in conjunction with extreme dryness is particularly harmful to larvae. However, as if to contradict this, in the same book it is also suggested that the preferred orientation of a sallow for laying must be open to the winds from the North East quarter as ' it seems probable that there is a quality in the combined cold and dryness in the North East air which helps to inhibit virus infection without injury to the larva.' As Dave says all part of the ongoing fascination I guess? Mark.
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