Hibernating Larva

Photo from Dennis Dell, showing clearly the silken pad spun to keep the leaf attached during winter.

Winter dessication in other insects

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 a reasonable hypothesis.

Derek

Dessication

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.

Piers Vigus

Losses of hibernating larvae

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?

The two larvae that have succumbed were in very different positions; one was in a fork, the other on a bud.

So, at present I seem to be losing one a week to predation, and now have 29 left. Nearly all apparent predation has been in one area towards the northern end of the site, where flocks of tits are particularly active.

The things one stoops to in the interests of science! My natural inclination is to put little gauze cages round each larva to keep the predators off...

Matthew

Losses of hibernating larvae

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?

The two larvae that have succumbed were in very different positions; one was in a fork, the other on a bud.

So, at present I seem to be losing one a week to predation, and now have 29 left. Nearly all apparent predation has been in one area towards the northern end of the site, where flocks of tits are particularly active.

The things one stoops to in the interests of science! My natural inclination is to put little gauze cages round each larva to keep the predators off...

Matthew

Larvae predation

Checked all 37 hibernating larvae today, Sun 31st. Regret that 4 had vanished without trace, presumably eaten, and another had shrivelled up and died - this one looked poorly on my previous visit, Sun 17th. So 5 were lost in late Jan, which is disappointing as none had been lost since about Christmas when Percy Bysshe Shelley vanished.

There is a mortality hot spot along the Roman Road. My guess is that there's a flock of tits regularly using that area. The remedy is simple, next time I'm taking Flea with me, my cat. All tits must be crunched.

Of the 4 presumed predated, 3 were by buds and were yellow-green in colour, and the other was on a scar and grey in colour. The latter is (was) one of those photoed by Brother Neil in early Jan - the 3rd larva illustrated in the blog for Jan 13th.

These new losses include Christina Rossetti and Philip Larkin, renowned for being the first poet to use the F word in verse. This is most distressing.

So far, I have lost 9 hibernating larvae to assumed predation, 6 of them in the Roman Road area. I've lost 2 from scars, 7 from buds. A distinct little seat pad is all that remains. And one of my captive larvae has withered up and died. This was an odd larva that never changed into autumn colours.

I now have 32 left. Those by buds are greening up. Please remember them in your prayers.

Matthew.

Bred Specimens emerge early

Hello Dennis

I have always felt that the micro climate of a bred species will encourage faster development.

When we checked the national history museum collection for ‘local’ specimens it was quite apparent that many of the bred pinned specimens from across the country had considerably earlier dates (within the flight period) to those that had simply been captured and then had a pin stuck in them.

This was one of our arguments for why the individual seen in West Norfolk in the 2004 on the 26^th July was the genuine article!

Liz