2009 - A wonderfully clouded yellow year

Every so often we enter a clouded yellow year.  In this respect 2009 turned out to be wonderfully clouded, what delight!  The Clouded Yellow (Colias croceus) is a vibrant yellow and accomplished migratory butterfly. It is one of the most widespread butterflies in Europe and it is quite common in the northern parts of Africa through southern and eastern parts of Europe. Being a very able flier it can migrate thousands of kilometers in summertime, so it can be found in the more northern parts of Europe also, in scarce numbers. In good years it has been seen as far north as the northern Scotland and the Faroe Islands. Usually extremely rare in Denmark, some years not present at all, a small invasion hit us this summer, though it was nothing compared to tales of yonder where huge saffron yellow swarms blocked the sun from the sky or covered cliffs:

“While sitting on a cliff near Marazion in Cornwall, he perceived out at sea a yellow patch which, as it came nearer was seen to be a swarm og Clouded Yellows making for the land, flying low over the water and rising and falling over the crest of each wave, The cliffs were soon  covered with them, and they stayed in the neighbourhood  for several weeks.” - from Vere Temple

I was lucky myself to find a few fields of Lucerne with visiting Clouded Yellows. The individual above was photographed on a 70 ha field, energetically and determined cruising just above the flowers, dropping down here and there to lay a few eggs.

In their all year homes the Clouded Yellow can be found in all sorts of habitats, the most decisive factor seems to be the presence of a lot of flowers rich in nectar.  In most cases in Denmark, the individuals have been sighted on - or very near – cultivated fields of Alfalfa / Lucerne (Medicago sativa). Due to our cold winters the Clouded Yellow is not capable to survive hibernation here, though we do see several generations through summer when we are lucky enough to be invaded. I found a single mention of Clouded Yellows surviving hibernation in south England, so who knows how soon before we can see overwintering clouded yellows here, now with climate change and all.

I brought a few eggs home to rear, to see them through larval and pupal stages and hopefully getting good images of the imagos first seconds entering this world. I kept the larvae in a terrarium with fresh Lucerne, Red Clover (Trifolium pratense) and Greater Birdsfoot-trefoil (Lotus pedunculatus)(by coincidence I found some of these, and thought I’d give it a go, having read that the larvae could feed on Birdsfoot-trefoil (Lotus corniculatus)). It turned out the larvae preferred Lucerne, not feeding on Clover at all. I did observe it feeding on Greater Birdsfoot-trefoil a single stretch of a few days, before it moved back to Lucerne again.

Clouded Yellow (C. croceus) larvae.The larvae gains a bit more colour as it ages. Here the yellow stripe broken by orangespots is quite apparent.

As a funny little sidenote I observed some heavy feces flicking! I had only read about this function in Skippers (Hesperiidae), when they are hiding out in a rolled up grass leaf tube.  I have no idea how common it is amongst butterflies in general, but it appears that others than Skippers do this. The telltale sign was a perimeter of small dark fecal pellets all the way around the plants with the larvae on. As described in Skippers, the larvae have an anal compartment where they can raise blood pressure, and shoot of the fecal pellet from the anal plate. The odours seeping from the fecal pellets could possibly attract parasites and predators, so shooting the pellets away like this could be beneficial for the survival of the larvae in its stealthy shelter. The mechanism allows the larvae to flick the feces out of its home without having to move outside the safety of its shelter. Pretty cool, eh?!

Back to my pet butterfly… At day 28 after being layed as an egg by the mother yellow, the larvae hung itself up ready to pupate. It spun a silken thread around a Lucerne stem as a loop to support it, and spun a small patch to a leaf which it could cling on to. This was in the morning before I left for work. When I came back it was pupated.

Larvae and pupae of Clouded Yellow (C. croceus). A) The larvae has readied itself for pupation. B) The young pupae is fresh green, without much else colouring. C) The old pupae is green over yellow to orange with hints of pink. The butterfly emerged the day after this shot.

As with the larvae, the older the pupae gain more colour, especially in the orange and pink range.
After 12 days the pupae hatched. Low and behold, out came the most beautiful brightly coloured butterfly. The bright saffron yellow wings, with pink borders and a single silvery spot are amazing.

Colias croceus - a saffron yellow beauty

The pictures in this post are “captive photos” of the outcome of one of these eggs, which I brought home to rear, except the photo of the mother Clouded Yellow laying it.  They were simply too energetic for me to get any proper photos in their natural habitat, though others have succeded magnificiently (Clouded Yellows én masse) at this. I’ll just have to try a little harder next time!

Releasing the Clouded Yellow - Posing on Lucerne

The two spots of sunlight shining through the top corner of the wing reveals this individual to be a female

• “Faecal firing in a skipper caterpillar is pressure-driven” - Caveney, S., McLean, H. and Surry, D. – 1998 - The journal of experimental biology 201, 121-133

• “Good housekeeping: why do shelter-dwelling caterpillars fling their frass?” – Martha R. Weiss – 2003 - Ecology letters 6:361-370

• “Butterflies and Moths in Britain” - Vere Temple - 1945

• “Nationalnyckeln DE 50-54 Fjärilar: Dagfjärilar” - Arsdatabanken - 2005

• “The Butterfly Book” - W.J.Holland - 1905