Saturday, June 9, 2012

LATE IRIS My iris have not really caught up. Three will not pupate before the third week in June at the earliest, and a fourth looks like an 'April larva'. The picture shows one of the mature larvae near to the underdeveloped fellow


Guy said...

That reflects very much the state in my local woods in 2010, when one caterpillar didn't progress to 5th instar until June 21st, the same day I found my first iris pupa. Another caterpillar I was following in the same woods made 5th grade on 28th May. I put it down to different aspect and shading by other trees but yours will have lived the same conditions, presumably. Is this a natural staggering technique? Guy

irisscientist said...

Guy. It has long since been documented that the males of many butterfly species emerge before the females (for examples, see Wiklund, 1977, link below) and this situation is especially prevalent in species where the males need to sequester essential compounds prior to mating. The larvae in the images posted by Dennis (and no doubt have been mirrored by many larvae in captivity and the wild) however definitely show much more than a 7-10 day delay which would be expected in such a male only, pre-emergence scenario. In this particular occasion I propose that the high degree of larval development variance currently being observed this year can certainly be explained due to exceptional weather conditions, bordering around key diapausal rousal temperatures early on in the season. Key factors controlling larval emergence from their diapausal states being temperature and photoperiod. Consequently, inconsistent and transient early high season temperatures, followed by a sudden and sustained temperature drop within a narrow photoperiod range most likely would have resulted in some larvae being triggered to emerge ‘early’ from their diapausal states, whilst other ‘late’ larvae (even within the same larval batch), remained firmly within their diapausal states until warmer weather once again returned. The essential factors to induce such a variance in larval development would definitely be the exact temperature and duration of such a rise and sudden fall in temperature within a specific photoperiod range, both of which I believe may well have occurred this year.

The link to Wiklund (1977) can be found here: