The CY Field and Clouded Yellow irruptions

Regular readers will recall that a friend and I discovered a patch of set-aside rich in wildlife. The patch is on farmland adjacent to a nature reserve that we frequent and aside from the roe deer, Chinese water deer, Great White Egrets, Grey Herons, Marsh Harriers and Hobbies, one day, had a couple of dozen Clouded Yellow butterflies. There were also the more common Common Blue, Whites, and Brown Argus on this same patch, nectaring and mating on it in the summer of 2022. We dubbed this patch the CY Field, as an abbreviation for our chats about wildlife photography.

The same field was savagely mown in 2023 and again in 2024, but in 2025, it was left as set-aside once more. By the autumn, there were plenty of nectar sources and wildlife photographer friend Andy spotted Clouded Yellows on a couple of visits. I made a trip with Mrs Sciencebase to what we call the CY Field on 2nd November within a few seconds of arrival Mrs Sb had spotted a male CY, I scanned the patch for half an hour or so and counted another half a dozen individuals.

I was curious as to what the chances were of a new influx of CYs finding this exact same patch three years later. They would seem to be vanishingly small. However, the chances of the butterflies we have seen these last few days being descendants of the 2022 irruption are even smaller. Successive generations would have to have survived the razing of the ground two years on the trot, as well as three winters. While the winters of 2022, 2023, and 2024 were fairly mild with barely any snow between them, the CY does not have the physiology to move into diapause, it can’t hibernate like a Red Admiral, as I mentioned earlier. Moreover, the pupae are susceptible to damp and any frost and simple could not reach maturity in the following spring-summer.

The CYs I saw yesterday and ones that others have seen on this patch over the last few weeks must just be a minor irruption and somehow they’ve found this same tiny patch of (different) flowers. It is known that migrants sometimes turn up from the Continent in the autumn, but chances are these are offspring of migrants that arrived in the spring and bred locally.

Either way, butterflies that emerged as adults thousands of miles away, perhaps in the South of France, or maybe even Italy, have flown across the channel and the width of East Anglia to end up on the same tiny patch of wildflowers that their relatives nectared and mated on back in the summer of 2022.