Large heath © Pete Richman
It is always a nervous moment when we step onto site at the start of the large heath survey season. Winmarleigh Moss may be a stronghold for the butterfly, but June and July of 2024 were a washout, resulting in few surveys being completed and even fewer sightings recorded. Just how much of an impact did the persistent inclement weather of last summer have on the population?
I suppose we are about to find out.
Janet Packham
A single large heath butterfly sits on a blade of dry, dead purple moor grass, basking in the late morning sunshine. The fact that I am kneeling close by and waving a camera phone in its face does not seem to faze it at all.
Sitting with its wings closed, the underwings are a mix of brown, white and orange and I can clearly see the black ‘eye’ dots that are ringed in yellow. The large heath is a true bog species, which makes it one of the UK’s rarest butterflies. It is always a thrill to see the first of the year. However, it is unusual to spot one in the first ten minutes of the new survey season; they normally make us work a little harder.
It is a joy to see them. It is also a relief to know that they have come through another winter.
Cotton grass on Winmarleigh Moss - Credit: A.J.Critch Wildlife
As I stagger to my feet, the butterfly takes to the air in its tell-tale erratic flight pattern, disappearing into the heather on the other side of the bund.
Halfway across the first section of the survey transect, the same route that we will walk for the whole season, two more are on the wing, then a third, while the lattice heath moths are also vying for some attention.
In recent years, large scale re-wetting works have been completed across Winmarleigh Moss, resulting in the natural peatland vegetation such as cotton grasses, heathers and sphagnum mosses recolonising areas of the site where the water table is now higher and stable.
It is a joy to see them. It is also a relief to know that they have come through another winter.
The last section of the transect was, not so long ago, a large expanse of bare peat. Today, the white heads of common cotton grass bob in the wind that is keeping the morning temperatures in check. Vast numbers of round-leaved sundews have moved in to cover the exposed peat and cross leaved heath – the key food plant for the species – is in bloom.
Round-leaved sundew - Credit: A.J.Critch Wildlife
Cross-leaved heath by Ross Hoddinott/2020VISION
And the butterflies have followed.
Sitting on one of the bunds, that were constructed across the site to prevent water loss and raise the water table, to eat lunch, a large heath zigzags around and over the heather close to my outstretched feet. Only a couple of years ago, we never recorded these butterflies in this area. Now they are here in abundance.
Of course, raising the water table across the site has been important to protect the peat and reduce the carbon dioxide emissions that dried out peat emits, but it has also created conditions that has allowed the large heath butterfly to march across Winmarleigh Moss to expand its range and significantly increase its chances of survival.
As I return to the car, this is the first survey of many over the coming weeks, and I am looking forward to seeing large heath numbers soar across the peatland as we head into the summer.
Large heath © Pete Richman
