Brimstone (Gonepteryx rhamni)
Cheshire local information
When to see: April-September
Where to see: Scrub, wetland, broad-leaved woodland
Caterpillar food plant: Alder Buckthorn (Frangila alnus)
Cheshire status: A fairly common resident in the east of the county, but scarcer in the west. As recently as the mid-1970s there were no known breeding sites in the county. Single-brooded flying from late July to September, but most frequently recorded following emergence from hibernation in early spring. Breeds on Alder Buckthorn.
It is thought that records of Brimstone in Cheshire prior to the mid-1970s were wanderers from breeding colonies beyond the county borders as the butterflies (especially the males) roam over large distances in search of females or Alder Buckthorn.
This butterfly breeds on Alder Buckthorn or Buckthorn, although there are only a few isolated trees of Buckthorn in Cheshire. Alder Buckthorn has always been a widespread tree in the east of the county, but absent from the west.
It is not certain why the Brimstone became established in Cheshire from the mid-1970s, but soon after that it is known that a former member of the Cheshire and Peak District branch bred and released many in the Holmes Chapel area - possibly this has helped the species become established in many parts of Cheshire. No doubt the only thing stopping its spread further westwards is the lack of its larval foodplant.
In the 1980s another branch member managed to get hold of many Alder Buckthorn plants that members planted in their gardens - Brimstones seem to quickly find these plants and breed on them. The branch has also been involved with helping rangers plant Alder Buckthorn on their reserves. The last time this happened was some planting near the Anderton Boat Lift in Northwich.