Our County Butterfly Recorder, Robin George writes: Portland Bird Observatory alerted us to a question about Brimstones recently in a post on their website :

“We’re forever puzzled by the status of the Brimstone butterfly at Portland. As far as we understand it – or at least this is what Good’s Flora of Dorset tells us – the two larval foodplants, Buckthorn and Alder Buckthorn, are absent from the island so you’d expect the butterfly to be quite a scarcity; however, every year seems to produce a really pretty decent spread of records – this year alone we’ve heard of at least half a dozen sightings to which this one today in the Obs garden is the latest addition. The butterfly’s a powerful flyer so are these all strays from the mainland – or even, like the one watched arriving in off the sea last year during a migration of whites, Red Admirals and Painted Ladies – immigrants from the continent, or are we overlooking a small but mobile resident population that are established on specimens of one or other of the foodplants that have escaped the attention of the botanical recorders?”

I looked through the Dorset records and find that with the exception of kilometre square SY6871, every kilometre square on Portland has records of Brimstone. The southern tip of Portland, round Portland Bill has fewer records, but the main area of Portland has regular sightings. Brimstones have been seen at King Barrow Quarry, Broadcroft Quarry and  Tout Quarry. There are several sightings from Fortuneswell. There are no sightings along the road and tracks between Weymouth and Portland, even though I find that the Brimstone is one of the few butterflies that can easily be recorded while driving.

Distribution of Brimstones on Portland 2020-2024. Yellow squares are for a single record, orange squares are for 2-9 records.

We’re wondering if we have Dorset Butterfly Conservation members, or other butterfly lovers who have planted Buckthorn plants in their gardens to attract Brimstones. If you know anything, we’d love to hear from you. Brimstones are on the wing most months of the year, but numbers grow March to May, then there is a second smaller peak in July and August; see our Brimstone page for more information about this species.