Brimstone

Gonepteryx rhamni

General Distribution and Status

It can be found south of the line between Cheshire and Lincolnshire although the occasional vagrant may occur further north as far as Scotland. As far as its range and abundance are concerned it is probably the most stable of all our butterflies in Britain during the last century or so in spite of habitat destruction due to farming intensification. Being a strong flier, it can travel long distances to colonise new sites if existing breeding grounds are no longer suitable. In Hertfordshire and Middlesex, changes in the butterfly's distribution and abundance are usually quite small year by year although populations fluctuate more widely at each locality.

UK distribution map
UKBMS Species summary (2020)

Habitat Requirements

The Brimstone can be found almost anywhere in the wider countryside although in the early spring they are more likely to occur in wooded areas where most adult specimens hibernate. In the late summer and autumn, they are often observed feeding on nectar sources in readiness for hibernation. Its main larval foodplant, away from calcareous grassland, is Alder Buckthorn Frangula alnus. In the north west of the county, on Hexton Chalk Pit for example, the chalky soils support Purging Buckthorn Rhamnus catharticus, another most widely used foodplant.

Larval foodplants

Alder Buckthorn Frangula alnus, Purging Buckthorn Rhamnus catharticus. Other foodplants which are very rarely taken for egg-laying are Curled Dock Rumex crispus (Sawford) and possibly Dogwood Cornus sanguinea (Murray & Wood 2010).

Adult Food Sources

Buddleia Buddleja davidii (124), Wild Teasel Dipsacus fullonum (112), Runner bean Phaseolus coccineus (80).

Historical Records

The butterfly was listed in Gibbs' 1902 survey and Foster's 1934 report notes it as 'common'. Ferry found the Brimstone in 'numbers' on 26 April 1948 at Knebworth Great Wood. Bell's report for 1972 states 'not seen so many second-brood Brimstones as in 1972 but they were scarce in spring 1973 after a long mild and dry winter'.

Local Distribution and Abundance

The Brimstone occurs widely in the Stevenage area as shown on the map. The last record reported in the TL22B tetrad was on 24 April 2004 but I suspect few searches have been made since then. The wooded areas around Langley should be suitable sites for finding the butterfly. 2021 and 2022 saw some very early sightings due to some mild weather in the winter and early spring including one reported from Watery Grove on 27 January 2022, the earliest ever date in the Stevenage area.

Transect weekly counts

Stevenage (South Fairlands Valley Park) transect 1993-2023

Although the chart shows fluctuating fortunes on a year by year basis the butterfly's abundance on my transect is comparatively stable. The total for 2014 was boosted by a count of 10 individuals on 29 March, before the official start of the transect season! The insect is often found along the edges of Whomerley Wood and Monks Wood with the vast majority appearing in the spring.

Knebworth Park transect 1996-2010 and 2017-2023

Similar variations in numbers are apparent as for the Stevenage transect but the status here is stable. In 2017, 13 were recorded which is slightly above the average of about nine in the 1996-2010 period. Most in 2017 were seen in the north-western part of the park. In 2019, 5 specimens were seen on 29 March, before the start of the transect season. On 9 August 2023 7 individuals were seen, 6 of which were together at Burleigh Bottom.

Knebworth Woods transect 2017-2023

Norton Green Common and other open areas in the woodland are the best places to find the butterfly. In 2021, 4 specimens were counted on 29 March.

Pryor's Wood transect 2000-2022

The butterfly is sometimes seen here, mostly in the spring; the best count was on 2 May 2008 when three specimens were recorded.

Life History

Earliest date: 27 January 2022 at Watery Grove
Latest date: 14 December 2016 at Burleigh Meadow

This species can be seen in every month of the year as it overwinters as an adult. It is most active in the spring when peak numbers usually occur in April. The hibernating individuals may even be flying in early July when the emergence of the new generation begins. The eggs are usually laid singly under a buckthorn leaf in early May with caterpillars hatching out by the end of the month. When larvae are fully grown they usually pupate away from the foodplant. Just before emergence of the adult butterfly, a spot on the forewing can be seen through the pupal case.
More details on the UK Butterflies website

Behaviour/Observation notes

As noted above, the Brimstone is more active in the spring when both sexes seek out a mate. Males spend time patrolling hedgerows and woodland edges to search for females which generally emerge from hibernation later than the males. Because of the more frenetic activity of the butterflies at this time they are more difficult to approach. However, after mating has occurred, males spend more time feeding and females egg-laying, facilitating an easier approach for close-up photographs.

Variations/Aberrations

There are a large number of aberrations mostly to do with the wing colour and the central spots but are very rare.
Find out more on the UK Butterflies website