The Brimstone has the longest adult life of any Irish (or British) butterfly. The emergence of the adult butterfly begins in late June or more usually in early July and the butterflies may live up to 12 months. It is not active throughout this time, spending much of the time from September to March in an inactive state usually referred to as hibernation. Typically, the butterfly seeks out a safe over-wintering site from mid-September and remains there until around mid-March from when mating and egg-laying occur. By mid-June all or nearly all adult Brimstones are dead and we await the new adults.
However, occasionally, probably as a result of good weather, some survive long enough to see their own descendants fly. In 1934, the great English entomologist and zoological artist, Frederick Frohawk (1861-1946) recorded seeing old, very worn hibernated Brimstones on the wing in the second half of July, in company with numbers of freshly emerged specimens. For the first time in my experience, I observed this generational overlap in Lullybeg in County Kildare. A newly emerged male (male butterflies generally emerge before females) was seen flying energetically along a ride at Lullybeg on June 3rd. On June 7th an old, worn female, which is shown below, was sighted on the same track. She fluttered to feed low down on young bramble before fluttering away to look for buckthorn plants to lay whatever eggs she still holds.
While this overlap in the Brimstone may have occurred before, it is a rare event. There is no reason why a male Brimstone from last year’s hatch would not survive just as long. However, the parents will not mate with their offspring. Brimstone butterflies do not mate in the year they hatch but delay breeding until spring. By that time, only Brimstones that hatched the previous summer will be alive. Nevertheless, living a full year is a great achievement for the adult butterfly and if the climate develops to give us warmer summers Brimstone lifespans may increase.