The Wall Brown and the ‘lost generation’

Wall Brown (female). Photo: Bob Eade
Wall Brown (female). Photo: Bob Eade

The dramatic decline of one of Britain’s butterflies may be because climate change is creating a “lost generation” according to research by Belgian scientists.

The disappearance of the Wall Brown (Lasiommata megera) from swathes of southern England has mystified conservationists for two decades but new evidence suggests that the butterfly has seen a serious decline (-86% since 1976) because warmer weather is causing generations to hatch out too late in the year to survive.

In recent years, instead of the offspring of the wall butterflies found flying in July and August spending winter as a caterpillar before emerging as a butterfly the following year, warm conditions encourage the caterpillars to quickly turn into a butterfly by September and October.

Wall Brown caterpillar. Photo: Pete Withers

Wall Brown caterpillar. Photo: Pete Withers

By emerging so late in the year, these butterflies fall into what researchers call a “developmental trap”. By autumn, it is too cold or there are not suitable plants for their offspring to eat before winter which in effect means these autumn butterflies are a lost generation, leaving no caterpillars that can survive to become butterflies the following spring.

In research published in the international journal Oikos, enclosed pots of captive-bred wall caterpillars were placed at coastal and inland sites in Belgium of similar latitude during the summer. All the caterpillars placed at inland sites quickly developed to become a third generation of wall butterflies which emerged in the autumn. At coastal sites, however, just 42.5% developed into a third generation, with the majority following the species’ traditional life-cycle – spending winter as a caterpillar before emerging as a butterfly the following year.

Wall Brown (Upperside). Photo: Brian Arnold

Lost generation Wall Brown (Upperside) photographed at Ballard Down on 25/10/2016. Photo: Brian Arnold

The scientists found that the micro-climate at the inland sites was on average 1.2C warmer than at coastal sites, suggesting that cooler conditions by the sea enabled the butterfly to maintain its traditional life-cycle.

While climate change should benefit many sun-loving butterfly species, the fate of the wall could be shared by many other insects struggling to adapt their lifestyles quickly enough to rapid changes in temperature.

3 thoughts on “The Wall Brown and the ‘lost generation’

    1. Brian Arnold

      I saw and photographed the female Wall Brown at Ballard Down on 25th October. I was not expecting to see a Wall and my first assumption when I saw it from a distance was that it was something else such as a Speckled Wood, although it did look too bright a colour so I was unsure until it landed close enough to see clearly. Someone else who was there shouted out that they had seen a Gatekeeper, but it was obviously too large and equally unlikely a sighting. I wonder if there have been other sightings of Walls in Dorset but like me people would make the assumption that it is a different species. Brian Arnold

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