Butterfly of the Week 26th August 2020

Male Wall

Wall:  This is another ‘brown’ and, like its relatives, its caterpillars are grass feeders.  However, it needs sparse, open, grassy areas with bare patches where sunshine enables the butterfly to bask and warm quickly.  It is a strong flier in these exposed habitats.  The male has a thickened dark area on its forewings which are the sex brands.  In inland areas of UK, in times past, it used to be common but has now become rare, only remaining relatively common at coastal sites around England, Wales, southern Scotland and Ireland.  It occurs in small numbers on Westbury Beacon and along the Mendips but in much larger numbers on Brean Down.  It does occur in Westbury gardens, perhaps one or two a year, and this year it has been seen on Broadhay during our particularly sunny and warm May.  Typically, it has two generations a year with the first emerging in late April or early May.  The eggs laid by this generation will go through their life history to emerge in July or August.  The offspring of this second generation need to get to the third instar caterpillar stage at which it is then able to hibernate.  The following spring the caterpillars will feed again and then moult to the 4th instar after which they pupate hidden deep amongst the grass tussocks to emerge in late-April or early-May.   While habitat loss inland of suitably sunny, sparsely grassy, rocky sites has occurred, it is not believed to be the main reason for its dramatic decline.

Female Wall

The suggestion is that, with a warming climate, the first generation is emerging earlier in April and the corresponding second generation in late June and early July such that it continues to produce a third generation in late September. However, eggs laid by this third generation produce caterpillars that do not have sufficient time, before it gets cold, to reach the third instar, which is necessary for it to be able to over-winter.  This means that there is a significant loss to the population every time there is an attempt at a third generation.  The cooler habitats around UK coasts mean that there is less opportunity for the butterfly to produce this abortive third generation and thus they remain relatively common there.

Photos    Peter Bright

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