Westbury Wildlife

News and information about wildlife in and around the parish.

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

Strawberry Line Bird Survey

Those keen on wildlife will be interested to see this report on breeding birds along the Strawberry Line.  It suggests that the concentration of breeding birds along the line is greater than in much of the farmland the route passes through – another reason to support the development of off-road paths.

Butterfly of the Week 19th August 2020

On the Beacon at the end of February 2019

Painted lady:  This is, perhaps, our most well-known migratory butterfly which arrives every year in the Spring or Summer. The adult butterflies breed here, and their offspring are being increasingly recorded as flying south again in the autumn.  Painted ladies are a very strong flying butterfly and have been shown to cross from sub-Saharan Africa to North Africa and to reach even the most northern parts of Europe.  Similar migrations occur across other continents.  Seemingly during the northern winter, Painted ladies breed in North Africa and the offspring of these head northwards spreading across southern Europe and may even arrive in the UK.

 

 

More usually this generation will breed across southern France and Spain and it is the offspring of this generation that continue northwards arriving in the UK in May.  Here they breed, with caterpillars feeding on various kinds of thistles and may go from egg to adult in as little as 6 weeks.  This next generation may then continue northwards to breed again.  In the Autumn something about the shortening days and falling temperatures triggers the newly emerged adult butterflies to head south.

Huge numbers have been recorded heading south and are able to once more return to the north African breeding grounds from which their great or great great grandparents left in the previous January.  This strategy is so successful that the numbers estimated to return are much greater than the numbers that left.  The benefit of this migration is believed to be related to a very effective wasp parasitoid that each year more or less exterminates any remaining north African Painted lady caterpillars.  Migration means that the butterfly can evade this parasite and the successfully returning butterflies can take advantage once again of the ideal breeding conditions in north Africa over the northern winter before the wasp parasitoid multiplies once more.  The extent to which sub-Saharan originating butterflies are part of the European annual immigration is not yet clear.  While the occasional overwintering butterfly has been found e.g. in Cornwall, it seems clear that, as it has no ‘hibernation’ stage and caterpillars die when the temperatures fall below 5oC, it does not survive to any useful extent in the UK. The numbers that arrive in the UK each year are hugely variable and are, presumably, related to the particular conditions in the breeding grounds along their migration routes.  2009 and 2019 were the last ‘Painted lady years’ in which large numbers arrived in the UK.

 

This year a few have been seen in the Parish, but it has been only the odd one here and there.  The fashion for purchasing captive bred caterpillars means that more people can see these wonderful animals up close and personal, but complicates knowing whether any particular butterfly has come from North Africa or from a local butterfly farm. They are a spectacular butterfly, and all the more so, because of their entirely migratory life history.

Photos  Peter Bright