The Fall and Rise of the Wall Brown

While I was delighted to see the arrival of the Wall Brown, Lasiommata megera, in 2010 and its subsequent spread across East Lothian I note that it has declined severely in Southern England. Strangely, it is hanging on around the coast of England, but since the mid-1980s, what was once a very common butterfly has become a rare sight in an area centred around Buckinghamshire and Oxfordshire.

The Wall Brown normally produces two generations of butterflies a year. The first generation emerges in May and June and a second generation occurs in August and September. This species over-winters as a caterpillar.

A study, published in December 2014, The lost generation hypothesis: could climate change drive ectotherms into a developmental trap? By Hans Van Dyck, Dries Bonte, Rik Puls, Karl Gotthard and Dirk Maes looked at the declining number of Wall Browns in Belgium.

Their theory is that due to climate change the season for this butterfly is extending. If the first generation is emerging earlier and the subsequent generation is therefore appearing earlier then there is a potential for a third generation to emerge late in the season. However, there is not enough time for this third generation to breed, or for their eggs to hatch in time to make it through the winter. Presumably, eggs and chrysalises can’t survive frosts. It would be interesting to know if all instars of the caterpillar can survive cold weather.

This theory sounds very plausible to me and it could also help to explain why the Wall Brown is now spreading north into East Lothian. We have certainly been experiencing less severe winter weather here over the last 20 years. Possibly before that our season wasn’t long enough for two complete generations to survive and go on to produce caterpillars before the winter set in. In effect we are now experiencing the sort of weather that was once more common in the Home Counties.

Number of Wall Brown records received over the last five years
2013	2014	2015	2016	2017
54	78	70	129	273

2017 was the best year we have had for Wall Browns. The number of records I have received has increased each year since they were first recorded in East Lothian in 2010. Last year we recorded more than twice as many Wall Browns as we had in 2016. They have now made their way right around the coast and to various inland sites.

They are a very welcome addition to the butterflies of East Lothian.

Speckled Wood - Pararge aegeria - 2017

In a previous post I explained how the Speckled Wood, Pararge aegeria, had arrived in East Lothian in 2009 and then colonized much of the county over the next five years. Well, I am pleased to say that they are continuing to do well. In fact in 2017 they were the secondly most commonly recorded butterfly here.

I have been looking back at the butterfly records for all species recorded here over the last five years and then comparing the 2017 figures with the average figures for that period. In the graph below the average number of Speckled Woods recorded over the last five years is shown in blue, with the 2017 records shown in red.

You will see that Speckled Woods did exceptionally well in the spring, but the poor weather in June and July impacted on their numbers. However, they made a bit of a resurgence later in the year.

Speckled Woods are interesting in that they can spend the winter as either a caterpillar or chrysalis. Those that we see here in April and May are thought to have spent the winter as a chrysalis. The larger peak in population from mid-June to mid-July are those that spent the winter as a caterpillar. There is usually then a much bigger second generation later in September. 

Looking further back, the number of Speckled Woods recorded in East Lothian has shown a steady increase since they were first recorded in 2009. 2014 was a very good year for many species, and Speckled Woods did particularly well then, but even with the poor start to the summer we had in 2017 they did better than ever.

Year	No of Speckled Woods
2009	2
2010	1
2011	16
2012	11
2013	134
2014	1019
2015	465
2016	692
2017	1174

I have been mapping where Speckled Woods have been recorded and the previous maps are shown on my earlier post. This year the Speckled Wood has continued to spread and it can now be assumed it occurs anywhere there is suitable habitat in East Lothian.

It is interesting that in a year when many species of butterflies have done so badly, this species has done well. The new arrivals in East Lothian appear to be getting on better than many of the species that have occurred here for many years.