Friday, August 28, 2020

The 2020 Season at Knepp, and Prospects for 2021...

This should be a tale of great and greater glory, but is sadly a tale of great woe, with a nasty sting in its tale. More woe is to come.

We were expecting His Gloriousness to emerge in fantastic numbers, as post-hibernation larvae had enjoyed fantastic feeding-up weather, from late March through to early June. But therein lies the problem, there weren't that many larvae.

Each winter I follow a number of wild larvae through, in order to measure winter survival / predation rate. However, this winter I didn't manage to get a measurement - the Forestry Commission inadvertently trashed my breeding area (I can't be critical: they left most of the sallows, it's just unfortunate that those bearing my monitored larvae got felled...). The winter was mild and horribly wet, and such winters are fairly disastrous for hibernating larvae, with predation rates as high as 85%.

Ben Greenaway, who was following a large sample of wild larvae in West Sussex, recorded a predation rate of about 80%. Had I known that (we thought the rate was more like 40-50%) I would not have predicted an annus mirabilis.  

In early June, the Emperor then got stuck in the departure lounge, as the weather deteriorated as the bulk of the brood was pupating. That may have done some minor damage. 

The first Knepp Emperors were seen on June 13th. My first sighting was a rubbish sighting, which was deeply ominous as great Emperor years kick off in spectacular fashion. Numbers failed to build well.

We had a short heatwave around Midsummer Day, wherein the Emperor flourished modestly. Then the wheelnuts came off, followed by the wheels. 

His Gloriousity is vain enough to roost in treetop sprays, and ignores the weather forecast. If He was to roost on the sheltered side of trunks and branches He would have survived the late June and early July gales quite well. As it was, He got shredded, and the equally dim-witted females. A number of crippled Emperors were found on the ground.  This one at Knepp -




This one in Sherwood Forest, Notts (which has recently re-declared itself Purple), on July 5th -



Here's the data from the Knepp Wildland Purple Emperor Transect (a 2km long single species transect walked weekly on non-windy afternoons, using a 50m recording box):-

2015 =  112                       
2016 =    95 (adjusted by Basian statistician, gappy data set)

2017 = 117

2018 = 201

2019 = 101
2020 =   61

The net result was a very poor egg lay - seemingly everywhere.


That in itself is not the end of the world, for low levels of larvae tend to lead to low winter predation rates - the tits don't find the hibernating larvae, and the insect recovers.

But Knepp got hit by a double-whammy: the poor egg lay was followed by a horrific drought, during the early August heatwave. The bulk of Knepp's sallows are on former arable fields, they're growing densely together on damaged soils - not on the woodland soils they're designed for. They are drought prone. They wilted badly in 2018, when no rain fell between May 31st and the end of July. But that was after a massive egg lay, so we got away with it.

This year's Knepp sallow drought is an order of magnitude more severe than that of 2018, and occurred after a rotten egg lay.  

No way is the Emperor going to abound at Knepp in 2021.

This is what many of Knepp's breeding sallows were looking like on August 13th - 





The rains then arrived too late. Oddly, sallows in the woods a mile to the north were not badly affected, but there may be other sites in the Low Weald that were similarly affected.  

In all, I think this was the worst Purple Emperor year since 1990, when a mild, wet and stormy winter gave way to a magnificent early spring and May, only for the weather to collapse in early June - probably as the first adults were emerging - Only, in 1990 heatwave conditions returned in mid-July, too late for the adults, and persisted.  

Pray very hard...  We need a wet autumn, a bitterly cold and frosty December to de-tit the woods, a cold winter, a steady non-early spring, a fine May, and a decent June and July. 

The last of the 2020 Knepp Purple Emperors was seen on July 23rd. Elsewhere, sporadic sightings were made into early August, mainly in the north of the Empire, with the final sighting coming from near East Midlands Airport in newly-Purpled Leicestershire on Aug 18th (a female).

The butterfly seems to have done appalling badly at all sites, with the notable exception of Abbots Wood Inclosure in Alice Holt Forest (I'm not sure why, it may be due to an upsurge in recording). In one top grade site, a privately owned wood known as 'Bucks Best Wood', only a single sighting was made.

The message to 2020 is simple: Don't come back!  





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