Knepp 2019 PE Data

The Purple Emperor flew for fifty days at Knepp in 2019.  That is, seven weeks and a day - the longest Purple Emperor flight season on record. This means that it had a long, flat peak (as opposed to the normal five day peak season pinnacle). The first was seen on June 25th, the last on August 13th (though it was rare after July 24th).

Monitoring Data

We have two monitoring data sets at Knepp: 1) derived from arduous day counts during the peak season period, when we try to cover as much of the 450ha / 1100 acres as possible, and 2) from the single-species transect down Green Lane. The second is seriously kosher.

Day count data suggests that numbers at Knepp were about one-third of those of 2018, but Neil and I were leading so many guided walk groups that we did not manage to conduct a thorough count in fine weather during the peak season period. I managed two day counts of 108, both of which can be calibrated up to 130 to allow for annoying cloud, and Neil’s peak count was 113. This compared to three day counts of over 300 in 2018 (with a maximum of 388 on July 2^nd, by Neil) and a peak day count of 148 in 2017. 

The Green Lane transect data is far more accurate. It indicates that numbers were half those of 2018. This is a single species transect, walked weekly (for six weeks) during reasonable weather (wind needs to be < Moderate strength), during afternoons when males are on territory, and with a 50m recording box. 

Knepp Green Lane Purple Emperor transect data

	2015	2016	2017	2018	2019
Total Counted	112	61	114	201	102
Peak Count	40	18	34	66	33

It seems that Purple Emperor numbers stood up remarkably well at Knepp in 2019, considering the number of factors impacting against the butterfly* and how poorly it fared elsewhere. This may be because the Knepp population is still building. 

*  2018 had been an annus mirabilis for the Purple Emperor.  However, no rain was recorded at Knepp between 31^st May and 28^th July, and many sallow bushes dropped sub-canopy leaves holding eggs and young larvae. By mid-July the ground beneath most of the young sallow stands was carpeted in fallen leaves. It was inevitable, then, that 2019 was going to see a drop in Purple Emperor numbers, though this was mitigated to some extent by 2018’s profuse egg lay.

The second half of February 2019 was very mild, causing some hibernating larvae to waken and expend valuable energy, long before sallows had started to leaf. Some would have perished. 

Worse, the mild February (and a mild winter in general) led to a population explosion of Umber moths (Mottled Umber and possibly Dotted Border and Scarce Umber). Their larvae feed profusely on sallows during mid- to late May, lacerate the foliage and set Purple Emperor larvae back, or even out-compete them (I'm starting to work on this area of Purple Emperor ecology; it’s crucial, as these moths may well be increasing due to mild winters generated by climate change).

Furthermore, as a result of drought stress, a great many sallows at Knepp Wildland and elsewhere came into leaf unusually late in 2019 – long after Purple Emperor larvae should have begun to feed. Sallows had put their efforts into flowering instead. Indeed, 2019 was a prolific sallow flowering (and seeding) year. Many sallows, particularly the narrow-leaved varieties, did not come into leaf until late May. Purple Emperor larvae on them would have struggled, or died.

Finally, after good weather for larval development during April and May, June saw a lengthy spell of cool wet weather from the 2^nd to the 18^th inclusive, when many Purple Emperor larvae would have been pupating. Evidence suggests that the longer Purple Emperors spend pupating and the longer they're forced to stay in the pupal stage, the fewer of them fly. Predation / mortality seems to be high in the pupal stage.

Fortunately, three factors were in the Purple Emperor’s favour: 1) the profuse egg lay of 2018, 2) a relatively low predation rate on hibernating larvae (by titmice), and 3) fine feeding weather during April and May. Also, flight season weather was good, until late July.