How Good Was 2019 at Knepp?

Neil and I run a Purple Emperor (single species) transect along a 2km stretch of green lane at Knepp Wildland. 

Method: weekly count throughout the PE season, afternoons only (when males are on territory), 50m box (the UK BMS 2.5m box mainly records males on dog poo), and windy weather restrictions (c/f tree tops). We developed it in 2014 and  2015, and launched it in 2016.  

Given that the 2019 season lasted just five weeks (most seasons do, though 2018 stretched to six weeks), the data set now runs -

                    2016      54
                    2017    114
                    2018    201
                    2019      98

It is hard to explain why 2019 was so much better than 2016, other than by suggesting that the Knepp population is still in building phase. 

Why were numbers so down on 2018? First, no rain fell at Knepp between 31st May and 28th July 2018, causing many sallows to drop their sub-canopy leaves - on which most eggs are laid. Secondly, Purple Emperor numbers are heavily influenced by June weather, and June 2019 was cool and wet (bar the last week).