2021 figures for a Newton Longville garden. Part 1: totals

I delayed finalising the analysis of my 2021 records until the very end of the year: the mild weather allowed me to run the traps on both 29th and 31st December.  Those two nights yielded a grand total of four moths: a late Scrobipalpa costella was the only Lepidopteran point of interest, but two diving beetles (Colymbetes fuscus) last night were the first I have seen here.

During 2020, I ran an experiment to compare the effectiveness of an actinic light against my own design of LED light.  I posted several times about this, summarising the year here.  I continued this comparison during 2021, but I will discuss that analysis in a couple of posts over the coming days.

In all of my analysis, it is important not to read too much into it: these are figures for moths caught in just one garden, comparing just two years.  There is a lot of chance involved as well as other factors such as weather.

Here is a comparison of the total 2021 records for my garden and a comparison with those for 2020. These figures include moths found in association with the moth traps as well as moths and larvae found elsewhere in the garden or indoors.  Commentary on individual species will be in another post.

The keen-eyed may spot that the 2020 figures presented above are slightly different from those that I originally published a year ago, mainly because I'm including non-trapped moths.  I have spent part of the Christmas break automating the annual analysis job, so it only took me 5 minutes to re-state the 2020 figures on a comparable basis to 2021.

Numbers were down in 2021: many fewer moths of slightly fewer species.  There were two main factors behind this.

One real-world factor was the poor spring in 2021 compared to the fine, sunny spring during lockdown in 2020.  On the nights on which I ran the traps (which is hardly a random sample: I select nights of better weather), it was 4½ °C colder in April 2021 than a year before, and 1½ °C cooler in May.  Met Office data for whole months at Cambridge shows somewhat smaller temperature differences than this, but it also shows just 3 days of frost in 2020 compared to 14 in 2021.  This was probably why in these two months of 2021 the traps caught just one quarter of the number of moths per night compared to what they had achieved in 2020, and between a third and a half of the number of species.

The second and larger cause of difference was a reduced amount of trapping effort in the peak June/July/August period.  A very rough calculation indicates that the effect of this on the total number of moths for the year is perhaps twice as big as the effect of the spring weather.  In normal years I am away a lot in spring and summer; the last two years have not been normal, but I was away rather more in June and July 2021 than in the same period in 2020.  Consequently, I only ran the traps at home seven times during June-August 2021 compared to thirteen times in the same months of 2020.

Complicating the analysis is the fact that there was a difference in lights between the two years which only applied between April and July and which seems to have made the 2021 figures better than they would otherwise have been.  I am still trying to separate the effect of this factor ("Synergetic" versus normal actinic lights), which will make for another post later.

Tim Arnold
Newton Longville, Bucks