Phenology of Meliscaeva auricollis

I was prompted to take a look at the phenology of Meliscaeva auricollis after a comment on the UKH Facebook page that it had been much scarecer than in 2017. The result is quite interesting because we can see that the graphs are roughly the same shape but with differeing numbers (Figure 1).

Figure 1. Phenology of Meliscaeva auricollis in 2018 and 2017.

I think ther main  factor behind the differences is that there was a much bigger spike in winter 2017 and that this is likely to have had some bearing upon the size of the population in mid-summer. This difference is, I suspect, a reflection of the longer colder winter in 2018 and looks to be one of those cases where there was probably a relationship between winter temperatures and the abundance of a single species during the summer. What is potentially less clear is the effect of the heatwave on any second generation in the Autumn.

So, lets consider 2018 against a 3-year average for the species (Figure 2). Here, we can see that there must have been other years when the summer peak was either later or longer.

Figure 2. Phenology of Meliscaeva auricollis in 2018 against the average for 2015 to 2017.

We therefore need to look at the three years that make up the average for 2015 to 2017 (Figure 3). Here, we see that there is huge variation in the timing of summer peaks and the degree to which those peaks express themselves. Is the exceptionally productive year in 2016 a factor behind the size of the summer spike in 2017? It might, but then we might not have expected such a high number of records in 2016 after what seemed to be a firly uninspiring 2015!

Figure 3. Yearly phenology of Meliscaeva auricollis between 2015 and 2017.

What these graphs fail to express is the difference in the numbers of records that make up the data. Numbers of records assembled in 2016 and 2017 are broadly similar. Since 2017 numbers of direct contributors have dropped as many contributors have moved over to spreadsheets. Also, the Autumn of 2016 was exceptionally warm and the recording season went on well into November. Do yearly proportions of records tell us anything else (Figure 4).

Figure 4. Meliscaeva auricollis represented as a proportion (%) of yearly records from 2013 to 2018 based on data extracted from social media

I have always felt that M. auricollis was a species that has many natural fluctuations and there was a period in the early 2000s when it was seemingly very scarce. Recent years have been something of a surprise so I do wonder if there are other regulating factors such as levels of parasitism? Such phenomena are common enough elswhere in the insect world, so why not amongst hoverflies?