Effects of the heatwave

Sometimes one gets a gut feeling that something is happening but it is difficult to work out why. That is the case for Volucella pellucens, which has been remarkably scarce this summer. I've noticed this in my own field work and in the posts on the Facebook group. It is a great subject because it is an animal that is noticed by novices and experienced recorders alike, and is probably over-represented in the data because it is big and relatively easy to photograph. So, when there seems to be a drop in numbers it is readily noticeable. The overall phenology (Figure 1) for 2018 compared against the past 3 years seems to tell a story but is it the heatwave or something else that is causing a problem?

Figure 1. All social media records for Volucella pellucens in 2018 compared against the three-year average of records extracted from social media.

What we see is the first peak being strongly attenuated before the second peak commences. We can be reasonably sure that the population was behaving quite normally until early June when the heatwave started to take a hold. BUT, thereafter numbers have been very low.. The second peak seems to be coincident with the normal peak, but again at much lower numbers. What is going on?

I think the answer lies in the emergence times of males and females. In most Volucella, males start to emerge a week to ten days before females and there is a very definite male peak followed by a female peak that is delayed by a couple of weeks. This is likely to be linked to delayed female emergence which is fairly common in insects. So, how does the phenology of males and females in 2018 differ from the three-year average? this we can see in Figures 2 and 3.

Figure 2. Phenology of male Volucella pellucens in 2018 compared to the three-year average 2015 to 2017.

Figure 3. Phenology of female Voucella pellucens in 2018 compared with the 3-year average from 2015 to 2017

These results are quite startling. We can see that emergence in 2018 was delayed by about a week and is likely to be a response to the longer winter. Once emergence had commenced it followed a normal trajectory for the first month in the males but females seem to have started to respond after just a couple of weeks.

What is interesting is that there appear to be double peaks in the phenology of both sexes. Prior to this year I would probably have thought that the second peak was a second brood and that larval development was quite rapid. I rather doubt that is the case now and suspect that there is always a partial delay in emergence from north to south but I don't think we really have enough records to test this theory.

What does seem to be clear is that the male population has been substantially attenuated by the heatwave, whilst there may have been some bounce-back amongst females. We know from our own mark-release-recapture work that some individuals survive for as much as six weeks, but that the vast bulk of the population only survives for about 3 weeks. So, there could be doubt about the numbers of females fertilised by the much-diminished male population.

My instincts are that these animals probably don't stay in diapause in response to external temperatures and that the drop in numbers in 2018 is related to survival rates and adult longevity. Perhaps this could be tested if we can secure sufficient larvae and breed them up in controlled conditions?

Time will tell, but there is clearly some interesting research potential in this species.