16 June 2023
By Dr. Richard Comont, Science Manager
The summer of 2022 was HOT – 40+°C for the first time, during an extended heatwave that certainly left Trust staff in need of air conditioning. Without that even as an option, how did the bumblebees fare? Luckily, that’s a question that we can now answer with the help of the Trust’s BeeWalk project.
Quite a few species did quite well in the spring, which was generally warm, albeit with a cold spell in April-May. The spring-specialist Early bumblebee had its best year in almost a decade! Several other species – notably the Common and Moss carder bumblebees, Buff-tailed, Bilberry and Garden bumblebees – all recorded noticeably above-average monthly counts at least once by the end of June.
Then came the heatwave. Many of our rarer bumblebees prefer warmer conditions: the Ruderal bumblebee actually had its best BeeWalk year on record. The Shrill Carder reached twice the normal peak abundance, but numbers fell in August and were below-average in September and October, suggesting a shorter season for the species and possibly suggesting that drought became a limiting factor during 2022. Overall, even the warmth-loving, southern-biased species generally did better in 2018, 2019, and 2020 than they did last year – all years with heatwaves, but none as hot for as long as 2022
However, other species didn’t enjoy the heat. After June, bumblebee numbers dropped off, with the 4th-lowest July count to date, followed by the lowest BeeWalk August count ever. For later-flying species, these are the peak months for foraging, and the production of males and queens. The Common carder and Red-tailed bumblebees both saw markedly and unusually (for the time of year) decreased abundance over the heatwave period, as did the rare Moss carder bumblebee, a species that likes it cooler and damper.
The mechanism for these decreases is twofold. Firstly, the heatwave caused a drought, reducing both the number of flowers available to bumblebees and the amount of nectar in the remaining flowers – less food means fewer bees.
Secondly, bumblebees overheat very quickly and struggle to fly above c. 30°C. Bumblebees usually forage from dawn until dusk, but couldn’t fly during the very hot period in the middle of the day, reducing their foraging time. Overall, bees couldn’t forage as much and couldn’t find much food even when they could get out.
We were concerned about the potential effect of this on the number of queen bumblebees going into hibernation, and thus on population numbers during 2023. When the meadows remained eerily quiet deep into May and then June, we had a preliminary look at the 2023 BeeWalk numbers to see if the missing species were those that had had a poor 2022. Surprisingly, it wasn’t – and in fact, emerging queen numbers were broadly average across the board. Instead, the missing bees were the workers, who hadn’t appeared in anything like their usual numbers. This seems to tie in better with the cold, wet spring that we’ve just had, slowing down colony establishment and worker production. It’s impossible to rule out the effect of weather in 2022 completely – perhaps queens have struggled because they were under-nourished going into hibernation, or the vegetation has been affected? But more work is required – and luckily, in BeeWalk we have the key tool for the job!
Read the full 2023 BeeWalk Report.
To take part in BeeWalk, visit the BeeWalk Website.
