2 July 2024
By Dr Richard Comont, Science Manager, Bumblebee Conservation Trust
Well, we’re currently in a gap between ‘peak queen’ bumblebees (spring, when there’s lots of overwintered queens looking for nest sites) and ‘peak worker’ bumblebees (mid summer). This year the gap seems to be deeper and more pronounced than normal. That’s worrying, but there are reasons for hope as well.
Early analysis of 2024 BeeWalk data suggests that overall queen bumblebee numbers were actually above average in March and April, and May numbers are almost exactly average. But that disguises patchiness between species and (we suspect) regions as well. For instance, the Early bumblebee (Bombus pratorum) is doing well this spring, with particularly high numbers reported during May. In terms of overall numbers, that balances out the fact that the Red-tailed bumblebee (Bombus lapidarius) and White-tailed bumblebee (Bombus lucorum agg.) are down, nationally.
Other species are showing broadly average population trends, but all show more of a downturn than usual in May. Looking at the caste data in BeeWalk, it’s clear that many species have fewer workers being reported than is usual for the time of year. Workers are where the real abundance is – a typical bumblebee next might have one queen and a hundred workers, and on BeeWalk the number of bumblebees seen per kilometre walked jumps from 4-5 in March and April as queens appear, to 7-8 in May, then 20-30 in June, July and August as the nests of most species reach their full size.
While we know many species have undergone large-scale, long-term declines (and in many cases still are declining), that doesn’t explain why bumblebees are suddenly thin on the ground over the past few weeks. This is likely to be down to the weather. Much of the country had a very wet spring, and it looks like the wet weather has delayed colony formation. Wet weather makes it hard for bumblebees to forage, and means queens need to spend more time in the nest incubating rather than being out foraging. That means that it will take longer for nests to produce workers. So even though there were plenty of queens out early, looking for nests in the warm weather at the start of March, colony development slowed to a crawl as queens had to divide their time between incubating their developing brood (the ideal bumblebee nest internal temperature is around 30°C) and foraging for pollen and nectar from soggy, bedraggled flowers in the comparatively brief windows between rain showers. Sadly, this is also likely to mean more queens have died than normal.
Looking forward, there will be queens in nests with workers on the way, especially now that the weather is improving, so the overall picture may change for the better (as it did last year). Something worth bearing in mind is that in spring 2023, bumblebee numbers were much lower than they have been this year, but 2023 bumblebees still rebounded to have a good year overall (aided by a mild, damp summer). They have the potential to catch up again this year if the weather plays ball, but that means that they’re currently hostage to fortune and in need of a good summer to thrive. A repeat of the 2022 heatwave, which had a massive impact on late-summer bumblebee numbers, would be the worst possible outcome.
We can only see this level of detail thanks to the BeeWalk dataset, run by the Bumblebee Conservation Trust, with data collected every month from March to October across Britain by 100s of volunteer BeeWalkers. The BeeWalk scheme has run since 2008, and was opened as a public citizen science scheme over 10 years ago. It’s a standardised bumblebee-monitoring scheme recording the abundance of each bumblebee species seen between March and October.
Read the latest BeeWalk Report.
