2021 Counting Bumblebees

Some of you saw me over the spring and summer walking along the Coastal Path and recording counts of each of the bumblebees seen and which flower they were visiting. You may well ask – Why count bumblebees? - How do you count them? - And what has the counting shown? This note will provide some answers those questions.

Counting wildlife is now quite a familiar activity, with many taking part in the Big Garden Bird Survey or some of the many other counts of different wildlife groups. The aim of all these counts is to keep an eye on the abundance of the different sets of species and whether they are going up or down. Apart from often producing a lot of anguish and hand wringing, the results are valuable to help with information on actions that are needed to halt further declines or pass on information of successes and how they can be repeated.

Bumblebees as a group are easy to count as they are familiar and easy to recognise as they move from flower to flower.Counting the individual species that are seen is rather more difficult.After leaving my insect ecology studies in the late 60s to find a paying job, I was able to return to insects again when I started volunteering with SNH counting bumblebees and butterflies with a group of other interested and enthusiastic people in 2008.Thanks to guidance from SNH staff and others in the group I joined, I learned to identify the species that are most commonly found in our area and also improved my identification of the flowers they were using.The technique used at Loch Leven was to steadily walk one way along a set route on a specified day each week from April, when the queens emerge from their winter sleep, to September when the new queens stock up before they hibernate ready to repeat the cycle the following year.Bumblebees on both sides of the path were counted and, in some years, (2013 – 2015) we also recorded which flowers each individual bumblebee was visiting to seek either nectar or pollen.The same method was applied here in 2015 when I did my first assessment of Dalgety Bay bumblebees on a path through the areas we had started to manage for people and wildlife as part of the newly created DBCWG.My 2021 count was a repeat along the same route and this is shown in Fig 1.To help with counting and to look at any changes in specific areas, the route, as in 2015, was divided into 5 sections.Unlike the Loch Leven counts of one day each week I was able to do my counts on up to 4 days each week.This allowed more flexibility in choosing suitable weather conditions when bumblebee activity would be high and so build up a greater body of data to help with analysis. Each survey took a little over one hour. One difference from the previous survey was the use of a hand held dictating machine to enable faster and more detailed recording in the field, though the data still had to be transcribed onto paper for input into Excel spreadsheets for the analysis that is used in this note to show the information gleaned from the counts.The data presented in the graphs show the maximum number recorded in any of the counts each week.

Fig 1

Which Bumblebees were Counted?

In the list of the bumblebees counted I will give both the common name and the specific scientific name. The latter is important as this universally accepted name allows for them to be clearly identified by anyone else despite any variations in what they might commonly be called in different areas. There were 6 species being counted in 2015, but they have since been joined by a 7th, the Tree Bumblebee (Bombus hypnorum), which has been steadily colonising the UK from first appearing in the south in 2001.

Of these 7, three are fairly straightforward to identify from their colour patterns Red-tailed (Bombus lapidarius); Common Carder (B.pascuorum) and Tree Bumblebee (B. hypnorum), but the other four require particular care to distinguish between them as they can have very similar bands of black, white, and yellow.

These four species are: Early (B. pratorum) distinguished by a pink/buff tail; Garden (B. hortorum) with two yellow bands in the middle; Buff-tailed (B. terrestris); and White-tailed (B.lucorum). Examples of workers of all these species are shown in the accompanying photographs. The last two can be very difficult to tell apart as workers so, although a separate identity was attempted and recorded for each one seen, many researchers often count them together as terrestris/lucorum (Buff-/White-tailed), and this combination will be also used in some of the figures discussed here.

Workers of the different bumblebees are illustrated in the photos in the gallery below.

Altogether the 2021 survey logged 2677 individual bumblebees from 53 walks, along with 59 Honey bees (Apis mellifera) and 95 Solitary bees of a range of species. This compares to 2152 individual bumblebees from 43 walks counted in 2015.

There is a wealth of information in the two surveys, but for now I will just give the main conclusions regarding abundance.

Overall Abundance

To get a good idea of the maximum population size a Peak Count was calculated separately for each species an the combined population by taking an average of the 3 highest count numbers. These “Peak counts” of all the species together were broadly similar for the two years, so it appears that within our managed area, bumblebees are faring quite well. (Fig 2) However, given the increased abundance and diversity of flowers as a result of our further expansion of the wildflower area by the seed sowing at the Heritage Viewpoint, I would have expected higher overall peak numbers in 2021. This year was however a difficult one for bumblebees and other pollinating insects. The see-sawing weather in the early part of the year with early very warm conditions on some days followed by plummeting temperatures; the cold wet May; and long periods with no rain at all during some of the summer months has made it hard both on the bumblebees themselves and the flowers on which they depend for pollen and nectar over all of the year.

Fig 2

Species Differences and Consistency.

Al the bumblebees counted do not follow identical patterns of presence and abundance.The counts show that the biology and ecology of the 7 species counted can be divided into three broad categories.Early, Garden and Tree have a short colony cycle and low peak numbers early in the summer. Three others, Red-tailed, and Buff/White-tailed, have moderate to high peak numbers later in the year and one, Common Carder, also has a high peak number, but even later in the season.These differences between species are consistent across the two surveys both in number and timing, though the later peaking species were slightly earlier to reach their maximum in 2021 (Fig 3 & Table 1

Fig 3