So, (almost) the end of the 10th season of bumblebee monitoring at the RSPB reserve and it is time to evaluate the results I gathered with Pete Akers. A strange year. On one hand the total of 240 bumblebees counted on the timed walks was the third lowest since monitoring began (last years count was 361). However this was due to a steep decline in the numbers of our three commonest species, whereas the rarer species had a good year, and two new species were found on the reserve.
The brown banded carder bee Bombus humilis had it’s second best year with 43 bees recorded (up from 20 last year) and it’s close relative the moss carder bee Bombus muscorum was also up with a count of 14 bees (double last years total). This insect seems to be struggling at Dungeness, possibly because it it a northern species that performs less well at higher temperatures, so it is good to see it increasing here. The garden bumblebee Bombus hortorum also registered it’s second highest count of 48 bees, and was narrowly the second most common bee on the reserve. Given that this is a long tongued species that, in common with the rarer species, likes legumes this is a good sign. The tiny heath bumblebee Bombus jonellus - see picture above, also continued it’s recovery from the poor year of 2007 when only 1 bee was found (16 recorded this year).
So much for the good news. The buff tailed bumblebee Bombus terrestris and the white tailed bumblebee Bombus lucorum had a wretched year with a combined count of only 42 workers (down from 141 last year). Numbers of queens recorded on the timed walks were also down on last year (the buff tailed falling from 10 last year to 3, and white tailed’s fell from 3 last year to 1 only). It is difficult to know what this means as it could be that there were no nests near the survey areas. However, my subjective impression is that this species has been less common across the marsh, particularly early in the season. This insect has been found foraging throughout the winter months in recent years. Did the last cold winter prevent the early establishment of nests and build up in numbers early in the season?
The red tailed bumblebee Bombus lapidarius also had a bad year, down from 104 specimens last year to only 51 in 2010. Like most of the species it had a slow start following the harsh winter, with none encountered in May (compared to 18 in 2009). By June it was performing well and looked likely to have a record year, however by July a drought had set in, and much of the bird’s-foot trefoil, a favoured food-plant at Dungeness, had stopped flowering. Fortunately there seems to have been more rain in the past month which increased bumblebee numbers in August.
So, as usual, a year of mixed fortunes, but good signs that the legume-rich fields are producing good results. Hopefully the shrill carder bees Bombus sylvarum will breed successfully, and an additional timed walk is planned for September to check if this recolonising species has persisted. The finding of one male large garden bumblebee Bombus ruderatus does not indicate local breeding, as males can disperse widely, but is an encouraging sign that this formerly struggling insect may be on the way back too.