Monitoring bumblebee pollinator visits to the medicinal plant Gentiana asclepiadea L. (Gentianacese) – a comparison between the periods 1990–1994 and 2017–2020

Abstract

Ever increasing data continue to indicate the decline of bumblebee populations. The key factors causing declines in their abundance and diversity are: 1) habitat destruction, 2) loss of floral resources, 3) emerging diseases, 4) increased use of pesticides (particularly neonicotinoids). The aim of this study is to monitor bumblebee visits to Gentiana asclepiadea L, recording pollinator species, and taking measurements of seed set. This plant species is chosen for two reasons: 1) similar data is available from our previous research in the 1990’s and 2) this montane plant species is supposed to be less exposed to hazards from pesticides and habitat destruction. Three study sites were chosen in Mt. Vitosha (SW Bulgaria) where natural populations of G. asclepiadea occur in 1990. The observations of bumblebee activity in the flowers of G. asclepiadea were conducted during the flowering seasons (August and September) of 2017 – 2020 at the same study sites and compared to the data obtained in the previous period (1990–1994). The free pollination fruit set was tested by monitoring of 100 G. asclepiadea flowers each year for development of fruit capsules. The seed set was tested by counting the matured seeds and non-fertilised ovules of 10 fruit capsules each year. A slight decline in bumblebee activity was recorded in 2017 – 2020 in comparison to 1990–1994. This is reflected in the fruit set and the seed set. Our data demonstrates that even in a mountain habitat, where there are fewer direct hazards to bumblebees, that pollination effectiveness has been suppressed over time. This corresponds to a research study which provides evidence that insect biomass fell by 76% in German nature reserves between 1989 and 2016.