Effectiveness of two monitoring methods (transect walk versus pan traps) in recording species richness of wild bees: A case study in Northern Serbia

Bees are considered the most important group of pollinators, playing a crucial role in maintaining ecosystem stability. A worldwide decline in the diversity of wild bees underlines the urgent need for their conservation. Addressing this challenge in Europe, there begins the development and implementation of effective monitoring strategies (European Pollinator Monitoring Scheme - EUPoMS). Together with transect walks, pan traps were initially used but recently excluded from the EUPoMS core scheme for financial reasons. However, as a survey method, pan traps offer several advantages for monitoring pollinators. In line with global needs, the monitoring of wild bees was conducted in semi-natural grassland and forest fragments in Serbia’s northern province, by using these two methods. The data from nine sampling sites, collected during 2022 and 2023, were used to evaluate the effectiveness of these two monitoring methods, as well as the effectiveness of three different colors (yellow, white and blue) of the pan traps in recording species richness. A total of 199 wild bee species were recorded: 109 during transect walks (55%), 25 caught in pan traps (12%) and 65 species detected using both methods (33%). This suggests that, although the transect walk was more efficient in recording species richness of wild bees, colored pan traps should not be overlooked but used as complementary method in order to provide more complete data. Most species were captured in blue pan traps (64), followed by white (52) and yellow traps (36). The correlation between flower cover and the number of wild bee species caught in pan traps was -0.3 (p = 0.052). This indicates that when floral cover increases, the number of species caught in traps tends to decrease. However, the connection is weak, suggesting that there are probably more factors that altogether affect the effectiveness of pan traps.