Developing methods for field experiments using commercially reared bumblebee colonies – initial colony strength and experimental duration as influential factors

Semi-field and field experiments with commercially used bumblebees (e.g. Bombus terrestris) gain more and more importance for both ecological studies and trials on potential side effects of plant protection products. However, standardized, replicable experimental methods are lacking so far and need further development. For example, initial strength of bumblebee colonies may vary across experiments but may be a key factor in successful colony development under field conditions. Further, trial duration and termination may impact results on total reproductive output (e.g. number of newly produced queens). In this study commercially purchased bumblebee colonies of different initial strengths (number of worker bees) were placed along the field margin of each of six field sites. Each site was nested within one of two seasons and planted with one of two arable crops (Brassica napus and Phacelia tanacetifolia). Each colony was spaced approx. 50 m apart from the next colony, and its development was monitored once a week. While the development of half of the colonies was terminated at the first sighting of newly emerging queens within the nesting area, the other half of the colonies was left to develop further until the end of their natural colony cycle. Newly emerging queens were kept within the colonies using queen excluders. Queen production (the number of hatched queens plus the number of queen cells) of early terminated and naturally terminating colonies within the same initial colony strength category was contrasted, and colony development parameters were compared between colonies of different initial strength. Queen production within small and large colonies was significantly higher in early terminated than in naturally terminating colonies. Results may help to develop optimal parameters for standardized field tests.