Larger pollinators deposit more pollen on stigmas across multiple plant species-A meta-analysis

Many insect species provide essential pollination services. However, the amount of pollen deposited onto a stigma when visiting a flower ('single visit pollen deposition', SVD) can vary greatly among taxa depending on morphological traits of pollinators. Further, SVD is commonly measured using one of two methods ('static': waiting for an insect to visit a flower present on plant, and 'active': removing the flower and presenting it to a flower visitor) that may also differ in their effectiveness. To gain a more comprehensive understanding of how SVD compares among pollinators, we conducted a hierarchical meta-analysis using data from 28 studies identified by a systematic review. These contained SVD data for 94 bee and 33 fly taxa (hereafter 'wild pollinators'), across 30 plant species from which we included 127 observations. In the analysis of each study, we used the western honey bee Apis mellifera as a comparator species. Wild pollinators deposited more pollen onto stigmas per single visit than honeybees, and those with larger body deposited significantly more pollen than smaller ones. Of the two methodological approaches to assess SVD, 'static' versus 'active', we found no significant difference regarding the amount of deposited pollen. Synthesis and applications. Our meta-analysis highlights the breadth of wild pollinators that contribute to pollination effectiveness via their delivery of pollen to many crop and non-crop plant species. However, just 25% of the observations assessed the amount of pollen deposited by fly species. Our findings point to the need to further quantify the pollination effectiveness of non-bee pollinators as studies have largely focused on managed and wild bee species.