Abundance of short- and long-tongued bees, and their impact on red clover seed production in four cultivars grown across a large latitude range

Red clover ( Trifolium pratense L.) is an important insect-pollinated forage crop. Low and variable seed production is a problem in many red clover cultivars, especially tetraploid ones, limiting their marketing potential. The roles of short- and long-tongued bees for red clover seed production, specifically in tetraploid cultivars, are not fully known. Here, we investigate the impact of short- and long-tongued bees, cultivar differences and plant traits for seed production, in one diploid and three tetraploid red clover cultivars managed for seed eating weevils ( Protapion sp.) over two years. Since we expected the abundance of short- and long-tongued bees to vary across a latitudinal distribution, we included six sites distributed in southern and northern Sweden. We found no clear southern vs. northern pattern for the abundance of short- and long-tongued bees, while seed eating weevils were more abundant in the south. Seed yield (weight per area) was positively related to long-tongued bees, whereas seed set (proportion developed seeds per flower head) was positively related to both short- and long-tongued bees. Cultivars differed in seed production across sites. Moreover, some investigated plant traits - flowering flower head density and florets per flower head - were positively related to seed production. Total flower head density and number of florets per flower head varied among cultivars, sites and years. In conclusion, our results support the significance of both short-, and especially long-tongued bees for red clover seed production, and also suggest the importance of studying underlying genetic and non-genetic influence on cultivar differences. • Red clover seed production and pollinators were studied in south and north Sweden. • Abundance of short- and long-tongued bees varied by year rather than latitude. • Long-tongued bees were important for seed yield and all bees affected seed set. • Poor seed production in tetraploid cultivars was not linked to pollinator rejection. • Study site affected seed production in tetraploid cultivars and two flower traits.