Mass Seasonal Migrations of Hoverflies Provide Extensive Pollination and Crop Protection Services

Summary Despite the fact that migratory insects dominate aerial bioflows in terms of diversity, abundance, and biomass [ 1 , 2 , 3 , 5 , 6 ], the migration patterns of most species, and the effects of their annual fluxes between high- and low-latitude regions, are poorly known. One important group of long-range migrants that remain understudied is a suite of highly beneficial species of hoverfly in the tribe Syrphini, which we collectively term “migrant hoverflies.” Adults are key pollinators [ 7 , 8 , 9 , 10 ] and larvae are significant biocontrol agents of aphid crop pests [ 11 ], and thus, it is important to quantify the scale of their migrations and the crucial ecosystem services they provide with respect to energy, nutrient, and biomass transport; regulation of crop pests; and pollen transfer. Such assessments cannot be made by sporadic observations of mass arrivals at ground level, because hoverflies largely migrate unnoticed high above ground. We used insect-monitoring radars [ 12 ] to show that up to 4 billion hoverflies (80 tons of biomass) travel high above southern Britain each year in seasonally adaptive directions. The long-range migrations redistribute tons of essential nutrients (nitrogen [N] and phosphorus [P]) and transport billions of pollen grains between Britain and Europe, and locally produced populations consume 6 trillion aphids and make billions of flower visits. Migrant hoverfly abundance fluctuated greatly between years, but there was no evidence of a population trend during the 10-year study period. Considering that many beneficial insects are seriously declining [ 7 , 10 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ], our results demonstrate that migrant hoverflies are key to maintaining essential ecosystem services.