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Despite the fact that migratory insects dominate aerial bioflows in terms of diversity, abundance, and biomass [ 1–6 ], 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 [ 7–10 ] 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, 13–19 ], our results demonstrate that migrant hoverflies are key to maintaining essential ecosystem services. • Between 1 and 4 billion hoverflies migrate into and out of southern Britain each year • These migrants provide important pest control by consuming 3–10 trillion aphids • They also provide extensive pollination services and long-range pollen transfer • Migrant hoverflies play a vital role due to declines of other beneficial insects Wotton et al. show that hoverflies have seasonally adaptive migration directions, allowing colonization of high-latitude summer breeding areas before their progeny return in autumn. Hoverflies deliver vital biocontrol and pollination services and are increasingly important, as their numbers are stable while other beneficial insects are declining. [ABSTRACT FROM AUTHOR]