Mass seasonal migrations of hoverflies provide extensive pollination and crop protection services

Abstract

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 which remain understudied are a suite of highly beneficial species of hoverfly in the tribe Syrphini, which we collectively term ‘migrant hoverflies’. Adults are key pollinators [7–10] while larvae are significant biocontrol agentsof 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 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 (N and P) and transport billions of pollen grains between Britain and Europe,while 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 the10-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.