Abstract
The seasonal migration of huge numbers of hoverflies is frequently reported in Europe from mountain passes or spurs of land. The movement of such large numbers of beneficial insects is thought to provide significant ecosystem services in terms of pollination and pest control. Observations from the East Coast of the USA during the 1920s indicate the presence of migratory life histories among some hoverfly species there, but 90 years have now passed since the last reported observation of hoverfly migration in the USA. Here, we analyse video footage taken during a huge northward migration of hoverflies on 20 April 2017 on the West Coast of California. The quantification of migrant numbers from this footage allows us to estimate the passage of over 100 000 hoverflies in half an hour over a 200 m section of headland in Montaña de Oro State Park (San Luis Obispo County). Field collections and analysis of citizen science data indicate different species from the previously reported Eristalis tenax migrations on the East Coast of the USA and provide evidence for migration among North American hoverflies. We wish to raise awareness of this phenomenon and suggest approaches to advance the study of hoverfly migration in North America and elsewhere.
Highlights
Insect migration is an environmentally significant phenomenon that provides both advantages and challenges to human populations
Northward movements of migratory insects are known to occur throughout North America during the spring in response to seasonal changes [1], yet over 90 years have passed since the last reports of hoverfly migration in the USA
Our results demonstrate the scale of hoverfly movement, which we estimate reached a minimum of 34 000 flies in the 30 min of observation but probably totalled in the hundreds of thousands over the complete migration front
Summary
Insect migration is an environmentally significant phenomenon that provides both advantages and challenges to human populations. The migration of hoverflies is best understood in Europe where seasonal influxes into northern regions begin around May and are followed by often-huge southerly migrations during August – October. These migrations take place on a broad front and, in general, observations are restricted to locations that concentrate large numbers of migrants—spurs of land bordered by water bodies [6,7] or mountain passes [8,9,10]. On 20 April 2017, BVB witnessed a large migration of hoverflies, this time on the West Coast of California in San Luis Obispo County (figure 1).
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