Using cloud radar to investigate the effect of rainfall on migratory insect flight

Abstract

1. The fate of migrating insects that encounter rainfall in flight is a critical consideration when modelling insect movement, but few field observations of this common phenomenon have ever been collected due to the logistical challenges of witnessing these encounters. Operational cloud radars have been deployed around the world by meteorological agencies to study precipitation physics, and as a byproduct, provide a rich database of insect observations that is freely available to researchers. Although considered unwanted 'clutter' by the meteorologists who collect the data, the analysis method presented here enables ecologists to delineate co-occurring signals from insects and raindrops.2. We present a method that uses image processing techniques on cloud radar velocity spectra to examine the fate of migrating insects when they encounter precipitation. By analysing velocity spectra, we can distinguish flying insects from falling rain and compare the relative density of insects in flight before, during and after the rainfall. We demonstrate the method on a case of insect migration in Oklahoma, USA.3. Using this method, we show the first reconstructed images of migrating insect layers in flight during rainfall. Our analysis shows that mild to moderate rainfall diminishes the number of insects aloft but does not cause full termination of migratory flight, as has previously been suggested.4. We hope this technique will spur further investigations of how changing weather conditions impact insect migration, and enable some of the first of such studies in regions of the world that are underrepresented in the literature.

The small hive beetle’s capacity to disperse over long distances by flight

“…The differences with the results obtained for marked SHBs are likely explained by the different spatial scales and diffused distribution in the surrounding environment in relation to the locations where unmarked SHBs were captured. While rainfall could be a trigger or predictor of flight initiation and explain seasonal population dynamics, it is likely that rainfall as a weather condition in itself has an adverse effect on flight 71 . Similarly, wind speed affected the number of unmarked SHBs captured.…”

Monitoring aerial insect biodiversity: a radar perspective

“…Although a variety of other radar types exists, e.g. frequency-modulated continuous wave radar [24], cloud radar [25,26], wind profiler [27] and airborne doppler radar [28], these have only sporadically been employed in insect studies and are therefore not discussed further. Small-scale (vertical-looking) biological radars use fixed-beam sampling with a beam angle of only a few degrees, which increases beam intensity, detectability of weakly reflecting targets, spatial resolution and altitudinal range (figure 2).…”

“…Although a variety of other radar types exists, e.g. frequency-modulated continuous wave radar [ 24 ], cloud radar [ 25 , 26 ], wind profiler [ 27 ] and airborne doppler radar [ 28 ], these have only sporadically been employed in insect studies and are therefore not discussed further.…”

Temperature and water availability drive insect seasonality across a temperate and a tropical region

“…Water availability can also affect insect abundances, either directly by increased risk of desiccation during droughts, or indirectly by reducing the availability of nectar, foliage or pollen or by promoting the spread of fungal pathogens across insects [18][19][20]. Conversely, extreme rainfall will limit periods of insect flight, and could increase insect mortality [21,22]. In tropical climates, temperatures are more constant throughout the year, and unlikely to fall below the thermal minima of insects [23].…”