Effects of nocturnal celestial illumination on high-flying migrant insects

Radar networks hold great promise for monitoring population trends of migrating insects. However, it is important to elucidate the nature of responses to environmental cues. We use data from a mini-network of vertical-looking entomological radars in the southern UK to investigate changes in nightly abundance, flight altitude, and behaviour of insect migrants, in relation to meteorological and celestial conditions. An abundance of migrants showed positive relationships with air temperature, indicating that this is the single most important variable influencing the decision to initiate migration. In addition, there was a small but significant effect of moonlight illumination, with more insects migrating on full moon nights. While the effect of nocturnal illumination levels on abundance was relatively minor, there was a stronger effect on the insects’ ability to orientate close to downwind: flight headings were more tightly clustered on nights when the moon was bright and when cloud cover was sparse. This indicates that nocturnal illumination is important for the navigational mechanisms used by nocturnal insect migrants. Further, our results clearly show that environmental conditions such as air temperature and light levels must be considered if long-term radar datasets are to be used to assess the changing population trends of migrants.

Methods
Two vertical-looking insect radars (VLRs) located at three sites in southern England between 2000 and 2009 collected insect abundance and flight behaviour data. One VLR was located at Rothamsted Research, Harpenden, Hertfordshire, for the whole 10-year period. The other VLR was at Malvern, Worcestershire, from 2000 to 2003 and then at Chilbolton, Hampshire, from 2004 to 2009. All statistical analyses were conducted in the R software environment (version 3.6.3).