Radio-tracking reveals how wind and temperature influence the pace of daytime insect migration

Abstract

Insects represent the most diverse and functionally important group of flying migratory animals around the globe, yet their small size makes tracking even large migratory species challenging. We attached miniaturized radio transmitters (less than 300 mg) to monarch butterflies ( Danaus plexippus) and common green darner dragonflies ( Anax junius) and tracked their autumn migratory movements through southern Ontario, Canada and into the United States using an automated array of over 100 telemetry towers. The farthest estimated distance a monarch travelled in a single day was 143 km at a wind-assisted groundspeed of 31 km h^-1 (8.7 m s^-1) and the farthest estimated distance a green darner travelled in a single day was 122 km with a wind-assisted groundspeed of up to 77 km h^-1 (21.5 m s^-1). For both species, increased temperature and wind assistance positively influenced the pace of migration, but there was no effect of precipitation. While limitations to tracking such small animals remain, our approach and results represent a fundamental advance in understanding the natural history of insect migration and environmental factors that govern their movements.

Keywords: Anax junius; Danaus plexippus; Motus; common green darner; monarch butterfly; radio-telemetry.

Figure 1.

Sequential detections (black points) of radio-tagged monarch butterflies (orange lines) and common green darners (green lines) between Motus Wildlife Tracking System towers in (a) eastern North America and (b) zoomed in to southern Ontario, Canada. Grey points indicate active Motus towers in 2015 and asterisks (*) indicate the deployment sites. Lines connect individuals between Motus tower detections but do not indicate the actual path of travel between points. Map projection is Mercator.

Figure 2.

(a) Circular plot displaying the direction of movements between detections. Arrow shows the circular mean direction. (b) Histograms displaying the estimated mean distance of travel each day between detections and (c) the estimated mean groundspeed of travel between detections for radio-tagged monarchs (i) and common green darners (ii). Dark bars indicate distances and speeds calculated from detections between two towers within the same day, whereas lighter bars indicate calculations from detections between multiple (2–3 or 4+) days.