Nocturnal migration of dragonflies over the Bohai Sea in northern China

1. A sudden increase and subsequent sharp decrease of catches of dragonflies in a searchlight trap, with Pantala flavescens Fabricius (Odonata: Libellulidae) predominating, observed at Beihuang Island in the centre of the Bohai Gulf, in 2003 and 2004, indicated a seasonal migration of these insects over the sea during the night in China. The movements were associated with the onset of fog. 2. Simultaneous radar observations indicated that the nocturnally migrating dragonflies generally flew at altitudes of up to 1000 m above sea level, with high density concentrations at about 200-300 or 500 m; these concentrations were coincident with the temperature inversion. 3. During early summer, the dragonflies oriented in a downwind direction, so that the displacement direction varied between different altitudes. In contrast, during late summer, the dragonflies were able to compensate for wind drift, even headwind drift, so as to orient south-westward no matter how the wind changed, and thus the displacement direction was towards the south-west. 4. The duration of flight, estimated from the variation of area density derived from radar data and hourly catches in the searchlight trap through the night, was about 9-10 h. The displacement speed detected using radar was approximate to 5-11 m s(-1). Therefore, the dragonflies might migrate 150-400 km in a single flight. 5. The dragonflies were thought to originate in Jiangsu province and they migrated into north-east China to exploit the temporary environment of paddy fields in early summer. Their offspring probably migrated back south during late summer and autumn.