THE LONG-DISTANCE MIGRATION OF NILAPARVATA-LUGENS (STAL) (DELPHACIDAE) IN CHINA - RADAR OBSERVATIONS OF MASS RETURN FLIGHT IN THE AUTUMN

1. A case study is presented of the autumn migration of the brown planthopper, Nilaparvata lugens (Stal), in the area of Nanjing in the People's Republic of China. The study was made using a high frequency (8 mm wavelength) radar and a net suspended from a kytoon. 2. The observations confirmed that long-distance return migrations occur in China in mid and late September, with N.lugens being carried on the prevailing north-easterly wind towards the autumn infestation and overwintering areas of the species. 3. After mass take-off in the late afternoon or at dusk, the migrants flew for several hours during the evening, often in a dense layer which formed at heights between about 400 and 1000 m above ground. These layers often had well-defined ceilings corresponding to an air temperature of about 16-degrees-C. The migration height was above the top of the surface temperature inversion, i.e. the migrants did not fly at the height of the warmest air. 4. The dense layer concentrations overflying the radar were back-tracked to source areas up to 240 km away in the north-east of Jiangsu Province. Planthoppers observed emigrating from the Nanjing area would reach areas in south Anhui Province or north Jiangxi Province if they flew for 12 h. 5. There was a second period of mass take-off at dawn. Insect layers sometimes formed but did not last longer than 1-2 h. 6. The present results were strikingly different from those previously observed in the dry season in the Philippines, where migratory flight durations were largely confined to periods of about 30 min at dusk and dawn. 7. Our observations are discussed in relation to the equator-wards return migrations undertaken in autumn by other insect species, and the importance of these migrations for the maintenance of long-flying genotypes in the overwintering populations is considered.