The incidence of high-flying insect fauna passing over Rothamsted, Harpenden, U.K. (51°49′N, 0°22′W) has been monitored for the past 3 years. Measurements, made with vertical-looking radar (Smith et al., 1993, 2000), record the displacement vector of individual overflying insects, their altitude, and their radar scattering properties, every 15 min, 24 h a day. The work reported here describes how a subset of these data was used to investigate the migratory flight of the diamondback moth Plutella xylostella (L.) (Lepidoptera: Yponomeutidae). For the first time in a radar study of insect migration, a unique combination of size and shape data embedded in the radar signals was used to identify the target species.
Plutella xylostella is highly migratory, and its seasonal movements have been well documented ( French & White, 1960 ; Lokki et al., 1978 ; Talekar & Shelton, 1993 ). It has a cosmopolitan distribution and is believed to be the most universally distributed species of Lepidoptera, occurring wherever crucifers are grown ( Talekar & Shelton, 1993 ). In tropical and subtropical areas, it has continuous generations. By contrast, in the northern- and southern-most parts of its range (e.g. Scandinavia, Canada, northern Japan, southern Argentina), where it is incapable of surviving winter temperatures, outbreaks are entirely the result of regular, long-range immigrations ( Talekar & Shelton, 1993 ). In temperate regions, P. xylostella may be able to overwinter in small numbers ( Marsh, 1917 ; Hardy, 1938 ) but it is widely believed that annual occurrences in these regions result largely from wind-borne immigration of adult moths from warmer regions ( Talekar & Shelton, 1993 ). In the U.K., it is usually assumed that if there is a small overwintering population, it is augmented to a greater or lesser extent by immigration from continental Europe on an annual or near-annual basis ( French & White, 1960 ). Thus, in years when P. xylostella is abundant, its high numbers have been attributed to influxes from continental Europe, and in a few years there is no doubt that very substantial immigrations occurred: e.g. 1958 ( French & White, 1960 ), 1966 ( Shaw & Hurst, 1969 ), 1978 ( Lokki et al., 1978 ). The 1958 outbreak, which was the most severe, is the best studied migration event of this species ( French & White, 1960 ; Johnson, 1969 ; Shaw & Hurst, 1969 ). There was an enormous influx of P. xylostella into the north and east of the U.K. in June and July 1958. Back-tracks indicated that the moths originated in the countries bordering the eastern Baltic Sea, and that they had flown more than 2000 km. The June outbreak of 1966 was attributed to immigration from the same region ( Shaw & Hurst, 1969 ). Although this scenario has been demonstrated only for two exceptional influx years, it has nevertheless come to be assumed as the most probable cause of P. xylostella populations in the U.K. ( Talekar & Shelton, 1993 ). That is, high-density U.K. populations generally arise from immigrations from Scandinavia in mid-summer. The focus of this study was to use the radar data (supplemented by aerial trapping and light trap data) to test this assumption and to attempt to determine the balance between putative overwintering and immigration in a more typical year. Thus, the study focused on the origin of the U.K. population in 2000, and in particular on the early-season observations.