Weather associated with autumn and winter migrations of rice pests and other insects in south-eastern and eastern Asia

Trajectory analysis was used to determine the direction and extent of windborne migrations of insects trapped in autumn and winter in south-east-and east Asia for 12 years between 1968 and 1987. Nearly 2600 trajectories 12 ere drawn upwind at 10 m and 1.5 km from 15 catching sites and within the time (less than or equal to 60 h), wind speed (greater than or equal to 5 km h(-1)) and temperature (greater than or equal to 10 degrees C) thresholds used, only 5% of the trajectories failed to locate a possible source and over 90% were completed in 40 h or less. The delphacids Nilaparvata lugens Stal and Sogatella furcifera (Horvath) dominated the trap catches (96%) with Cicadellidae (2.3%) and Diptera (0.5%) as the nest most numerous. Very few Lepidoptera were reported (<0.3%). Nearly 80% of the trajectories were constructed in the prevailing Winter Monsoon and Trade winds, resulting in a southward displacement of insects towards putative overwintering areas as reported in previous studies. Tropical cyclones in autumn, produced trajectories which differed in both direction and extent from those in the prevailing winds, supporting the suggestion that the contraction of the distribution areas of rice pests at this time of year is the product of a series of movements in different directions. The results suggest that migrations continue throughout the year in the tropics and sub-tropics and indicate that this may be one way the capacity for long distance migration is maintained in some rice pest populations.