Introduction

Of over one hundred species of planthoppers (Homoptera: Delphacidae) occurring in East Asia, only three, the Brown Planthopper Nilaparvata lugens (BPH), the White-backed Planthopper Sogatella furcifera (WBPH), and the Small Brown Planthopper Laodelphax striatellus, both reproduce successfully on rice and migrate long distances (Kisimoto, 1981). Two of these species, BPH and WBPH, are serious pests of rice throughout the region. Outbreaks of both species have been recorded in Japan since the mid-1700s (Suenaga & Nakatsuka, 1958), but in the tropics, where BPH is the more serious pest, outbreaks did not occur until the mid-1960s (Kisimoto, 1984).

The significance of migration in initiating BPH and WBPH outbreaks was first tentatively recognised by Murata & Hirano (1929) who proposed that infestations in the Japanese mainland (i.e. the four main islands of Japan: Hokkaido, Honshu, Kyushu and Shikoku (Fig. 3.1)) are initiated each year by immigrations from the south during the Bai-u (Mei-yu in the Chinese literature) rainy season (June-July). This hypothesis was revived following the observation of a mass of WBPH and a few BPH swarming around a weather ship at location ‘Tango’ (29° N, 135° E) in the Pacific Ocean (Fig. 3.1) in July 1967 (Asahina & Turuoka, 1968). At about the same time, catches of planthoppers in pole-mounted tow-nets set at Chikugo (33°12’ N, 130°30’ E), on the western coast of Kyushu, were found to be correlated with warm and moist southwesterly or west-southwesterly winds that occurred in the warm sectors of northeastward-moving depressions (Kisimoto, 1976).