The Role of Migration in The Pest Status of Brown Planthopper in Temperate and Tropical Areas

Abstract

A radar study of the migration of brown planthopper (Nilaparvata lugens Stal) during the autumn in east central China revealed that the flight duration was strikingly different to that previously found for the same species during the dry season in the Philippines. Flights were largely confined to periods of about 30 minutes at dusk and dawn in the Philippines, with minimal activity at other times, whereas in China, migrants which took off in the period from late afternoon to dusk continued in downwind migratory flight for several hours and some may have flown all night. These different durations of migration may be characteristic of populations from the humid tropics and from the temperate zone, respectively. The relative importance in the ecology of N. lugens is correspondingly different in the two zones. In temperate areas, the initial long-distance migrants arrive in localities where rapid and sustained population increase is possible because of the ineffectiveness of regulatory factors, particularly natural enemies. In contrast, in the tropics, mortality due to natural enemies will usually prevent the build-up of damaging populations which might otherwise follow any immigration from a distant source area. However, migration could still be a major determinant of outbreaks in the tropics in certain situations, for example where short-range mass movements occur between asynchronously-planted contiguous rice cultivations. Migration of brown planthopper may also have an impact on its pest status through the spread of virus diseases, or of N. lugens genotypes which carry resistance to insecticide or which are able to overcome the varietal resistance of rice plants. The effect of the newly-arrived genotypes is often difficult to assess, however, and in some cases immigration may be beneficial, hindering the local evolution of virulent genotypes.