Comparative analysis of population dynamics of the brown planthopper between the tropical and temperate region.

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Distinct differences in the basic features of population dynamics of the brown planthopper were detected between the different rice cropping systems in the tropical and temperate regions : (1) In the synchronous cropping area in the tropics, BPH populations multiplied rapidly in the period from initial to peak generation, reaching quite often the destructive level despite the low density of initial immigrants, as shown in the temperate regions. However, in the staggered cropping area and also in the second crops in the synchronous area, the population growth rate and the peak population density were much lower. The abundance of natural enemies such as parasitic wasps playd a major role in determining such a difference in seasonal population development. (2) The density at the peak generation or the occurrence of outbreaks in each paddy field was predictable in the formers with fairly high accuracy on the basis of the density at the initial or previous seasonal generations. In the latters, however, the rate of population growth and the peak population density widely varied among the fields depending on the water status in each field. (3) In the formers, the regulatory processes were only detected in such high densities as cause the considerable deterioration of host plant, because of intra-specific competition. However, in the latters, the regulatory processes operated at a much lower density in the earlier stages of the crops. Density-dependent dispersal of macropterous adults playd an important role to stabilize the population fluctuation among the fields.