Migration of insect pests and natural enemies and its ecological significance

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Recent researches on insect migration and dispersal were critically reviewed and significance of the migration and dispersal was discussed with special reference to evolutionary aduptation of insects. Some migratory insects have the wing polymorphism. The winged or macropterous form plays a role of the migrant specialist and apterous or brachypterous form does the sedentary and reproductive specialist. Generally the former has a lower reproductive potential than the latter. The comparison of the intrinsic rate of increase between the two forms showed that the diffecence in the reproductive rate of the phase polymorphic species was less than that of the seasonal polymorphic species. The factors influencing the wing polymorphism were studied for rice planthoppers and veriid water striders, Microvelia spp. Genetic mechanism operated in the determination of wing polymorphism as well as the environmental factors like population density for both insect groups. Flight activity and biotic perf … More ormances of adults in relation to the migratory traits were investigated for the migrant skipper, Parnara guttata. Flight activity of the adults of the overwintered and the 2nd generations was higher than that of the lst teneration. Hether or not the skipper uses the earth magnetism for the orientation of the flight direction was examined. However, the hypothesis was not supported in the present experiment. Intergenerational changes in egg size of the migrant skipper P. guttata suggest that a parallel evolution probably occurs between egg size of skipper and leaf toughness of host grasses. The hypothesis was supported by the correlation analysis of the actual data. The green leafhoppers, Nephotettix spp. are non-polymorphic but disperse in the density-dependent manner. The sensitivity of 4 species to the density was measured in terms of the reduction of reproductive rate as increase of the density. The temperate species, N. cincticeps was more sensitive than the three other tropical species. Less