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Application of nitrogen fertilizer affects natural control of the rice brown planthopper( Nilaparvata lugens Stl) by arthropod natural enemies

ZHU Pingyang;HEONG Kongluen;Sylvia VILLAREAL;L Zhongxian;State Key Laboratory Breeding Base for Zhejiang Sustainable Pest and Disease Control,Institute of Plant Protection and Microbiology,Zhejiang Academy of Agriculture Sciences;Jinhua Plant Protection Station;International Rice Research Institute;Institute of Insect Sciences,Zhejiang University;  
Rice crop yields often increase with applications of nitrogenous fertilizer. However excessive nitrogen use can also increase the susceptibility of the crop to pests and diseases. In addition excess nitrogen in water systems is a pollutant that can threaten aquatic fauna and human health. Increased crop susceptibility to insect pests also affects the relationship between pests and their natural enemies and thus can reduce natural control that can potentially result in outbreaks. To evaluate these effects of nitrogen on the natural control of insect pests by arthropod natural enemies in rice field,we experimentally explored the predation ability and natural control capacity by arthropod natural enemies on the rice brown planthopper( BPH),Nilaparvata lugens Stl using different nitrogen fertilizer regimes in field plots conducted in the International Rice Research Institute,Philippines. The natural enemy exclusion method was used in filed plots wherenitrogen( 0,200 kg N/hm~2 and 200 kg N/hm~2) was applied at and repeated for both the dry and wet season. BPH and natural enemy densities of inside the exclusion cages were monitored using the Blower-vac insect suction device. The predatory capacities on BPH nymphs by generalist predators,and the functions of natural biological control on BPH populations by natural enemies including predators and parasitoids were studied. Results showed that the predation ability of natural enemies and the natural control functions were significantly weakened with the increasing use of nitrogen fertilizer in dry season. Predation rates of natural enemies with the highest nitrogen use( 200 kg N/hm~2) were all lower than 40%.Nitrogen levels,rice growth stages and their interaction had significant effects on the predation ability. The rate of natural control( RNC) in rice fields with no nitrogen application( 0 kg N/hm2) in the different growth stages was more than 95%,highest among the 3 nitrogen fertilizer levels. The natural biological control functions in wet season had no significant effects on the predation ability in the different levels of nitrogen fertilizer. The different nitrogen fertilizer levels had no effects on the natural enemies. Meanwhile obvious higher densities of the natural biological control functions of BPH in rice fields applied with high usage of nitrogen fertilizer than that with low. This phenomenon indicates that a high nitrogen fertilizer application decreased the control ability of the natural enemies,on the other hand,the BPH population increases too fast with the high nitrogen fertilizer application,that beyond their natural enemies' control ability. Our results strongly supports the conclusion that excessive applications of nitrogen fertilizer in rice fields make them vulnerable to BPH outbreaks through weakening of the natural control functions of the natural enemies.
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