氮肥对水稻叶冠层捕食性天敌种群及其自然控制能力的影响

Dynamics of predators in rice canopy and capacity of natural control on insect pests in paddy fields with different nitrogen regimes

为了公正地评价氮肥对整个稻田生态系统的影响,在国际水稻研究所试验场分别设立0、100和200kg/hm2三个氮肥水平,测定了水稻主要特性参数和叶冠层捕食性天敌的种群动态,同时以果蝇蛹作为替代猎物的方法在田间模拟评价了叶冠层捕食性天敌对害虫的自然控制能力。结果表明,虽然施用200kg/hm2氮肥稻田的稻株高度、叶面积和稻草干重显著比施用100kg/hm2氮肥稻田的高,但每丛分蘖数和稻谷产量则没有明显差异。在水稻叶冠层中,肖蛸、圆蛛、黑肩绿盲蝽、稻红瓢虫和捕食性蟌等为稻飞虱主要捕食性天敌。肖蛸在水稻孕穗期的数量最高,圆蛛的种群随水稻的生长而下降,但黑肩绿盲蝽、稻红瓢虫和捕食性蟌的数量却均随水稻的生长而增加。施用氮肥后稻田中肖蛸和稻红瓢虫的种群数量显著减少。在水稻孕穗期,叶冠层上蜘蛛网的数量和蜘蛛网总面积均随氮肥施用量的提高而显著减少。在水稻孕穗期和乳熟期,不施用氮肥稻田中的捕食性天敌对替代猎物果蝇蛹的捕食能力显著高于其它两个施用氮肥的稻田中的捕食能力。而在乳熟期施用100kg/hm2氮肥稻田的捕食能力又显著比施用200kg/hm2氮肥稻田的高。结果说明过量施用氮肥可削弱天敌对害虫的自然控制作用。

To evaluate reasonably the impact of nitrogen fertilizer on rice-based ecosystem, the main biological characteristic parameters of rice plants and population dynamics of predators in rice canopy were determined in paddy fields applied with 0, 100 and 200 kg/hm^2 nitrogen fertilizer, respectively, and the predation capacity of generalist predators on leaf- and planthoppers was also estimated by monitoring the predated number of alternative prey, pupae of fruit fly, Drosophila sp. , in a special predation trap. The results showed that plant height, total leaf area and dry straw weight of rice plants in field with 200 kg N/ hm^2 were markedly higher than that in field with 100 kg N/hm^2, however, no obvious difference between two fields was found in the number of tillers and yield of rice. Predators in rice canopy in all three tested fields were dominated by Tetragnatha sp., Araneus sp., Cyrthorhinus lividipennis, Micraspis sp. and Agriocnemis sp. The most abundant Tetragnatha sp. was sampled at booting stage of rice, while the number of Araneus sp. declined and Cyrthorhinus lividipennis, Micraspis sp. and Agriocnemis sp. increased with the growth of rice. Significantly abundant Tetragnatha sp. and Micraspis sp. were recorded in the field without nitrogen fertilizer than those with nitrogen fertilizer. Both number of spider webs and total web area on rice canopy at booting stage were strongly decreased with the increase of nitrogen fertilizer used in the fields. Predation capacity of generalist predators on pupae of fruit fly at booting and milky stages of rice significantly decreased in the fields applied with nitrogen fertilizer compared to that field without nitrogen fertilizer, and the lowest predation capacity was found in the field with 200 kg/hm^2 nitrogen fertilizer. It implies that excessive application of nitrogen fertilizer in paddy field maybe weaken the natural control of generalist predators on insect pests.