Effects Of Nitrogenous Fertilizer On Ecological Fitness Of The Brown Planthopper, Nilaparvata Lugens, And Its Relationships With Natural Enemies And Rice

Brown planthopper (BPH), Nilaparvata lugens (Stal), characterized by its monsoon migration and r-strategy life pattern in East Asia, is an important insect pest in the Asian rice grow n region. It has increased in abundance and shifted from minor to major pest since the mid-1960', due to the adoption of new rice productive practices such as planting widely of semi-dwarf, early mature, wide blade and high yield varieties, increasing cropping index, applying chemical fertilizers and pesticides. The excessive application of nitrogen fertilizer was considered to be the key factor to trigger the increase of BPH populations and frequencies of "hopper burn". However, the ecological mechanism on the influence of nitrogen nutrient on tritrophic interactions between BPH and its host rice and natural enemies is still unclear, especially on the changes in ecological fitness of BPH and its natural enemies continuously fed on rice plant with high nitrogen content, and on the relationship between nitrogen nutrient and the effective natural biological control functions.The main objectives of this research are to quantify the effect of nitrogen nutrient on the populations of BPH in fitness and their tolerances to adverse environmental stresses, and to assess the impact of nitrogen in the natural biological control functions, for a better integrated pest and nutrient management (IPNM) for rice.1. Effects of nitrogen regimes on the SPAD readings of rice leaves and the relationship between SPAD readings and the nitrogen content of rice plantsThe SPAD (the content of chlorophyll) readings of rice leaves at different rice growth stages were positively related to the rates of nitrogenous fertilizer, however the significant differences in SPAD readings were recorded among the replications of rice plants with the same rate of nitrogenous fertilizer. The nitrogen content of rice plants increased significantly with the increase of leaf SPAD readings both at different rice growth stages and different rice growing seasons, and the relationship between SPAD readings and nitrogen content of rice plants can be expressed as: N%= 0.1151 SPAD - 1.2772.2. Effects of nitrogenous fertilizer on water content, sap flow and tolerance to the brown planthopper of rice plantsBoth water content (WC) and relative water content (RWC) of rice plants were significantly increased, while the sap amount flowed from leaf sheath of rice plants were statistically reduced, with the increase of nitrogen content in rice plants. The RWC, which is more sensitive than WC to the change in nitrogen content of rice plants, in rice plants applied with high nitrogenous fertilizers strongly decreased by the injury of BPH nymphs. These may be the key factors to decrease the tolerance to BPH injury of rice plants with the increasing application of nitrogenous fertilizer.3. Effects of nitrogen content of rice plants on the oviposition and feeding behavior of the brown planthopperIn the free choice tests conducted in greenhouse, all adult females of BPH populations continuously fed on the rice plants both with high (N2) and low (N0) nitrogenous fertilizers preferred to laying their eggs on rice plants with a higher nitrogen content. The relationshipbetween numbers of BPH eggs laid (E) and the nitrogen content of host plants (N%) can be statistically regressed as: E = 85.036 N%-82.602. The numbers of eggs laid on the bottom of leaf sheath increased and those laid on the leaf blade decreased, when the nitrogen content of rice plants was increased. More adults selected to feed on rice plants with high nitrogen content than on those with low nitrogen content, while no obvious differences were found in feeding preference of BPH nymphs.Both nymphs and adults more frequently fed on NO rice plants than on N2 rice plants. The highest feeding frequencies of nymphs were found on NO rice plants in those nymphs that reared successively on NO rice plants. The feeding rates of newly emerged adult females were significantly increased with increase of the nitroge...