Effects Of Nitrogen Fertilization On Development Of Spodoptera Litura Larva And Its Parasitoid Meteorus Pulchricornis

Application of nitrogen fertilizers to plants can affect development and reproduction of herbivorous pests that feed on the plant directly and their parasitoid indirectly. Investigation of effects of nitrogen fertilization on herbivorous insects and their parasitoids can provide important data for the integrated control practices of the pests in agricuture. In this thesis, a series of experiments were conducted using soybean-Spodoptera litura-Meteorus pulchricornis as the model system, to examine performances in development of the host larva and its parasitoid under treatments of nitrogen fertilization applied to the potted plant. The main results obtained from this study were summarized below.1Effects of nitrogen fertilizer on consumption and food utilization efficiency of S. litura larvaTo understand the impact of nitrogen fertilization on food consumption and utilization of potted soybean plants, the fertilization was manipulated as5categories (20,50,100,200and300mg·L-1) as N1, N2, N3, N4and N5. The4th instar larva was tested. Results showed that food consumption and food degested of S. litura4th larva had cubic relationship with the nitrogen fertilization levels, the frass excreted increased by34.71%from the N1to the N5level, nitrogen significantly affected the final weight of S. litura4th larvae which increased by0.02g while nitrogen increased by1g. There was a non-linear relationship in the relative consumption rate (RCR) across nitrogen fertilization levels, increasing by47%from the N1to N3, and then decreased. There was no significant relationship between RGR and nitrogen fertilization levels. The approximate digestibility (AD) increased from39.59±1.69%(Mean±SE, the same below) under the N1to58.77±2.29%under the N3treatment, and then decreased to38.71±2.24%under the N5. The efficiency of conversion of ingested food (ECI) decreased and then increased with increasing nitrogen application. The efficiency of conversion of digested food (ECD) decreased from22.90±1.78%under N1to9.69±1.42%under N3, and then increased to26.08±2.60%under N5. The results of this experiment suggested that, the food consumption increased and then decreased and finally stabilized, frass dry weight increased, the final weight of S. litura larva increased with the increasing nitrogen fertilization.2Effects of nitrogen fertilization on developmental performances of S. lituraLarval developmental performances were observed while feeding the soybean plant treated with treatments of different nitrogen fertilization levels (five). Results showed a non-linear relationship between egg-to-pupation time and nitrogen fertilizer levels, where the time first reduced from21.07±0.29d (Mean±SE, the same below) under the N1to18.39±0.25d under the N3treatment and then increased to19.38±0.21d under the N5. The nitrogen fertilization did not significantly affect pupal duration, and pupal weight increased by26.9%from the N1to N5treatments. There was a significant linear relationship between adult longevity and nitrogen fertilization, the longevity extended by1.31d from the low to high fertilization levels. With the increasing nitrogen, adult-body dry weight increased by21.4%. The results of this experiment suggested that, with the increasing nitrogen fertilization, the size of S. litura body increased and life extended, but under the excessive nitrogen fertilization, larval period extended, which increased the risk of its exposure to natural enemies.3Effects of nitrogen fertilization on developmental performances and host selection of M. pulchricornisNitrogen nutrition in the host plant can influence the parasitoid that attacks the host in developmental performances of offspring parasitoids. Nitrogen fertilization was manipulated as5categories, and imposed on soybean plants, which were used to feed S. litura neonate larvae until either2nd or4th instar when they were exposed to M. pulchricornis for parasitism. The results showed that nitrogen fertilization had a significant influence on the parasitoid in its development. Compared with the4th instar larvae, when the2nd instar larva was parasitized, the offspring parasitoid performed with increasing N fertilization as:(1) decreasing larva-to-pupation developmental time,(2) increasing pupal weight from5.59mg under the low N fertilization level (N1) to6.43mg under the high N level (N5),(3) extending adult longevity from3.69d under the low N fertilization level to5.75d under the high N level,(4) increasing hind tibia length from1.40mm under the low N level to1.49mm under the high N level, and (5) increasing pupation rate. The results of this study suggest that increasing nitrogen fertilization may benefit the parasitoid in development, and the effect can vary with host instars that were attacked. There is no significant preference for the host larva feeding on soybean leaves fertilized by nitrogen.