Studies On The Effect Of Soil Water Stress And Fertilization On Spodoptera Litura (Fabricius) And Its Potential Mechanism

Soil Water and nutrient was not only necessary for the growth of plant, but also had profound effects on insects by the morphology and physiology change of plant. In the experiment, we investigated the effects of soil nutrient and water availability on the performance of Spodoptera litura (Fabricius). A 2×3 fully factorial design was used on the cabbage, which consisted of two level nutrient treatments (high and low) and three water treatments (dry, suitable and flooded). The treated cabbage leaves were used to feed the larva of S. litura and tests were conducted. Otherwise, in order to making the mechanism clearly, the size and rigidity of different treated leaves were measured and quantitative analysis were done not only on the primary nutriment including water, soluble sugar, starch and free amino acid but also on the important secondary chemistry such as glucosinolates. The aim of the study were to determine:1)whether soil water and nutrient influence the development and population of S. litura; 2) whether S. litura show some preference to different treated cabbages and whether the larva has different utilization of cabbage nutrition under different treatments; 3) the potential mechanisms of the effects of soil water and fertilization on S. litura.The experiments demonstrated that water stress and fertilization to cabbage had profound effects on the development and population dynamics of S. litura. The larva which fed on drought stressed cabbages had shortest development time and heaviest weight than that under other water treatments. Otherwise, the survival rate of the larva and water content of the larva body increased under the drought treatment which also led to high level innate increase rate and finite increase rate to the population dynamics of S. litura. In contrast, flooded stress to cabbage limited the development of S. litura which significantly decreased the growth rate of larva and resulted to the decline of population. However, the abundance of nutrient was beneficial not only to the larva development but also to growth of its population. Under the 2×3 factorial design experiment, S. litura were be with best performance at suitable water with abundance nutrient (Suitable Water*High Fertilization) was most advantage to the reproduction and population increase and the average number of eggs laid by per female could reach 2727 and the innate increase rate reached 0.1874.The results showed that not only the nutrition utilization to different treated cabbages by larva but also the preference behavior including food selection by larva and oviposition preference by adult were mediated by water stress and soil nutrient. Except the amount of treated cabbage consumed by larva and relative consumption rate of the larva, different water treatment had profound effect on the nutrition utilization. Drought stress to cabbage promoted the efficiency of conversion of ingested and digested food of 3rd larva but reduced the approximate digestibility (AD). Compared to drought stress, the relative growth rate and efficiency of conversion of digested food (ECD) were in low level under the flooded stress. In the condition of abundance nutrition, the 3rd instar larva consumed more cabbage leaves and developed more quickly than that in nutrition scarcity condition. As far as the herbivore's host preference was concerned, the 1st instar larva showed no preference to the different water and fertilization treated cabbages, but 3rd instar larva preferred drought stressed cabbage to flooded stress, indicated that the drought stress made the cabbage much more lure to 3rd instar larva than the flooded stress. S. litura adult showed no preference for different treated cabbage to oviposite, although the cabbage with flooded stress could be seen a little more eggs.The performance of S. litura to different treated cabbages may associate with plant traits, nutrient and secondary chemistry changes of leaves under different water and fertilization treatments. The results indicated that soil with abundance nutrition and water was propitious to the outspread of cabbage but the rigidity of its leaves was not significantly different under different water and nutrient treatment. The water content of cabbage leaves which under the treatment of flooded stress or high fertilization was higher than other water or nutrition treatment. It seemed that the soluble sugar fluctuation in our experiment had much more connections with different water treatments and it went up under flooded stress which was opposite to drought stress. The starch content was increased under the drought stress and low level of fertilization. For the nitrogen nutriment of plants, soil fertilization and water to cabbage was beneficial to the synthesis of free amino acid, which was consistent with the change of total nitrogen under different water and fertilization treatment. The total glucosinolates content declined both under drought stress and flooded stress, whereas the influence of water stress and nutrition on each glucosinolates was different.Finally, we analyzed the relations between the performance of S. litura and the chemistry dynamics of cabbages under different water and nutrient treatment. It turned out that change of several chemistry played an important role in the development, reproduction and population dynamics of S. litura. First of all, it was the nitrogen especially the abundance of free amino acid that mostly benefited the development and population of S. litura. Secondly, the glucosinolates also affected the interaction between the cabbage and S. litura. It stimulated the feeding behavior of larva but was harmful to S. litura. The third important substance was carbohydrate which shortened the duration of development.Besides, analysis indicated that the effects of soil water and nutrition were interactive on cabbage and S. litura. Referring to water stress, though we did some research on the indirect influence of soil water on S. litura, it was so complex that need much more study especially the field experiment.