Swarming Advantage Of Loxostege Sticticalis (Lepidoptera:Pyralidae) On The Growth, Fecundity, Migratory And Its Physiological Causes

The beet webworm, Loxostege sticticalis L.(Lepidoptera:Pyralidae) is an important migratory insect pest of crop and fodder plants in northern China. Larval population density plays an important role in affecting the population dynamics of L. sticticalis. The aim of all the research is to clarify the sparse and outbreak population, supply the theoretical basis of the prediction and forecast skills for L. sticticalis. In this paper, on the basis of existing research results we mainly investigated the effects of larval density on the growth, development, cellular immunity and adults migratory behavior by the ecological and physiological technique on intact plants. The main innovative results obtained are:To better understand its outbreak mechanism, we conducted a laboratory experiment on intact plants to determine the development of L. sticticalis which larvae were reared at different larval density (1,5,10and20larvae/jar), corresponding to field densities of130,650,1300and2600meadow moth m-2. Several studies have shown that larvae grow fast and adult have better flight performance on a proper group feeding larval density. However, why the gregarious morph have the better performance is still not clear in L. sticticalis. According to this study on intact host plants, individuals reared at isolated conditions (1larva/jar) showed the longest larval and pupal developmental time, and they decreased with the larval density increasing. The pre-oviposition period was significantly longer at the density of5-10larvae/jar than1larva/jar. The survival rate from larvae to prepupae and weight of S^-instar larvae increased with the larval density increasing. There were no significant differences in the pupation rate, abnormality rate of adults and eclosion rate at different larval rearing density. The ratio of female and male of adults L. sticticalis increased with the larval density increasing. The larval and pupal developmental uniformity decreased with the larval density increasing. However, there was no such variation trend for the meadow moth under these rearing density fed on excised host plants.To better understand its outbreak mechanism, we conducted a laboratory experiment on intact plants to determine the fecundity of L. sticticalis which larvae were reared at different larval density (1,5,10and20larvae/jar), corresponding to field density of130,650,1300and2600meadow moth m-2. There were no significant differences in the oviposition period, mating percentage, mating frequency, longevity of female and egg hatch rate on intact host plants at different larval rearing density. According to this study, the beet webworm has more reproduction with the gradual increase of rearing density among1,5,10and20larvae/jar fed on intact host plants. However, there was no such variation trend for the meadow moth under these rearing density fed on excised host plants. The innate capacity for increase (rm), the net reproductive rate (R0), and the finite rate of increase (λ) increased with the increase of the rearing density. However, this density-dependent effect did not occur either when the meadow moth was fed with excised plants at the same rearing density. By comparing the groups fed on intact and excised host plants we found that host plants condition is an important factor in affect the density-dependent variation trend in meadow moth. These results provide further evidence of larval group feeding dominance for L. sticticalis. This supports the hypothesis that larval group feeding dominance occurs in L. sticticalis. These results were provide a scientific basis to differentiate the characteristics between the sparse and outbreak population and to pursue the causes for existence of the cyclic outbreak of L. sticticalis.To understand the roles of larval density played in the cellular immunity and population dynamics of the beet webworm, the type, number and composition of haemocytes in the5th instar of larvae developed from the density of1,5,10, and20larvae/900mL jar were investigated by using the light microscopy, Giemsa stain method, and neubauer haemocytometer on intact host plants. Five types of haemocytes were identified for larval L. sticticalis. They were prohemocytes, granulocytes, plasmatocytes, spherulocytes and oenocytoids. The number of plasmatocytes, granulocytes and total haemocytes increased as the larval density increased and their number at the density of20larvae/jar was significantly greater than that at the density of1and5larvae/jar although the number of prohemocytes, spherulocytes, and oenocytoids was insignificantly affected by the rearing density. The proportion of each haemocyte type to the total haemocetes ranked at similar order in the4treatments but the ratio of plasmatocytes at the density of10and20larvae/jar was remarkably greater than that of the isolated larvae. These newly obtained results suggested that to increase the plasmatocytes and granulocytes number and immune capacity of larvae is one of the physiological strategies for L. sticticalis to adapt the increased larval density, which provided a basis to differentiate the characteristics between the sparse and outbreak population and to pursue the causes for existence of the cyclic outbreak of L. sticticalis. To understand the roles of larval density played in the flight potential of adult beet webworm, the total flight duration, flight distance and flight average velocity from the density of1,5,10, and20larvae/900mL jar were investigated by using the tethered-flight techniques on intact host plants. The larval density had a significant effect on the flight potential of the L. sticticalis on intact host plants. Larval density had no significant effect on the total flight duration. Flight distance and flight average velocity increased with larval density increased. But there was no significant for the meadow moth under these rearing density fed on excised host plants. Adults reared at a density of10-20larvae per jar on the intact plants displayed the greatest flight capacity, relative to reared on the excised host plants. These results suggest that the host plant had significant influence on flight potential of adult L. sticticalis.