Effects Of Larval Density On Disease-resistant Ability And Immune Function Of The Diamondback Moth,Plutella Xylostella(L.)

The Diamondback moth (DBM), Plutella xylostella L.(Lepidoptera:plutellidae) is a worldwide pest. Its main host is the cruciferous vegetable, reaching more than40kinds of species. The insect population density affects the important parameters of its life history and physiological indicators. In order to ascertain the diamondback moth variation of its disease resistance and immune function under different larval densities, in this paper, we mainly investigated the effects of larval density on the disease resistance, humoral immunity and cellular immunity of diamondback moth using the theory and method of insect ecology and physiology. The main results are as follows:1Systematically studied on effects of larval density on the disease resistance in DBM We conducted an experiment on vitro host to determine the disease resistance, feeding, pupation and eclosion of DBM which were reared at different larval density (1,2,5,10,15,20larval per petri dish). The results showed that there were significant effects of larval density on the survival rate, survival time, average food consumption, pupation rate and eclosion rate of DBM which were vaccinated with the plutella xylostella granulovirus (Plxy GV). As the density increased, the survival rate and survival time of DBM were first increased and then decreased. The highest survival rate was at the larval density of10larval per petri dish and DMB owned the longest survival time at5-10larval per petri dish. Larval density had a significant effect on the feed intake. As the density increased, the food consumption of the inoculated group and uninoculated group both showed a downward trend after the first increase and owned the maximum at5-10larval per petri dish which was consistent with the survival rate and survival time trend. The larval stage at2-5larval per petri dish was relatively short, the overall performanced of the first decreased and then increased with the increase of density. The pupation rate and eclosion rate were the highest at2-5larval per petri dish. Therefore, the larval density was one of important factors influencing resistance of the larvae DMB as well as growth and development.2Systematically studied on effects of larval density on the humoral immunity in DBM We conducted an experiment on vivo and vitro host to determine the protein content, phenoloxidase activity, lysozyme activity and antibacterial activity of hemolymph of DBM which were reared at different larval density. The biological test results show that larval density had significant effects on the larvae of DBM above humoral immune indices. The protein content showed the first increase in the fall with the increase of density, and was the highest at5-10larval per petri dish, which was significantly higher than that of the other density treatments on vitro host. But there was no significant difference among the various densities of protein content on vivo host; The phenoloxidase activity showed the same trend both on vivo and vitro host, and it was highest at the density of5-10larval per petri dish, significantly higher than that of the other density treatments; On vitro host, the lysozyme activity increased gradually with the increase of density, but it showed the first increase and then decrease on vivo host, which owned the highest lysozyme activity at5-10larval per petri dish; The antimicrobial activity of the larvae hemolymph showed the same trend, and it was highest at the density of5-10larval per petri dish, significantly higher than that of the other density treatments both on vivo and vitro host. These results indicated that the larva density-factor lead to disease resistance difference by adjusting the phenoloxidase activity and lysozyme activity to affect the humoral immunity.3Studied on effects of larval density on the cellular immunity in DBM We conducted an experiment on vivo and vitro host to observe haemocytes morphology, number and proportion of various types of DBM which were reared at different larval density. Five types of haemocytes were identified for larva of DBM. The mayor haemocytes were plasmatocyte, granulocytes, plasmatocytes, spherulocytes and oenocytoids. The volume of haemocytes increased along with the density increased at first and then decreased, and it showed higher levels at5-10larval per petri dish both on vivo and vitro host. The number of plasmatocytes, spherulocytes and oenocytoids on vivo host was higher than that on vitro host. However, these haemocytes possessed lower ratios. The plasmatocyte and granulocytes owned a higher ratio, which were up to49.06%and41.91%respectively on vivo host. These results indicated that plasmatocyte and granulocytes played a major role, while other types of haemocytes played a supporting role in cellular immunity of DBM. The larva density affected number of various types of haemocytes and composition ratio to make cell immunocompetent different.