Effects Of Larval Density On Biological Characters And Immune Function Of The Diamondback Moth, Plutella Xylostella(L.)

Insect density-dependent immune function has been reported at domestic and abroad. In order to explore the variation regulation of population growth and resistance to disease for larva of Plutella xylostella in different densities, the effect of larval density on its development, population growth, biochemicals and immune function, were studied in this paper. The results are as follows:1Studied on effect of larval density on growth and reproduction of the diamondback moth(DBM), P. xylostella(L.). The results showed that there were significantly effects on development duration, pupal weight and survival for larval of DBM in different densities. The development duration of larva was the shortest at10larvae per petri dish, the longest at1larva per petri dish. The pupal duration was the shortest at10larvae per petri dish, the longest at40larvae per petri dish. The pupal weight was the highest at1larva per petri dish, next at10larvae per petri dish. Both of them were significantly higher than the others. The survival of larva to pupa was highest at20larvae per petri dish, next at10larvae per petri dish. Both of them were significantly higher than1and40larvae per petri dish. There was significant influence on malformation rate, fecundity and longevity in different larval densities. Malformation rate of adults was the lowest at10larvae per petri dish and the highest at1larva per petri dish. The total fecundity of per female showed the same trend with the pupae weight as larval density increased. The longevity of female was the longest at40larvae per petri dish, the shortest at1larva per petri dish which was significantly shorter than other densities. Life table in lab at different larval densities indicated that the population growth index was the highest at10larvae per petri dish, next at1larva per petri dish. The higher population densities suppressed population growth. It is so concluded that larval density is one of the major factors influencing population growth of DBM.2Studied on effect of larval density on biochemicals of the DBM. The amount of the total carbohydrate and protein in the hemolymph and lipid of larva fed in5different densities were determined. The results showed that the total carbohydrate in the hemolymph of larva was the highest at20larvae per petri dish(23.84mg/mL) which was significantly higher than that of30and40larvae per petri dish. The amount of protein in the hemolymph of larva at10and20 larvae per petri dish was significantly higher that of other densities. The amount of lipid was the highest at1larvae per petri dish(63.72%) which was significantly higher than that of20,30,40larvae per petri dish. Biochemicals of larva of DBM treated by LC30of Bacillus thuringiensis(Bt) and Beta-cypermethrin in different densities was significantly affected. The total amount of carbohydrate of larva of DBM treated by Bt was11.39mg/mL at1larva per petri dish, which was significantly higher than40larvae per petri dish. The amount of protein of larva of DBM treated by Bt at20larvae per petri dish was significantly higher than at1,30,40larvae per petri dish. The content of lipid of larva of DBM treated by Bt was66.73%at1larvae per petri dish, which was significantly higher than that of other densities. The total amount of carbohydrate of larva of DBM treated by Beta-cypermethrin at10larvae per petri dish was significantly higher than that of other densities while there was no significant difference among the various densities of the protein content. The effect of larval density on biochemicals provided a scientific evidence for the density dependence of development and population growth of DBM.3Studied on the variation of immune function of the DBM larva in different densities. The number of total hemocyte, the activity of larval phenoloxidase(PO) and lysozyme(LSZ) of larva of DBM fed in5different densities were determined. The results were as follows: There were significantly effects of different densities of DBM larva on its immune function. The number of total hemocyte was the highest at10larvae per petri dish(90.67×105), which was significantly higher than that of1and40larvae per petri dish. The activity of PO was also the highest at10larvae per petri dish (56.76U/mL) which was significantly higher than that of other densities. The activity of LSZ was87.47U/mL at20larvae per petri dish which was significantly higher than that of other densities. Immune function of larva of DBM treated by Bt and beta-cypermethrin in different densities, the results indicated that there were significantly effects of two insecticides on immune function of DBM in different densities. The number of total hemocyte of larva treated by Bt was113.33×105at10larvae per petri dish which significantly higher than that of the1,30,40larvae per petri dish. The activity of LSZ at10and20larvae per petri dish was significantly higher than at40larvae per petri dish. There was no significantly difference of the PO activity among five different densities. The number of total hemocyte of larva treated by Beta-cypermethrin was122.67×105at10larvae per petri dish which was significantly higher than that of other densities. The activity of LSZ of larva treated by Beta-cypermethrin was170.56U/mL at10larvae per petri dish which was significantly higher than at1and40larvae per petri dish. Otherwise, the activity of LSZ of larva treated by two insecticides was significantly higher than the control group. These results suggested that there were effects of larva in different densities on immune function of DBM. These results were also of great importance for controlling population dynamics of DBM.