Influence Of Environmental Condition And Physiological Basis On The Host Plant Selection By Beet Webworm, Loxostege Sticticalis (Lepidoptera: Pyralidae)

The beet webworm Loxostege sticticalis Linnaeus (Lepidoptera:Pyralidae) is a world wide insect which has become one of the major disastrous serious insect pests to the economic crops in the northern, northeastern and northwestern of China. In order to elucidate the occurrence and damages of L. sticticalis, and provide a scientific basis for improving and enhancing its forecast and control technology, and reducing the losses caused by this pest, the research was done on the basis of a careful analysis of existing research results at home and abroad, which consisted of a series of outdoor combined indoor studies and eco-physiological experiments. The study involved with selection of oviposition host plants of beet webworm and the main influence factors, the choice of feeding host plant of the larval, the influence of main host plants on the growth and development of larvae, nutritional effect on shifting host of the larval. The difference of the activity of lipase, amylase and total protease of the larval, and the various proteases in the midgut of larval among 1-5 instars was tested. It demonstrated that the L. sticticalis had different host ranges between oviposition and feeding behavior, and the affecting factors were found out. It was affirmed the nutrition value of the larvae changing the host plants, and affirmed that the activy of the change of major enzymes after 3 instar in the L. sticticalis larvae was one of the main reasons why the larvae gradually increase the scope of host plants. Main results are as follows:The host plant species on which eggs laid were systemically investigated in the suburb of Kangbao County of Hebei Province on August,2008. Results obtained exemplified that adult females of L. sticticalis showed prominent preference to the ovipositing host plant species. There were 13 herbaceous species in 7 families were found to be with eggs of L. sticticalis accounts for 78% and 56.3% amongst the 23 species in 9 families encountered during the investigation period. Adults have significant preferences on the stinkgrass Eragrostis cilianensis (All.) Link ex Vignolo-Lutati. On E. cilianensis, Single plant egg amount, rate of oviposited plant, egg density on the same species per square meter, the relative number of eggs of L. sticticalis on different plant species, were all significantly higher than that of Chenopodium album Linnaeus and other plants, even more than 10 times to 100 times of the C. album. These results did not conform to the existing results that the meadow moth prefer C. album to oviposite. The selection of ovipositing host plant species is irrelevant to plant density, and leaf width of the ovipositing plant species but is positively and significantly correlated to the length and ratio of leaf length to the leaf width. That is, adult L. sticticalis preferred to select species with thread-like leaf as ovipositing host plants during this investigation period. The relative humidity during the adult peaking periods are considered to be the major factor that alternated the host plant species preference by the adult female L. sticticalis. These newly discoveries demonstrate that the selection of ovipositing host plant species by L. sticticalis is alterable and this alteration is an adaptive strategy to the variation of environmental condition.Based on the data of systematic investigation, weeds of 22 species of 9 families were identified foraged by L. sticticalis larvae, accounts for 90% and 84.3% in all investigated plants of 26 species in 10 families. The damaged plants were distributed mainly in Compositae (9 species), Gramineae (2 species), Chenopodiaceae (4 species), Polygonaceae (2 species), while only 1 specie was found to have larval L. sticticalis in some families, such as Leguminosae, Labiatae, Cruciferae, Convolvulaceae, Geraniaceae. The result indicated that the larvae can damage much more species of plants than the adult selected for oviposition, and preferred feeding dicotyledonous plants. The numbers of the larvae found on different plant species are significantly different. Medicago sativa Linn had the highest proportion of the larvae, C. album took the second place, and however, there were few larvae on Setaria viridis and E.cilianensis. These tesults indicate that the host selection of the adult was not always coinciding with the larvae. And results from correlation analysis demonstrated that the number of the larvae on the plant was influenced by plant density.The survival rate and pupal weight was determined with 13 species of plants oviposited and foraged by adult and larval L. sticticalis. It was found that different host plant species had a significant impact on larval growth and development. Firstly, the larvae fed on E. cilianensis which adults faviout to lay eggs on had the lowest survival rate of less than 2%, moreover, the 5th instar larvae can not pupate, the larvae fed on C. album had the highest survival rate of 83.7%, which is significantly greater than those fed on other host plants. Secondly, the larvae fed on C. album had the the largest pupal weight of 38.9 mg, significantly higher than that of other host plants apart from Artemisia sieversiana Ehrhart ex Willd. The result showed that C. album is the most favorable host plant to growth and development of L. sticticalis. Finally, results obtained also demonstrated that the pupal weight and survival rate were greatly improved when the larvae were shifted from C. album to a host plant after 3rd instar. These are more prominans for larvae that shifted from C. album to Sonchus arvensis Linn. These results indicated that food plants were the most important factor to the growth and development of the larvae before the 3rd instar, and C. album played a key role in the growth and development of the larval L. sticticalis. Modern physiological methods was applied to study the variation of activity of lipase, amylase and total protease of the larval, optimum PH value of primary protease in larval midgut, the difference between instars.The result further confirmed the variation regularity of these enzymes, and the increased activity and a wide range of PH value of protease activity is the reason of why the larvae had the capacity to adapt to a more extensive host range of L. sticticalis larvae with the increase of instar. A wide range of optimum PH of the corresponding protease systems always ensure a protease which have a maximum activity of enzyme to adapt rapidly to different host plants, thus it may be one of the physiological basis that larvae can forage majority of the herbaceoas plant species.