The Wasp Parasitoids And Their Regulating Roles To The Populations Of Beet Webworm, Loxostege Sticticalis (lepidoptera: Pyralidae)

The beet webworm, Loxostege sticticalis L. (Lepidoptera: Pyralidae) is one of the most destructive insect pests worldwide. Great yield and economical losses have been caused by this pest since the founding of new China. The species of wasp parasitoids and their regulating roles to the populations of L. sticticalis have been less investigated and were mostly unknown in the home and abroad. The species of wasp parasitoids for egg, larvae and hibernating larvae of L. sticticalis were investigated to clarify the occurrence and regulariting roles of the parasitoids for the populations of L. sticticalis in this thesis, which is of great significant to improve its forecasting and management technology. The number of wasp parasitoid species for the egg, larvae and hibernating larvae were clarified, the dominant species and their regulating roles to the main developmental stages of L. sticticalis were confirmed, and environmental factors affecting the parasitism rates of the host were well understand, and basic biology of the dominant species were also demonstrated by using ecological principal and methods through a two-years studies. The main results were summarized as follows:Totally, there were 16 species of wasp parasitoids for L. sticticalis were found through a tow-years field investigation in 5 provinces in northern China. Systematically, they were 1 species of trichogramma (Trichogramma sp.), 6 species of brachonids (Apanteles sp., Orgilus ischnus, 1 species of Orgilinae, Cotesia sp., Zele chlorophthalmus), 7 species of ichneumonids (1 species of Porizontinae, 3 species of Diadegma sp., Temelucha sp., Agrypon flexorius, and Mesochorus sp.), 1 species of perilampid (Perilampus nola), and 2 unidentified species. Developmentally, species in trichogramma was egg parasitoid, while those in brachonids, ichneumonids, and perilampids were larval parasitoids. Functionally, P. nola and Mesochorus sp. were hyperparasitoids while the remains were primary parasitoids. Among the wasp parasitoid species found, the Trichogramma sp. was the first egg parasitoid discovered in China, and remains species were almost completely new parasitoids of larval L. sticticalis to the world.Dominant species of the wasp parasitoids and their roles in regulating populations of L. sticticalis were confirmed through the investigations of eggs and 1-5 instars of larval L. sticticalis during 2nd generation of 2008 and 1st generation of 2009 in Kangbao County of Hebei province. Results obtained showed that the egg parasitism rate by Trichogramma sp. was around 1%, and the Trichogramma sp. preferred to select eggs with higher levels of maturation and smaller egg masses as host. Secondly, each instar of larval L. sticticalis could be parasitized by several species of wasp parasitoids. Number of the wasp parasitoid species found for the 1st, 2nd, 3rd, 4th, and 5th instar of larval L. sticticalis was 6, 8, 10, 8 and 8 and their respective diversity index was 1.18, 1.23, 1.64, 1.61 and 1.61 during the 2009 season. Correspondently, the parasitism rate of each instar of larval L. sticticalis by the wasp parasitoids was increased from the 1st to 3rd instar but decreased gradually from the 3rd to 5th instar. The parasitism rate in both of the 2008 and 2009 seasons showed the similar trend although the parasitism rate of each instar of larval L. sticticalis in the former season (2% - 18%) was much lower than that (52.7% - 90%) in the later season. Thirdly, roles the wasp parasitoids species played in regulation of the larval population of L. sticticalis are different and depended on what the developmental stages of the larvae they parasitized. Species of Apanteles sp., O. ischnus, Porizontinae, and Diadegma sp.1 mainly selected 1st to 3rd instar of larval L. sticticalis as their host. The host larvae died or cocooned advance the time in about 3rd instar, the larval L. sticticalis can be controlled before the overeating, the damage of crop by larval L. sticticalis was greatly decreased, these species, then have a great value to practical applications. The 2nd and 3rd species of Diadegma sp. and Temelucha mainly selected 3rd to 5th instar of larval L. sticticalis as their host. The host larvae would not stop eating and developing after they were parasitized. They could cocoon in the soil surface and died until the wasps completed their development. These wasp parasitoids, together with tachinids play an important role in reducing the population sources of L. sticticalis for next generation. Finally, length and weight of the cocoon made by larval L. sticticalis parasitized by different wasp parasitoids were decreased to some extent in comparison with that made by the un-parasitized larvae, indicating that the viability of the host larvae were greatly decreased.The species of wasp parasitoids and their parasitism rates, dominant indexes were illustrated for the 51 hibernating larval populations of 2008 located in Hebei, Shanxi, Inner Mongolia, Jilin and Liaoning provinces. Number of wasp parasitoid species and parasitism rate of the hibernating larvae of L. sticticalis varied greatly as the investigated population differed. Of the population investigated, number of the wasp parasitoid species found in Xingan District, Inner Mongolia was the highest (13), and followed by that (10) in Kangbao, Hebei Province, while the species found in northern Shanxi, central-western Jilin and Liaoning Province was only 3, 3, and 1, respectively. Results also showed that the areas with rich species of wasp parasitoids have a higher parasite rate for the hibernating larval populations. The dominant species varied as the geographical population differed, but species in Orgilinae and A. flexorius were the most common and dominant species among the populations investigated.The results obtained from the 2008 hibernating larval population of L. sticticalis also show that the parasitism rates for the wasp parasitoids were greatly affected by the environmental condition and farming practice. Firstly, the parasitism rate of L. sticticalis larvae at the field ridge was significantly greater than those in the field as demonstrated by results obtained from 3 investigation sites of 3 counties. Furthermore, in contrast to the untreated fields, the parasitism rate of the hibernating larvae in the field treated by insecticides was significantly decreased. Parasitism rate of the wasp parasitoids in the field treated by insecticides was only 1/3 and 1/2 of the untreated field. Vegetation types and farming systems also influenced the parasitism rate of wasp parasitoids to certain extent. Greater parasitism rate of the hibernating larvae by the wasp parasitoids are found in grassland and dicotyledonous crop fields in comparison with that in the monocotyledonous crop field although the parasitism rate varied greatly in the field with same type of vegetation.The host range, longevity of two wasp hyperparasitoid species and parasitism rate to the primary parasitoids were demonstrated after the basic biology for the primary wasp parasitoids of egg and larval L. sticticalis were illustrated. Trichogramma usually need 9-14 days to complete its development from egg to adult. Sex ratio of female and male increased as the decrease of ambient temperature. Mating activities occurred in the day of emergence (0-24h). The adult trichogramma lived only for 1 day under light condition, but male adult could live 4 days under completely dark conditions. The lifespan of all the virgin wasp parasitoids of larval L. sticticalis were between 2 to 5 days when they were fed with honey solutions, and female wasps lived generally longer than males. The hyperparasitoids of P. nola and Mesochorus sp. mainly parasitisized the primary parasitoids in the 3rd to 5th instar of larval L. sticticalis. The P. nola distributed widely and could parasitize many species of wasp parasitoids and tachinids found in all instars of the larval L. sticticalis. In addition, the lifespan of the adult P. nola was much longer than the primary wasp parasitoids, and could be suspended animation. The hyper-parasitism rate for primary parasitoids found in the 3rd to 5th instar of larval L. sticticalis in Kangbao County, Hebei Province, 2009 were 32.13%, 63.18% and 51.29%, respectively. The P. nola, is therefore a dominant species for the primary parasitoids for its great parasitism rates found on each instar of larval L. sticticalis.