| Ulmus glaucescens open forest is one of the important open forest vegetation in the Helan Mountain,accounting for 84%of the sparse forest area in the Helan Mountain which plays an important ecological role in soil and water conservation,reducing soil erosion and providing biological habitat in this area.Orchestes mutabilis is one of the most important monophagous leaf-eating pests in Grey elm.From April 2018 to June 2019,a field survey was conducted on the ratio of infested grey elm trees and the population density of O.mutabilis from 9 sampling transects in valley of the Helan Mountain.The spatial distribution patterns of O.mutabilis larvae and its mechanisms with by using six aggregation indexes examination and two regressions analysis were investigated.The effects of topography and climatic factors on the spatial distribution of the larvae based on univariate regression analysis were analyzed.We studied the biological characteristics of O.mutabilis and relationship leaf morphology damaged and larvae development based on field investigation and indoor observation.The main results and conclusions of this study are as follows:(1)We've identified this weevil is O.mutabilis as a serious pest of gray elm in Helan Mountain.The life cycle of O.mutabilis was one generation in one year in Helan Mountain,within adults emerged early April from last-year overwinter.Adult layed eggs in the middle of April,hatched the larvae in the middle and last ten days,and The duration of the 3rd instar of the larvae was 2 months.The larvae changed into pupae after 15 days or so,and the pupae becomed adults after 7 or 13 days.The second generation adults oversummered early July and overwintered in the middle of September in withered branches and leaves,bark seams and surface loose soil.(2)Through the analysis of leaf morphology damaged by pest,the results showed that the adults of O.mutabilis tended to lay eggs in the direction of semi-cloudy slope and cloudy slope with better growth than that of leeward elms,and to lay eggs above the middle of main leaf vein.The larvae feeded on the upper part of the leaf and form vesicles.In order to reduce individual competition,it is necessary to avoid multiple individuals feeding on the same leaf.(3)According to the relationship between the characteristics of leaf feeding area and the development of larvae,the results showed that the total area of vesicles,holes and vesicles formed by insect feeding larvae was the largest in the middle canopy except the west direction.There was no significant difference between different orientations and different canopy leaf pore areas,the ratio of vesicle and pore are different.There was a significant positive correlation between the body length of larvae and the vesicle area.The larvae of O.mutabilis on the low-altitude elm feeded on more leaves,making the leaves form a larger bubble area.(4)The spatial distribution of larvae and the relation of its influencing factors were analyzed.The result showed the larvae density of grey elm in the central channel of helan mountain was higher than that of the northern and southern channels.The average density of larvae on semi-cloudy slope and cloudy slope was significantly higher than on sunny slope and valley.There was difference in the average density of larvae in different directions of the crown of grey elm.The larvae of O.mutabilis had an aggregated spatial pattern and the basic component distribution was individual group.The spatial pattern showed density dependence,in which aggreation increased with increasing population density.Larvae density was positively correlated with tree trunk height,and positively correlated with elevation and slope.The mean temperature of coldest season and the mean precipitation of wettest month were the most significant climatic factors affecting the larvae density of elms. |
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