| Oedaleus decorus asiaticus is one of the dominant species in grassland and pastoral areas where locust plague is serious in northern China.With the gradual change of climate,the damage of this pest is getting more serious and would be a great threat to the agricultural and animal husbandry production.The grasshopper has the characteristics of long-distance migration,but the knowledge of this aspect is still quite few,which hindered the agricultural forecasting and scientific controlling.In this study,we will study the take-off behavior of the grasshopper by setting a net room in field;then analyzed a five-years’(2011-2015)historical data of the grasshopper in northern China to explore the occurrence rules;and last did a case study of the migration events on Yanqing,Beijing by using WRF mesoscale meteorological model and trajectory analysis method.The main results are as follows:1.Observations on the field take-off behaviour of Oedaleus decorus asiaticusThe take-off behavior of migratory O.asiaticus was investigated in a field cage in the Inner Mongolia region of northern China.The species shows a degree of density-dependent phase polyphenism,with high-density swarming populations characterized by a brown morph,while low-density populations are more likely to comprise a green morph.It was found that only a tenth of brown-coloured individuals engaged in migratory take-off,and fewer of the green-coloured individuals.Migratory grasshoppers took off at dusk,especially in the half hour after sunset(20:00-20:30h).Most emigrating individuals did not have any food in their digestive tract,and the females were mated but with immature ovaries.In contrast,non-emigrating individuals rarely had empty digestive tracts,and most females were mated and sexually mature.Therefore,it seems clear that individuals prepare for migration in the afternoon by eliminating food residue in body,and migration is largely restricted to sexually immature stages(at least in females).Further,it was found that weather conditions(particularly temperature,humidity,and pressure change)in the afternoon had a significant effect on take-off that evening,with O.asiaticus preferring to take off in warm,dry weather.2.The occorrence rules of Oedaleus decorus asiaticus in north ChinaWe analyzed a five-years’(2011-2015)historical data of the grasshopper sweeping data and relevant meteorological data in northern China,and it was shown that:(i)the grasshopper usually hatched from mid-to-late May,the population size reached its peak in June and July,and gradually declined from August;(ii)the distribution area includes Inner Mongolia,Gansu,Ningxia,Shaanxi,Shanxi,Hebei and Shandong provinces(autonomous regions),but mainly distributed in Inner Mongolia;(iii)the level of occurrence are correlated with the local temperature and total precipitation,indicating that high temperature and rainfall Conditions are conducive to the occurrence of the grasshopper;and(iv)the disappear period of the grasshopper in autumn is significantly positively correlated with the latitude and longitude,and this means the grasshopper receded gradually from west to east and from south to north.3.Analysis the source area of Oedaleus decorus asiaticus in BeijingAlthough Beijing is not a suitable area for the grasshopper,grasshopper can be found there each year.A case study was taken by analyzing a six-years’(2012-2017)searchlight trapping data at Yangqing.It was found that most grasshoppers were caught by the light trap in July and August each year,particularly in August.The result of trajectory analysis showed that the grasshopper mainly came from Inner Mongolia,Shanxi,Shaanxi and Hebei provinces,and the initial insect source may come from the eastern part of the Mongolia.Grasshoppers were mostly observed to be caught during wind shear,rainfall and downdraft. |
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