Study On The Migratory Behavior Of Scotogramma Trifolii(Lepidoptera:Noctuidae)across The Bohai Strait

Scotogramma trifolii(Lepidoptera: Noctuidae),the clover cutworm,is a partially out-breaking pest in farming and stockbreeding areas of northern China.Although S.trifolii is known as a long-distance migrant,the seasonal pattern and the migration behavior of this species in northern China is not well known.Insect migration in particular is an important behavior and ecological phenomenon used to ensure the continuation of the life history and reproduction adapt to the seasonality of their habitats in some species.Migration also plays a key role in the sudden outbreaks of crop pests including the clover cutworm.In this study,systematic monitoring of population fluctuations of S.trifolii in northern China was conducted using light traps,entomological radar monitoring and ovarian dissections on an isolated small island(named Beihuang,BH)in the center of the Bohai Strait,and the natal origin was studied by using the pollen on the proboscis of moths.Our work will provide effective information for the forecasting and management of S.trifolii and help clarify population dynamics of S.trifolii migration across the sea.The main results obtained are summarized as follows:Variations in night catches indicated that Scotogramma trifolii exhibited a regular pattern of seasonal migration across the Bohai Strait,and the size of this migration varied by more than an order of magnitude.The majority of catches was trapped in summer,followed by autumn and spring.The mean period when migrants were trapped on the island was 150.42 ± 5.48 days,with the shortest time span of 112 days and the longest time span of 181 days.In each month,the proportion of trapped moths that were females was significantly higher than that of males,and the development of ovarian and the mating rate was different in different month.The vast majority of females trapped in August and September were virgins with little or no ovarian development,which suggests that the natal origin of the S.trifolii may be in the north of China close to the catch site.However,most females trapped in May to July had mated and showed some ovarian development,suggesting that the natal origin of the S.trifolii may be in the south of China far away from the catch site.The entomological radar monitoring indicated that the flight altitude of S.trifolii migration across the Bohai Strait mainly concentrated in the 100-550 m above the ground level,and air temperature affects their flying height.The maximum volumn density of S.trifolii appeared in the inversion layer.The displacement direction and orientation direction of S.trifolii were not random,but had some degree of collective orientation.In the migration process,S.trifolii moths were not completely flying with the wind and showed a tendency to head with an acute angle to the wind direction in the three mass migration events.S.trifolii moths can adjust the head to the direction of displacement correction.After examining the pollen on the proboscis of S.trifolii moths,it was found that there were 31 species of pollens on S.trifolii moths.The natal origin of the S.trifolii in May of 2015 may be in East,Southern or Southwest China,because the distribution of Flueggea virosa mainly in East,South and Southwest China,meanwhile the carrier rate of this species of pollen reach to 25.00%;in June 2015,the natal origin of the S.trifolii may be in Central or Southern China,for the distribution of Castanea mollissima/Castanea sativa mainly in Central and Southern China.In August of 2016,the natal origin of the S.trifolii may be in Northern or Northeast China,for the distribution of Adenophora trachelioides mainly in Northern and Northeast China.To conclude,S.trifolii exhibited a regular pattern of seasonal migration across the Bohai Strait and the longest time span was up to 181 days.Immigrant population mainly concentrated in the 100-550 m flight altitude,and had some degree of collective orientation.Meanwhile,they could identify the direction when flight with the wind.In May and June,the natal origin of the S.trifolii may be in Central or Southern China,while in August the natal origin of the S.trifolii may be in Northeast China.These findings may be helpful for improving the forecasting system for S.trifolii control and provide a theoretical basis for the next step of the research for the natal origin and law of migration in different ecological regions of S.trifolii.