Ecosystem Functions And Orientation Strategies Of High-flying Insects

Every spring,hundreds of millions of insects migrate from low-latitude areas to highlatitude areas to obtain survival resources and reproduce their offspring.In autumn,they migrate from the breeding grounds at high latitudes back to low low-latitude areas for overwintering.Insect migration is the most ambitious migration activity in the ecosystem,affecting the ecosystem extensively and profoundly.Firstly,insect migration is a process in which a large amount of material and energy flow over long distances and redistribute them in large-scale spaces.Secondly,insects are the most important part of the ecosystem and play a vital role in connecting the food web.They participate in all aspects of the ecological cycle as decomposers,pollinators,predators,parasites,etc.In addition to migratory pests,the other migratory insects serve the ecosystem as pollinators,natural enemies of pests,and small vertebrates’ main food source.They service the ecosystem and function the ecosystem hugely;however,little is known about their ecosystem functions.At present,research on migratory insects often focuses on migratory pests.To prevent their outbreaks which cause devastating damage to agriculture and provide early warning,researchers attach great importance to how the migratory pests achieve long-distance migration.They conduct in-depth studies of the behaviour mechanisms of aerial insect faunas.Most pests migrate at night,however,there are still many daytime migratory insects that are very important to the ecosystem and we do not know how they migrate.Understanding how they realize long-distance migration is very beneficial to implement natural enemy control on pests and maintain the stable development of food production.Especially for most beneficial insects that migrate during the day,understanding how they can achieve long-distance migration is of great significance to control pests and maintain the stable development of food production.In summary,there are still insufficient understandings of the ecosystem functions and the aerial behaviour mechanisms of migratory insects.This study has conducted investigations and analysis,and the main findings are as follows:1.High-flying insect trends under global changeThe prerequisite for evaluating migratory insects’ ecosystem functions is to know whether the number of high-flying insects has changed.In the context of changes in the global ecosystem,the number and diversity of terrestrial insects continue to decrease.Are aerial insects also facing survival threats? In order to answer this question,the study analyzes the trend of insect trends over seven sites in Europe,North America,and Asia since the 1920 s by using historical monitoring data and high-altitude netting data.It reveals for the first time that high-flying insect trends have not declined with the changes in the global ecosystem.In the southeast of England,the density of high-flying insects in 1932-1935 is significantly lower than that in 1999-2007.In the south-central United States,the density of high-flying insects in 1926-1931 is almost equal to that in 2015-2016.In China,the Philippines,and India,the density of high-flying insets in 1984-1994 is lower than that in 2017.This study analyses the high-flying pest groups and finds that the key high-flying pests remain in a balance of fluctuating development even though they were prevented for a long time.In the southeast of England,the aerial density of aphids in the 1930 s was significantly lower than that in the 1940 s and early 2000 s,there was no difference between the aerial density of aphids in the 1940 s and early 2000 s.The study shows that the aerial density of aphids does not decrease over time in the southeast of England.In the south-central United States,the aerial density of Hemiptera in 1926-1931 is almost equal to that of 2015-2016,and the size of the high-flying Hemiptera population did not decrease significantly.This study makes a comparative analysis of the rice planthoppers collected by the highmountain net and ground searchlight traps in Xiushan,Chongqing from 1979 to 2015.It shows that the trend of high-flying rice planthopper is not consistent with that of ground rice planthopper.The high-flying brown planthoppers keep a fluctuating development,the overall trend has not increased or decreased significantly.The ground brown planthoppers showed a rapid decline from 1979 to 2001.Although they broke out in some years,the long-term trend tended to be stable in 2002-2015.The dominant species of the ground rice planthopper populations has changed;however,this phenomenon did not occur in high-flying rice planthoppers.The number of ground brown planthopper surpassed the number of ground white-backed planthopper from 1979 to 1995.The number of ground white-backed planthopper exceeded the number of ground brown planthopper from 1996 to 2015.However,the number of high-flying brown planthopper has long been higher than that of the highflying white-backed planthopper.Therefore,the high-flying pest trend is more stable than the ground pest trend.2.Ecosystem functions of high-flying insectsThe beneficial migratory insects are important contributors to ecosystem functions.However,previous studies usually focused on migratory pests,and we lack an understanding of beneficial migratory insects and their ecological functions.Many hoverfly species(Diptera:Syrphidae)have a strong migratory ability.The hoverflies are typical beneficial insects,their larvae of prey on aphids and adults spread pollens.Thus,this study takes migratory hoverflies(Diptera: Syrphidae)as the research object,and uses the insect vertical-looking radar monitoring data from 2000 to 2009 to measure the material flow and energy flow of the highflying hoverflies.Also,it evaluates the ecosystem functions of the high-flying hoverflies by measuring the number of aphids preyed by hoverfly larvae and the number of pollens transferred by hoverfly adults.The results show that 1 to 4 billion hoverflies migrate seasonally over southern England each year.These hoverflies transport30-80 tons of biomass over the UK every year,which equals 50-150 Gigajoules of energy and contains 1000-2500 kg of nitrogen and 100-250 kg of phosphorus.Therefore,the hoverfly migration is a process of the massive material and energy flow a long distance,which redistributes these materials and energy in large-scale space.The hoverfly migration is seasonally reversed north-south migration.In spring,the first generation of hoverfly adults migrate northwards from Europe and entered the UK and reproduce multiple generations after landing.In summer,the high-flying hoverflies do not prefer flying in any direction,their tracks distributed randomly.In autumn,the hoverfly offspring adults leave the UK to migrate south,return to lower-latitude and warmer overwintering region in Europe.The spring emigrated hoverflies account for 20% of the annual total,the hoverfly offspring in summer and autumn respectively account for 42% and38%.Aphids are the most important agricultural pests in the UK,the larvae produced by migratory hoverflies through reproduction prey on 3-10 trillion aphids per year,so the migratory hoverflies play a very important role in controlling local pests.In addition,the high-flying hoverflies transport 3-8 billion pollens from the southern continent to the UK each spring,and transport 3-19 billion pollens to return to the wintering region in autumn,this process affects the large-scale plant gene exchange.Additionally,there are no native wild honey bees in the UK now.There are only 5 billion managed honey bees,but around 4 billion migratory hoverflies pollinate in the UK every year.Thus,the local pollination of migratory hoverflies in the UK is also very important.Moreover,although the number of migratory hoverflies fluctuates significantly from year to year,there is no significant downward trend overall.Due to the decline of local pollinators such as bees,migratory pollinators may play a more important role in the future ecological services.3.Orientation strategies of high-flying hoverfliesUnder the circumstance that the number of local beneficial insects such as bees is declining,understanding the behavioural mechanism of migratory beneficial insects becomes particularly important.However,the objects of existing research on the behavioural mechanism of high-flying insects are often migratory pests.The pests usually migrate at night,while beneficial insects usually migrate during the day,their behaviour mechanisms are different.In order to clarify the behavioural mechanism of beneficial high-flying insects,the research takes the daytime high-flying hoverflies as the object,investigating and analyzing the aerial behaviour data of more than 155,000 hoverflies detected by entomological radar.The hoverflies do not completely drift with the wind in the air,and their migration direction will change seasonally.It is inferred that the high-flying hoverfly fauna can orientate at high altitude.And the following three hypotheses are put forward for their orientation strategies:Firstly,the migratory hoverflies have seasonal preferred migration directions.Secondly,the wind directions of mass-migration days are usually close to the preferred migration directions.Thirdly,the migratory hoverfly adopts the orientation strategies which is beneficial for correcting the crosswind drift to improve the success rate of migration.The study tested the above three hypotheses and found that the high-flying hoverflies adopt complex orientation strategies to achieve long-distance migration,but the high-flying migrants in spring and autumn use different orientation strategies.In spring,the airflows are warmer and run from south to north,which is favourable for the hoverfly migrate northward.The high-flying hoverflies adjusted their heads to a direction close to the airflows in spring and moved northward into the UK with the fast-running airflows.In autumn,the airflows run from the southwest to the northeast over the UK,which is not conducive to the southward migration of hoverflies,so the hoverflies have to adopt more complicated orientation strategies to overcome the impact of crosswind drift.Firstly,the high-flying hoverflies choose the days when the wind blows relatively southwards and the wind speed is relatively slow to migrate.Secondly,the high-flying hoverflies adjust their heads to the preferred migration direction to correct the influence of crosswind drift.Moreover,the head direction of the highflying hoverfly is between the direction of airflow and the preferred migration direction,so that most of the hoverfly’s self-powered speed is added with the airflow speed,which ensures its migration speed.Having a compass mechanism and the ability to recognize airflow clues are the prerequisites for hoverflies to adopt complex orientation strategies.Since the hoverflies are diurnal high-flying migrants,the sun’s position or changes in polarized light are the most possible orientation compass.This article focuses on the two issues of ‘the ecosystem functions of high-flying insects’ and ‘how the insect swarm identifies its direction at high altitude’.This study revealed highflying insect trends under global changes had not changed significantly since the 1920 s.It measured the number and biomass of migratory hoverflies and evaluated their ecosystem functions in controlling pests and pollination.The different directional strategies adopted by migratory hoverflies in response to different airflow environments in spring and autumn are clarified.This study provides a theoretical basis for the population management of migratory insects.