Flight Behavioral Mechanisms Of Aerial Migrating Insects

Many insect species undertake regular seasonal migration in order to exploit suitable breeding habitats. Migratory insects engage in high-altitude, windborne migration, often at heights of several hundred meters above ground level, at where they can take advantage of strong wind to fly considerable distance. The atmospheric transport of insects is worthy of study because many migrant species are serious pests of agriculture while other insects are important natural enemies. Knowledge of insect movement is necessary when formulating or improving management strategies for the species concerned. The development and use of entomological radar has made it possible to direct observe the high-altitude movements of insects. Entomological radars have revealed many fascinating phenomena, but among the most noticeable phenomena are the layer formation and common orientation of aerial fauna. Both of two flight behavior can cause a greater concentration of migratory pests in the fallout area and heavy loss of agriculture production. The aim of this study is to reveal the behavioral mechanisms of aerial migratory fauna, focus on layer formation and common orientation of nocturnal insects, and the results will provide very useful information for the remote forecast of migratory insect pests.Simulation test, radar observation, aerial netting samples, analysis of meteorological factors and trajectory were used in this study. Some behavioral mechanisms of aerial migratory insects were explained satisfactorily. In the first part of this paper, the behavioral mechanisms of layer formation and common orientation were explored in simulation test by means of independent design experimental equipments. In the second part, the flight behavioral mechanisms and migration dynamics of Cnaphalocrocis medinalis, Mythimna separate and other small size insects were revealed by radar observations and aerial netting. The main results are as follows:1. The flight behavior of cotton bollworm Helicoverpa armigera was studied in simulated wind and temperature fields to explore the stratification mechanism of airborne migratory fauna. In the simulated temperature field, the tested moths exhibited significantly active selection behaviors for temperature. The moths tended to select their optimal flight temperature (20-22℃) and low temperature threshold (13℃) for flight. The result of simulation test indicated that high-speed airflow had significant suction effects on the tested moths, and that there was obvious negative pressure around the high-speed airflow. The inward vertical airflow generated by the high-speed removing jet pushed and bound the moths into the high-speed horizontal airflow. Our results illustrated that the active selection behavior of the airborne migrants for optimal flight temperature promoted their tendency towards the nocturnal inversion layer. The inward vertical airflow from the negative pressure at the edges of the low-level jet located on top of the inversion was the key factor that formed sustained layer concentration and aggregation of aerial fauna. If the environmental temperature was significantly lower than the optimal flight temperature of insects, the flying insects preferred to climb to their flight ceiling, i.e., the height with the low-temperature threshold.2. The orientation behavior of M. separate and H. armigera moth was studied in simulated wind and magnetic fields. Most of tether moths maintained upwind heading on condition of different wind speed and flew upwind when we released them in high speed horizontal airflow. The compensation angle for cross wind of moth decreased with increase of wind speed. The experimental moths exhibited common oriented to same side of airflow only when there was an obvious point light on the side of moths. Furthermore, we tested whether migratory moths orient by a magnetic compass in simulated magnetic field. Experimental moths exhibited common orientation in local geomagnetic filed and changed their heading obviously in the stronger magnetic field. The orientation behavior was not affected by polar change of horizontal component of magnetic field. Magnetic inclination may be the compass cues of aerial fauna collective orientation.3. The wing beat frequency (WBF) of H. armigera moths were tested in different conditions. The result showed that sustain flight time, temperature and mating behavior significantly affected WBF of moth. The WBF decreased quickly with the increase of sustain flight time in the first three hours, and kept constant over the next three hours, and then decreased quickly in the later period. This dynamic of WBF is a pretty good description of natural migratory dynamic of aerial fauna. The reducing of WBF of male moth caused by mating could be recovered in a short time, but female moth not. Moths maintained the maximum frequency and the longest time of wing beat when the temperature was between19and23℃. The wind speed had an obvious effect on WBF of moths in downward wind situation, nearly no efficacy on WBF in upward wind situation. Most of experimental moth could hardly beat double wings when the downward wind faster than4m/s. 4. Doppler insect monitoring radar observations of rice leaf roller Cnaphalocrocis medinalis migration were made at Pukou district of Nanjing in2007and2009. Radar observations showed that large numbers of rice leaf roller moths migrated from dusk (18:00h) until about05:00h the following morning and moths mainly flew below500m (agl). The majority of the migrating moths on any one night usually aggregated in layers between100m and500m (agl), and sometimes two moth layers were present simultaneously. The stratification of moth density was closely related to the low-level nocturnal jet rather than air temperature. We analyzed the migration dynamics and the temperature and wind profiles on the migration routes by numerical simulation. The results showed that there were four migrations of the4th generation of the moth in Jangpu2007. The rice leaf rollers engaged in southwards’return’migration on the northeast winds following the cyclone. Two massive immigration peaks of the rice leaf roller on the night of18-19and25August2007was associated with the subsidence around the low pressure system.5. Day and night sampling of windborne insects at a height of200m (agl) was undertaken at Jiangpu, Nanjing, from August to September2009, using a net supported by a tethered balloon. Hemiptera was the most frequent order in our catches on account of the abundance of the rice plant hopper, aphids and leafhopper. The abundance of Diptera was generally the second and Hymenoptera was the third. The accompanying migration of some natural enemies, e.g. parasitic wasp, with their host insects was found in the aerial netting studies. There was a significant difference in composition of species in aerial fauna between daytime and nighttime. Every insect species has a preferred time of flight. Aphids and parasitic wasp are active in daytime, but rice plant hopper, leafhopper and moths fly in nighttime. The insect population dynamics of aerial netting were affected by temperature, wind speed and wind direction. The trajectory analysis of the rice plant hopper indicated that the insect population dynamics of aerial netting can reflect the natural migration dynamics very well.6. The spring migration behavior of the oriental armyworm M. separate was studied based on the previous radar data. The results indicated that most moths trended to aggregate near the inversion upper surface in order to select warm condition. Only a few moths’ layers were related to wind shear. M. separate moths had a lower temperature threshold (about13℃) during the migration period. The mass landing of aerial fauna could be caused by sudden cooling. Furthermore, there was a significant correlation between the dispersion of migration direction for M. separate and the wind speed. When the wind was strong (>5m/s) and closely aligned with the armyworm moths heading (i.e., toward the NE), M. separate exhibited common orientation close to the downwind; When the wind speed wasn’t high (<3m/s) and not closely aligned with the moths heading, M. separate can compensate for the cross-wind drift and even flight upwind. The oriental armyworm M separate maybe use some compass cues (e.g., geomagnetic compass) to orient general flight direction.