Flight behaviors promote optimal migration trajectories in high-flying moths

Background/Question/Methods

Billions of individual insects, of numerous species, undertake long-range seasonal migrations between winter and summer ranges to take advantage of seasonally available breeding resources. To cover the distances required (often thousands of kilometers), many insects rely largely on wind assistance, and routinely ascend hundreds of meters above the ground to migrate in fast-moving airstreams. However, it is not clear to what extent high-flying insect migrants are able to influence their migration directions, given that wind speeds are typically three to five times faster that the insects’ airspeeds at their preferred flight altitudes. To answer this question, we studied the flight behavior of nocturnally-migrating large moths in Europe (and in particular the Palaearctic noctuid Autographa gamma) with specialized entomological radars, and simulated their migration pathways with a meteorological trajectory model that had been modified to take account of moth flight behavior.

Results/Conclusions

Radar observations demonstrated that an ability to select favorably-directed airstreams (i.e. northwards in spring and southwards in fall) was widespread among high-flying migrant Lepidoptera, and thus the migrants gained considerable wind assistance for their seasonal migrations. Furthermore, A. gamma moths preferentially flew at the altitude of the fastest winds, and partially compensated for small degrees of lateral wind drift away from their seasonally-preferred migration directions. Trajectory simulations show that these flight behaviors results in a 50% increase in mean nightly migration distance compared to passively-transported particles (300 km versus 200 km) and a significant decrease in lateral drift. Comparison of moth migration parameters with those of nocturnal passerine bird migrants demonstrated that the moths’ highly efficient strategies resulted in them achieving the same travel speeds and directions as birds capable of flying three times faster. The flight strategies employed by A. gamma and other moths explain how such small, short-lived and relatively slow-flying species are able to cover such great distances in seasonally-beneficial directions.