2.3. Trajectory Analysis

Trajectory analysis is one of the common and effective methods to determine the origin and landing area of the migratory insects [24], which has been successfully used for many migratory noctuid moths such as Cnaphalocrocis medinalis (Güenée) [21,25], Mythimna separata (Walker) [26], S. frugiperda [22,23,27,28]. In this study, possible source regions of the fall armyworm invading Australia were determined by backward trajectory simulation. The date with fall armyworm moths captured by surveillance trapping was set as trajectory simulation dates, and the trajectories were obtained from three origins, Saibai and Erub Islands and Bamaga.

Based on biological and flight characteristics of fall armyworm, trajectories were calculated with the following parameters: (1) Fall armyworm flies downwind at a high altitude [29,30,31], without considering a directional deflection angle [23]; (2) Other similar-sized noctuid moths have a self-powered flight speed of about 2.5–4 m s⁻¹ [23,30]. In this trajectory modeling, the self-powered flight vector of 3.0 m s⁻¹ was assumed [22]; (3) The noctuid insects typically migrate at night, taking off at dusk and landing at the following dawn [21,25]. Fall armyworm can continuously fly 12 h every night and mostly fly for three consecutive nights, similar to most other noctuid moths whenever flying over land [32]; (4) While the fall armyworm flies over sea, nocturnal continuous flight duration is extended until it reaches land [22]. Therefore, when fall armyworm migrated in the archipelago of northwestern Australia, the backward trajectories were set as the 2 times flight and a single flight lasted for but did not exceed 36 h [22]. Then, the trajectories effective endpoints were selected by the departure time at taking off time and in a host planting region or at least from a terrestrial location [22]; (5) Radar observations show that moths usually fly in a low-level jet stream at altitudes with wind speeds greater than 10 m s⁻¹ [29,33]. However, the most appropriative flight height of fall armyworm before trajectory simulating was not determined. In this study eight possible different initial altitudes of 500, 750, 1000, 1200, 1500, 1750, 2000, and 2250 m above mean ground level (AGL) were assumed [22,23,24,25,26,27,28,29,30,31,32,33,34]; and (6) Preliminary flight ability test also found that the fall armyworm stopped flying after a short period when the ambient temperature reached approximately 13.8 °C (Chen H from Nanjing Agricultural University, unpublished data). So fall armyworm cannot fly when the air temperature at flight altitude falls below 13.8 °C, the minimum temperature for survival of fall armyworm [22,23]. The trajectories of the fall armyworm using meteorological conditions at different flight altitudes were simulated.