Tracking individual Bactrocera tryoni: Wind effects and natural movement

Determining the movement characteristics under real-world conditions of insect pests, such as tephritid fruit flies, is critical to increase the effectiveness of detection, response, and control strategies. In this study, we conducted two experiments using harmonic radar to track wild-caught male Queensland fruit flies (Qflies), Bactrocera tryoni (Froggatt) (Diptera: Tephritidae), a major horticultural pest in Australia. In Experiment 1, we continuously tracked individual Qflies, which were prodded to induce movement in a high-density papaya (Carica papaya, L., Caricaceae) field. We conducted Experiment 2 in a field with lower papaya density and tracked flies were allowed to move without disturbance. This latter natural movement experiment showed that Qflies move at a rate of (mean ± SE) 19 ± 3 m h⁻¹. In both experiments, overall and between-tree flight directions were found to be correlated with wind direction, whereas within-tree movement directions were not. Further, the effect of wind direction on fly trajectories varied by step distance but not strongly with wind speed, whereas step-distance distributions were consistent with Lévy walks (i.e., short random steps with occasional larger steps). Qfly movements were well fitted by two-state hidden Markov models, further supporting the observation that Qflies move differently within (short steps with random direction) and between (longer more directional steps) trees. Data on flight directionality, step distances, and movement speed determined in this study provide parameters that may help enhance current surveillance, control, and eradication methods, such as optimizing trap placements and pesticide applications, determining release sites for parasitoids, and setting quarantine boundaries after incursions.