Follow the flower: Approach-flight behaviour of bumblebees landing on a moving target

While landing on flowers, pollinating insects often have to deal with flower movement caused by wind. Here, we determined the landing performance of bumblebees on a moving artificial flower, and how they use their visual-motor system to control their landings. To do this, we built an experimental setup containing a physical model of a flower, moving sideways using sinusoidal kinematics at various oscillation frequencies (up to 0.65 Hz, at constant amplitude of 5 cm). We filmed the landings of Bombus terrestris bumblebees on this moving flower model and extracted the flight kinematics and trajectories using deep neural network-based videography tracking. The bumblebees were capable of compensating for the detrimental effects of flower movement on landing performance for flower frequencies up to 0.53 Hz. Only at our maximum frequency of 0.65 Hz, the percentage of successful landings decreased, but landing accuracy and duration were not affected. To successfully land on the moving flower, the bumblebees gradually slowed down, aimed towards the middle of the flower and aligned with its movement. Our results indicated that bumblebees use modular visual-motor control feedback to do this: (1) they slow down by maintaining an approximately constant average optic expansion of the approaching flower image; (2) they aim towards the flower by keeping the flower in the middle of their view;  (3) they align to the flower movement by minimizing the sideways optic flow of the moving flower image. Our findings increase our understanding of how flying insects land on flowers moved by wind.