A method for low-cost, low-impact insect tracking using retroreflective tags

Current methods for direct tracking of individual bee movement behaviour have several limitations. In particular, the weight and size of some types of electronic tag may limit their use to larger species. Radars and other electronic systems are also often large and very expensive to deploy. A tool is needed that complements these electronic-tag methods. In particular, one that is simple to use, low-cost, can have a high spatial resolution and can be used with smaller insects. This paper presents a candidate method that uses retroreflective tags. These are detected using a camera with a global electronic shutter, with which we take photos with and without a flash. The tags can be detected by comparing these two photos. The small retroreflective tags are simple and lightweight, allowing many bees to be tagged at almost no cost and with little effect on their behaviour. We demonstrate this retroreflector-based tracking system (RTS) with a series of simple experiments: Training and validation with a manually positioned tag, case studies of individual bees, tracking multiple bees as they forage in a garden, use of real-time monitoring to allow easy reobservation to enable a simple floral preference experiment and a very brief experiment with 3D path reconstruction (integrating two devices). We found we could detect bees to a range of about 35 m with the current configuration. We envisage the system will be used in the future to increase detection rates in mark-reobservation studies, provide 3D flight path analysis and for automated long-term monitoring. In summary, this novel tracking method has advantages that complement those of electronic-tag tracking which we believe will lead to new applications and areas of research.