Bridging behaviour and spatial propagation in minute insects: the double-spiral method and image analysis pipeline Mélina Cointe, Vincent Calcagno, Ludovic Mailleret, Mathias Launay, Victor Burte, Guy Perez Studying what is difficult to observe, has always aroused the curiosity of human beings and fostered methodological innovations. Minute insects such as parasitic micro-wasps have high basic and applied importance, for their widespread use as biocontrol agents, but they are difficult to observe both in the laboratory and in the field. Their dispersal i.e. their spatial spread that can have consequences on their reproduction and gene flow is a phenotype of particular interest. Classically, the dispersal of such insects is studied using field releases, but those are time consuming, costly, and the results highly variable, preventing high-throughput and repeatability. Alternatively, dispersal can be studied in small- scale behavioural assays. However, those neglect important higher-scale processes such as group dynamics. Consequently, proper evaluation of dispersal metrics is often complicated and lacking in the study of behavioural dynamics or, for example, biocontrol breeding programs. Here we introduce a new method, coupling a double spiral arena with computer vision aided insect counting and localization, which permits the study of spatial propagation at relevant scales (several hours and meters), retaining high throughput and experimental power. The method records the location of every individual at high frequency, providing precise estimates of Mean Squared Displacement and diffusion coefficients. This provides an affordable and easy-to-implement tool for studying insect movement and performing behavioural ecology, applicable to biocontrol research. We describe the method and apply it on 14 strains of a species of agronomic interest: Trichogramma cacoeciae, a parasitoid micro- wasp. Several experiments allowed us to identify two main types of spatial propagation dynamics. One is characterised by an initial phase of acceleration followed by a sustained diffusive regime. The other by an initial phase of acceleration followed by a phase of slowing down.