An experimentally tractable Arabidopsis–Swede Midge model to explore host reprogramming by gall-inducing insects

Summary Insect-induced galls are striking examples of host developmental reprogramming, yet their causal mechanisms remain poorly understood because few insect gall systems combine experimental tractability with precise control of insect and host developmental state. Here, we establish Arabidopsis thaliana and the gall-inducing Swede Midge ( Contarinia nasturtii ) as an experimental system for insect gall research. We show that the Swede Midge, a major pest of Brassica crops, completes its full life cycle on Arabidopsis, and induces characteristic inflorescence galls. To define and quantify galling while accounting for changes in developmental stage in both partners, we developed three complementary approaches. On the insect side, a larval phenotyping pipeline monitors how many larvae are present and how far they have developed, and a larval droplet assay delivers defined numbers of stage-selected larvae directly to host inflorescences, fixing the insect input and establishing a precise time zero for the interaction. We found that early-stage larvae can induce galls, whereas late-stage larvae (L3) cannot, defining the developmental window for gall induction. On the host side, we established a phyllotactic dissection and staging framework that enables galled and non-galled floral tissues to be compared at defined developmental positions. We found that galling enlarged floral buds, floral primordia and the inflorescence meristem while delaying floral developmental progression, with accompanying changes in cell size, shape and tissue organisation. Together, these tools combine Arabidopsis genetics with controlled insect manipulation, opening the way to mechanistic dissection of how insects reprogram plant pathways.