Pesticides are intensively used to protect cereals from pests, but they are known to increase pest resistance and affect the environment and non-target organisms, including humans, which points to the need for sustainable alternatives. Some of the main cereal pests are the weevil Sitophilus spp., which proliferate on a cereal-exclusive diet thanks to a mutualistic association with the intracellular symbiotic bacterium (endosymbiont), which provides the insect the nutrients that are scarce in the grains. Because weevils rely on this symbiosis to survive, a new, specific and sustainable control strategy could be to target not the insect itself, but the functioning of this association. Metabolic exchanges between the cereal weevils and S. pierantonius are at the core of the symbiosis, as both partners have evolved towards a metabolic complementarity and dependency on each other. How these crucial nutrients are exchanged between partners remains an open question. With this project, we will FOCuS on deciphering the mechanisms by which the specialized insect cells that host the bacteria are turned into highly specialized cross-kingdom ‘metabolic factories’, and to identify novel target mechanisms to disturb their function and, consequently, impact host fitness as a novel strategy to control crop pests.