Lead arsenate was introduced on a massive scale in agriculture in Spain in the early 1940s. With the support of a network of agricultural engineers, the new Francoist state encouraged the production and use of lead arsenate as the main weapon against a newly arrived pest, the Colorado potato beetle. In this paper I discuss arsenical pesticides as sociotechnological products which played a pivotal role in the joint production of both chemical-based agriculture and the emerging Francoist regime in Spain during the 1940s. I review the campaigns organized by agriculture engineers and the making of the new National Register for Phytosanitary Products in 1942. The new regulations promoted research in pesticide quality control but also contributed to concealing the health hazards. This invisibilization of the risks took shape in the confluence of interests of the emerging Francoist state, the new pesticide industry, and the large network of agricultural engineers.