This paper reviews the way in which Spanish agriculture climbed onto the pesticide treadmill. We claim that Fascist policies and expert advice assembled in the early 1940s accelerated the introduction of pesticides into Spanish agriculture and promoted the emergence of the Spanish pesticide industry in the times of autarky. Agricultural engineers were the key protagonists in this process, but other human and non-human actors also played a pivotal role: a new pest (the Colorado beetle), Francoist politicians, farmers, landowners and industry managers. Our focus is on the use of pesticides against the Colorado beetle (the main threat to the potato crop), and the transition from arsenical pesticides to DDT during the 1940s. We discuss how the politics of autarky offered new opportunities for developing agronomic programmes and the chemical industry and led to the creation of the Register of Pesticides in 1942. We also discuss the role of these regulations in concealing the risks of pesticides from farmers and food consumers. Arsenic pesticides became sources of slow poisoning and tools for social control while reinforcing the alliance of agricultural engineers and Fascist politicians in their autarkic and authoritarian projects. When DDT arrived in Spain, the agricultural engineers praised the low toxicity it had demonstrated (compared to lead arsenate) in its first uses in public health and in military campaigns in Italy. Indeed, the data concerning its potential dangers disappeared from view thanks in part to a large multimedia campaign launched to promote the introduction of the new organic pesticides in Spanish agriculture, which is described at the end of the paper.