Although Brazil is among the world's largest consumers of pesticides, their impacts on local populations have struggled to emerge as a public problem due to a political context that is particularly favourable to industrial agriculture. In this article, we investigate how the knowledge produced on the impact of pesticides contributes (or not) to the emergence of this public problem. We conduct a reflection based on a citizen science process we conducted in the region of Santarem, in the Brazilian Amazon, where soybean has been expanding significantly for the past 20 years. Since 2017, we have built an observatory in partnership with peasant unions to make visible the impact of pesticides used in soy crops on family farmers. Young farmers, trained to become "community researchers", have administered 544 questionnaires to family farmers to learn about their practices and assess the changes caused since the arrival of soy. We conduct an analysis at three levels: at the national level, we follow the emergence of pesticides as a public issue, its consolidation during the Labor government (2003-2016), and then its dismantling; at a local level, the survey reveals how the family farmers are impacted by soy pesticides and at the same time, not so aware of being "victims"; finally, at the territorial level, we question the weakening of the debate between science and politics, particularly within the Regional Forum for the fight against the impact of pesticides set up in Santarém.