Toxicity of rock dust and charcoal powder enhanced with diatomaceous earth against rice weevil in maize stored by smallholder farmers

Maize is a major staple food in Africa, its production and post-harvest generally occurs on smallholder farmers' properties, where significant food losses occur in the field during harvesting, processing and storage. Most smallholder farmers lack access to modern methods of harvesting, processing and storage. Generally, the storage structures used are ineffective against storage pests, leading to post-harvest losses due to insect pests that, in some cases, can reach about 40% of the total production. Pest control is generally scarce, and when adopted, it is based on local knowledge-based pest control alternatives or on the application of synthetic pesticides known to be harmful to the environment and human and animal health. So, alternative pest control methods commonly found include the use of inert dust as an integrated pest management approach, without residual effects and ideal for food safety. This study aimed to determine the efficacy of charcoal powder (CP) and rock dust (RD) alone or combined with diatomaceous earth (DE) in the control of rice weevil (Sitophilus oryzae). Maize grains were treated and infested by the addition of Sitophilus oryzae, on the day they were treated and, 30 and 60 days after treatment. Mortality was assessed at 10 and 20 days after each infestation. Emerged insects were counted 40 days after the second mortality assessment for each infestation period. At 90 days after treatment, the seed germination test was performed. The best treatments for Sitophilus oryzae control (mortality) and F1 emergence reduction were RD (0.5, 0.25 and 0.10 g/kg), RD+DE (0.5+0.05, 0.25+0.05 and 0.10+0.05 g/kg), and CP+DE (1.0+0.05, 0.50+0.05 and 0.25+0.05 g/kg). The RD and RD+DE remained efficient in maize treated and stored for 60 days. The treatments did not interfere with seed germination. Rock Dusts, RD+DE, and CP have the potential to be used as stored grain protectants, providing economically and environmentally friendly pest control alternatives, especially in small-scale agriculture. In developing countries, especially in sub-Saharan Africa, studies for optimization and introduction of inert dusts tested in this research, have the potential to reduce post-harvest losses, improving food security. Key words: Inert dust, stored maize, biological agriculture, storage pests, diatomaceous earths