Comparative insecticidal efficacy of five raw African DE samples against the stored grain insect pests, Sitophilus zeamais and Tribolium castaneum

Five raw African diatomaceous earth (DE) samples collected from Tanzania, Zimbabwe, Zambia and South Africa were compared in a laboratory bioassay studies at University of Zimbabwe and the Natural Resources Institute, UK, to determine their potential as locally available grain protectants. A commercially available DE sample, Protect-It®, and an untreated control were also included in the study. In UK, each sample was tested at two application rates 2500ppm and 5000ppm whereas in Zimbabwe an additional application rate of 1000ppm was included. These application rates are higher than those at which commercial DEs are commonly used, but much lower than the application rates at which farmers in sub Saharan Africa (SSA) typically admix traditional protectants such as ash and sand with their commodities during storage. The DE samples were admixed with maize or wheat prior to addition of 40-50 14-28 day old Sitophilus zeamais or Tribolium castaneum insects respectively, and adult mortality assessed after 7, 14 and 28 days of exposure. Progeny emergence was assessed after 7 weeks. The emergence of progeny insects was reduced by at least 80% after 7 weeks in all the African DE treatments in comparison to the untreated control except when applied at 1000ppm. These results suggest that if these African DEs could be easily accessed, were found to have low crystalline silica contents and the application rate of 2500ppm was acceptable to farmers, then they could offer small-scale farmers in SSA a new and possibly cheaper grain protection option.