Efficacy of wood ash and diatomaceous earth against <i>Sitophilus granarius</i>: influence of dose, environmental conditions, and geomorphological composition

This study evaluates the insecticidal efficacy of wood ash derived from various coniferous sources and a commercial diatomaceous earth product SilicoSec against adults of granary weevil, Sitophilus granarius (L.) (Coleoptera: Curculionidae). Morphological analyses revealed significant differences in particle sizes among treatments, with SilicoSec averaging 14.70 ± 1.85 µm, and wood ashes ranging from 160.51 ± 58.25 to 500.10 ± 183.58 µm. Geochemical analyses indicated that SilicoSec comprised 79.5% SiO2, whereas wood ashes contained 12.8% to 17.5% SiO2 and 31.0% to 37.6% CaO. Insecticidal assays demonstrated that all treatments achieved over 90% mortality; however, wood ash treatments exhibited greater variability, suggesting a dependence on concentration and environmental conditions. Probit modeling estimated that mortality increased by a factor of 2.77 with each concentration doubling. Analysis of variance indicated that neither temperature, nor relative humidity significantly affected lethal concentrations (LC50 and LC90). Notably, the diatomaceous earth product SilicoSec achieved consistent results at lower concentrations than currently recommended, suggesting potential for reduced application rates. Progeny emergence studies revealed that treatment efficacy was influenced by temperature and relative humidity, with higher temperatures and lower humidity levels enhancing mortality. Beta regression analysis confirmed that treatments and temperature significantly affected the proportion of dead beetles, whereas relative humidity and concentration did not. These findings underscore the importance of selecting appropriate treatment combinations and storage conditions to effectively control S. granarius populations.