Susceptibility of Sitophilus zeamais (Coleoptera: Curculionidae) to selected stored product insecticides in the Central region of Ghana

Background: Sitophilus zeamais is a major pest of stored grains in Ghana and its control can be achieved through an Integrated Pest Management approach, which includes the judicious use of effective and selective insecticides. However, persistent use of synthetic protectants can cause insecticide resistance, militating against control. It is therefore important to monitor the effectiveness of these products to justify their continuous use and to detect the emergence of resistance. This study evaluated the efficacy of two commonly used commercial insecticide products namely Agricombi® (30% fenitrothion + 10% fenvalerate) and Betallic Super® (80 g pirimiphos-methyl + 15 g permethrin) on adult S. zeamais populations from three districts in the Central region of Ghana. Results: The median knockdown times (KT₅₀s) of the products at their recommended label concentrations (RLCs) were 15.13–53.42 min for Agricombi® and 17.17–63.70 min for Betallic Super® while mortalities after 24 h of insecticide exposure were 52.00–80.00% and 21.33–58.67%, respectively. Concentration-response effect of the composite insecticide products showed median lethal concentrations (LC₅₀s) of 8149–12827 mg/L for Agricombi® and 2638–4411 mg/L for Betallic Super®. Comparisons of the RLCs of the products and their respective LC₅₀s and LC₉₅s indicate decreased susceptibility in the weevil populations. Conclusions: The insecticides were ineffective against the S. zeamais populations at their recommended label concentrations. The decreased susceptibilities pose challenges for effective management of the pest and hence the need for alternative effective management methods or products with different active ingredients in these communities.