Rhyzopertha dominica (Fab.) (Bostrichidae: Coleoptera) susceptibility to fumigant and contact toxicity of Secamone afzelii extracts as food grains preservatives

The study aims to investigate the ability of Secamone afzelii extracts to protect wheat grains against Rhyzopertha dominica infestation. Fumigant and contact bioassay were carried out, as 20 g of disinfected wheat was treated with 50, 100, 150 and 200 μL in triplicate of different extracts of S. afzelii, at different times of exposure. Contact toxicity on adult mortality increased as concentration and exposure time increased. Mortalities were 22.06% - 28.45%, 26.78% - 30.59%, 28.45% - 43.68% and 29.87% - 49.43% at 48 h’ exposure, as concentration increased. At 48 and 72 h, the highest mortality was achieved by acetone extracts at 150 μL (30.46% and 36.63% respectively) and 200 μL (34.20% and 37.05% respectively) concentration for fumigant toxicity. Extracts of S. afzelii significantly (p <0.05) reduce loss in grains weight and emergence of R. dominica. In the contact bio-assay, lowest LC50 (Lethal Concentration 50) toxicity on R. dominica was achieved with S. afzelii ethyl acetate extract at 24 and 48 h exposure (171.0 μL and 233.6 μL) respectively. Meanwhile, the acetone extract had the least LC50 (276.4 μL and 327.7 μL, respectively) in the fumigant bioassay. The present study indicates that these extracts can safeguard stored grains from insect pest infestations and damage