Field study of the repellent activity of 'Lem-ocimum'-treated double bags against the insect pests of stored sorghum, Tribolium castaneum and Rhyzopertha dominica, in northern Nigeria

•An improved method to protect grain with repellent plants was discovered.•Sweet basil and lemongrass pasted between layers of double bag was most effective.•The efficacy of the treatment was greatest in store-rooms with the most grain bags.•Participating farmers approved of the new method when at least 4% of grain treated.

A field study of the efficacy of a novel use of repellent plant material to protect stored sorghum from pest damage was conducted in Kebbi State, Nigeria. A combination of Ocimum basilicum (Sweet basil) and Cymbopogon nardus (Lemongrass) powdered dried leaves ('Lem-ocimum') was found to be significantly more repellent to the most common grain pest, Tribolium castaneum, when applied as a water-based paste between the layers of double storage-bags at a dose of 1% w/w (plant powder/grain) than untreated double bags (n = 30, P < 0.001). The efficacy of protecting a given percentage of grain in Lem-ocimum treated double-bags was tested in 120 store-rooms, each of which contained 15-35 × 60 kg single bags of sorghum that initially had moderate levels of beetle infestation (26-50 T. castaneum/bag). After 5 months in storage, the percent change in grain weight and levels of infestation by the two most prevalent pests, T. castaneum and Rhyzopertha dominica, inside treated double-bags were significantly lowest in the store-rooms with the highest percentage of all grain (4%) kept in treated double-bags (P < 0.01, n = 120 store-rooms). This result may have been due to the mass fumigation effect of adding 400-900 g Lem-ocimum to each of the store-rooms with 4% treated grain. Only the participant farmers that had stored 4% of their grain in treated double-bags felt the treatment provided significant protection. The findings suggest Lem-ocimum treated double-bags could improve the chances that a proportion of a farmer's grain would be of good enough quality to sell in the market mid-way through the storage season, when the price of grain would earn a good profit.