Insecticidal efficacy of botanical food by-products against selected stored-grain beetles by the combined action with modified diatomaceous earth

Farmers in Africa in general have repeatedly prioritized the urgent need for improved methods of storage pest control. Laboratory studies were evaluated on the toxicity of sublethal doses comprising of 25.0 x 9 10(3), 10.0 x 10(3), 20.0 x 9 10(3), 30.0 x 9 10(3), 40.0 x 10(3) and 50.0 x 9 10(3) ppm (wt/wt) paddy rice husk ash (PRHAP) and false yam Icacina oliviformis leaf powders (FYLP), alone or in combination with enhanced diatomaceous earth (DE) to adult Sitophilus granarius, Tribolium castaneum and Acanthoscelides obtectus in stored grains. Adult mortality was observed up to 7 days, while progeny production was recorded 6-10 weeks. PRHAP was more toxic to the three beetle species than the FYLP, irrespective of dosage or exposure period. Grains treated with 30,000-50,000 ppm (wt/wt) of PRHAP for 5 days killed all T. castaneum and S. granarius ranging from 93 to 100% and A. obtectus in 1 day, respectively, whereas false yam killed 46-73, 33-60 and 33-60 of S. granarius, T. castaneum and A. obtectus, respectively. LT50, LD50 for the adult test insects to the food by-products and diatomaceous earth revealed that A. obtectus was most susceptible towards the botanical byproducts (LD50) 308.4639 and 1410.6973 ppm (wt/wt) followed by S. granarius 3708.1338, 22,908.3521 ppm (wt/wt) of FYLP and PRHAP, respectively. Tribolium castaneum was the most tolerant (LD50) 33,240.3851, 2137.8481 ppm (wt/wt) to FYLP, respectively, but were susceptible to PRHAP compared to S. granarius. Fylp did not completely inhibit the progeny production of all the three insects, irrespective of dosage applied compared to Prhap which completely inhibited F1 production of all the three beetles tested irrespective of the dosage applied. Combination of powders of the botanical food by-products and diatomaceous earth controlled the beetles faster compared to the plant products in isolation. LT50 ranged from 13 to 84 h for T. castaneum exposed to doses of 20,000 ppm (wt/wt) PRHAP and 20,000 ppm (wt/wt) DE, and 10,000 ppm (wt/wt) FYLP and 20,000 ppm (wt/wt) DE, respectively. PRHAP alone with a dosage of 20,000 ppm (wt/wt), the LD50 for A. obtectus, S. granarius and T. castaneum ranged from 29, 69 to 115 h, respectively. Botanical powders caused significant reduction of F1 adults compared to the control. Combination of DE and botanical food by-products is promising approach of integrated pest management strategy.