Comparative lethality of rice husk ash and a diatomaceous eartht adults of four storage beetles

Lethality of rice husk ash (RHA) and a diatomaceous earth (SilicoSec) (DE) to adults of Sitophilus zeamais, S. granarius, Lasioderma serricorne and Callosobruchus maculatus was investigated under controlled conditions of 25 ± 2° C and 60 ± 3% relative humidity. Each product was tested at 0.05 g to 0.5 g/20 g of grain respectively in glass Petri dishes against 20 adults of each beetle. Adult mortality was observed up to 10 days post treatment. RHA/DE mixtures (1:1, 3:1 and 1:3 ratios) were also tested at 2% of grain weight. Additionally, RHA and DE were tested at low dosages (0.01 g to 0.04 g/20 g) against adults of C. maculatus alone. The DE generally produced significantly higher mortality of all the adult storage beetles and at earlier observation times, than RHA at the lower dosages (< 0.2 g). Adult mortality produced by RHA and DE in S. zeamais and S. granarius increased with increase in dosage from 0.05 g to 0.5 g. The RHA/DE mixtures generally produced similar mortality of all the adult storage beetles irrespective of post-treatment exposure time. The S. zeamais and S. granarius were generally more tolerant to the DE and RHA treatments than L. serricorne and C. maculatus. Percentage mortality of C. maculatus adults when DE was applied at low dosages (0.01 g to 0.04 g) was generally higher than RHA applied at similar dosages, up to 3 days-post treatment. All treatments produced 100% mortality of C. maculatus adults 4 days-post treatment. The data further confirm the efficacy of DE and RHA as insecticidal dusts at the dosage rate of 0.5 g or more per kg of grain.