Effects of different inert dusts on Sitophilus oryzae and Plodia interpunctella during contact exposure

The use of natural inert dusts against storage insect pests is increasing recently, as an alternative to conventional insecticides. Laboratory study was carried out to evaluate the contact effect of three inert dusts, diatomaceous earth (DE), kaoline (KA) and vermiculite (VE), at rates 5, 7.5, 10, 15 and 20 gm-2, against adults of Sitophilus oryzae (L.) and larvae of Plodia interpunctella (Hubner). Insect mortality was evaluated 1, 2, 3 and 7 days after the exposure. Insect mortality varied depending on the species, concentrations and exposure periods. The DE and KA caused 86.7-98% mortality of S. oryzae after 2 days of exposure at the highest rates, while at 5 and 7.5 gm-2, 100% mortality was achieved only after 7 days. The highest rates of inert dusts caused 42-50% (DE) and 60-75% (KA) mortality of P. interpunctella larvae only after 7days. The mortality of moths increased gradually with the concentration and 100% was achieved 3 days after the contact with DE and KA (10, 15 and 20 g m-2). However, inert dusts induced faster pupation of P. interpunctella, while adult emergence was reduced and adults had smaller body-sizes, compared to control. The VE caused relatively low mortalities (7-11% of S. oryzae adults and 5-8% of P. interpunctella larvae) at all tested rates during the entire experiment. Our results have shown good insecticidal effect of DE and KA against S. oryzae and P. interpunctella at 10, 15 and 20 gm-2. These products could therefore be used by small-scale farmers to protect stored grains against insect pest infestation.