Efficacy of diatomaceous earth against the rice weevil Sitophilus oryzae (L.) after a preventive treatment of wheat

Abstract: Since 2009, diatomaceous earth (DE) is authorized at the EU level as natural insecticide to control insects and mites in grain stores and also for the treatment of empty stores. It is also registered in different EU countries. Treatments with DE SilicoSec® were carried out in order to prevent wheat infestation by the rice weevil Sitophilus oryzae. Two doses were tested with an application rate of 1 and 2 grams of DE/kilogram of grains and the efficacy was compared to a contact insecticide treatment with pirimiphos-methyl, used in France at 4 mg/kg. For each test series, 50 kg of non-infested wheat were treated in a concrete mixer with diatomaceous earth (2 doses) and pirimiphos-methyl by spraying. Treated wheat was divided in two parts, 25 kg were stored at 15 °C and 25 kg were stored at 25 °C. For each test series, three samples of 1 kilogram of wheat where sampled 24 hours, two weeks, 1, 2 and 3 months after treatment in order to assess the efficacy of the treatment and its insecticidal persistence. After each post-treatment period 200 rice weevil adults were introduced into 1.5 litre glass jars filled with 1 kilogram of treated wheat with DE and pirimiphos-methyl. The same operation was repeated with non-treated wheat for the control. The observation of insecticidal efficacy of diatomaceous earth was done 14 days after the introduction of insects. After the first test series (24 h after treatments), samples of wheat were stored at 25 °C and 65% R.H. in order to assess the emergence reduction of insects in these conditions and to compare the results between DE, pirimiphos-methyl and control. Mortality was 100% for all treatments with pirimiphos-methyl. Corrected mortality was above 95% at 25 °C for all treatments with DE. Corrected mortality was below 22% at 15 °C for each test series with 1 g/kg and below 69% for samples treated with 2 g/kg. The temperature seems to be the main factor which affects the efficacy of DE in these conditions even after 3 months of storage. The emergence reduction of insects was around 99% for all samples treated with pirimiphos-methyl, between 63 to 76% for wheat treated at 1 g/kg and between 81 to 94% for wheat treated at 2 g/kg.