Efficacy of diatomaceous earth to control Rhyzopertha dominica (F.) (Coleoptera: Bostrichidae) in rough rice: Impacts of temperature and relative humidity

Laboratory trials were conducted to determine the effectiveness of diatomaceous earth (DE) against Rhyzopertha dominica (F.), the lesser grain borer, on stored rough rice. Two DE commercial products, Insecto™ and Protect-It®, were applied at their respective label rates of 500 and 400 ppm to long grain rice by hand-mixing the DE with the rice. R. dominica were exposed for varying time intervals, mortality was assessed, and rice was held at different temperatures and relative humidities (r.h.) for 8 weeks until F1 adult emergence. There was a significant difference in mortality between the DE treatments and untreated controls (P<0.01), but no significant differences with respect to the two DE products (P⩾0.05). Mortality increased as exposure interval increased, and ranged from 15.8% to 69.2%, depending on the exposure interval. Although the general ANOVA showed a significant difference for temperature and r.h., when mortality and r.h. were compared only 5 out of 30 comparisons were significant (P<0.05). There was extensive progeny production in all treatments (including controls) and more progeny were produced at 32 than at 27 °C. The overall ANOVA showed a difference for treatment and r.h., but again few comparisons were significant (P<0.05). Results showed that the two DE products did not completely suppress R. dominica on rough rice, and combination treatments with another insecticide may be necessary to give complete control.