Evaluation of diatomaceous earth, methoprene, and varietal resistance to control Rhyzopertha dominica (F.), the lesser grain borer in stored rice

Commercial formulations of diatomaceous earth (DE), Insecto RTM and Protect-ItRTM, did not completely control Rhyzopertha dominica (F.), the lesser grain borer on rough rice when used at the rates. Mortality after one week of exposure to both DE formulations was about 69% and there were no differences in effectiveness of DE among long grain, short, grain and medium-grain rice types. When adults were exposed on rough rice treated with combinations of 0 to 500 ppm Protect-It RTM DE and 0 to 1 ppm of methoprene, mortality of exposed adults was only about 20% at the highest combination. However, any combination of DE plus 0.5 to 1.0 ppm methoprene completely suppressed progeny production. In DE alone, mortality of adults and progeny production was lower on medium grain rice than either long or short-grain rice.; The hatch rate of R. dominica eggs was reduced by 40% on rough rice treated with 1 ppm methoprene compared to untreated rice. Larvae that hatched from these exposed eggs died inside the rice kernel. If the newly-emerged adults were exposed to the same rate on rough rice, fecundity of adult females was reduced about 80% compared to fecundity on untreated rice. Neonates could only infest rough rice with cracked or split hulls, and progeny production increased in direct proportion to the percentage of cracked and split hulls in different rice varieties. Varieties Bengal, Jupiter, Pirogue, and Wells were considered tolerant to R. dominica, as indicated by the Dobie index of susceptibility; varieties Cocodrie, M-205, and Rico were classified as susceptible. At 1,000 ppm, InsectoRTM gave complete mortality and progeny suppression on the resistant varieties, while on the susceptible varieties mortality did not exceed 80% and progeny were produced. These results demonstrate that resistant varieties of rough rice can enhance efficacy of DE for controlling R. dominica. The methoprene effectively eliminated progeny production on susceptible and resistant rice varieties. Combining varietal resistance with combination treatments of DE and methoprene could be a promising approach to control R. dominica in stored rough rice.