The Effects of Four Insecticides on the Population Dynamics of the Rice Water Weevil, Lissorhoptrus oryzophilus Kuschel2

The rice water weevil, Lissorhoptrus oryzophilus Kuschel, has been managed for the past 30 yrs using the soil insecticide carbofuran. The recent cancellation of the registration for carbofuran in rice has necessitated a shift to management strategies involving other insecticides, including lambda-cyhalothrin, fipronil, and diflubenzuron. Efficacies and effects on population dynamics of three alternatives to carbofuran (lambda-cyhalothrin and diflubenzuron as foliar sprays and fipronil as a seed treatment) were compared in two water-seeded and one drill-seeded field trials. Applications of lambda-cyhalothrin, but not of diflubenzuron or fipronil, resulted in decreases in the densities of rice water weevil adults and eggs. All three insecticides suppressed larval densities to levels comparable to, or lower than, densities in plots treated with carbofuran. All three alternatives to carbofuran differed from carbofuran with respect to their effects on the population dynamics of weevil larvae. Lambda-cyhalothrin, diflubenzuron, and fipronil were more effective than carbofuran at preventing early larval infestation of rice roots, but were less effective at preventing later infestation of roots. Yields from plots treated with fipronil, diflubenzuron, and lambda-cyhalothrin were generally higher than yields from plots treated with carbofuran, probably because prevention of early injury to roots has a more beneficial impact than prevention of later injury. This study also provided evidence for the utility of early planting and delayed flooding for management of the rice water weevil.