Risk assessment of insecticides used in rice on miridbug, Cyrtorhinus lividipennis Reuter, the important predator of brown planthopper, Nilaparvata lugens (Stal.)

The green miridbug, Cyrtorhinus lividipennis, an important natural enemy of the rice brown planthopper (BPH), Nilaparvata lugens plays a major role as a predator in suppressing the pest population. The study assessed the impact of certain potential insecticides used in the rice ecosystem on the miridbug predator and brown planthopper through contact toxicity. Eleven insecticides, including neonicotinoids, diamides, azomethine pyridines, carbamates, pyrethroids, organophosphates and cyclodienes were selected to test their toxicities against the nymphs of C. lividipennis and N. lugens. Median lethal concentration (LC50) was determined for each insecticide using an insecticide-coated vial (scintillation) residue bioassay, which revealed BPMC as the highly toxic chemical with an LC50 of 0.003 mg a.i L−1 followed by ethofenprox and clothianidin with LC50 of 0.006 mg a.i L−1 at 48 HAT against C. lividipennis and ethofenprox as the highly toxic chemical with an LC50 of 0.009 mg a.i L−1 followed by clothianidin with an LC50 of 0.211 mg a.i L−1 at 48 h after treatment (HAT) against N. lugens. Among the insecticides tested, the cyclodiene compound, endosulfan had the lowest acute contact toxicity (LC50 = 66.65 mg a.i L−1 at 48 HAT) to C. lividipennis. Among the insecticides tested, endosulfan, chlorpyriphos, acephate and methyl parathion are regarded as safer to C. lividipennis based on selectivity ratio, hazard quotient and probit substitution method of risk assessments.