Variation in susceptibility of eight insecticides in the brown planthopper Nilaparvata lugens in three regions of Vietnam 2015-2017.

The brown planthopper (BPH), Nilaparvata lugens, is a serious threat to rice production in Vietnam and insecticides are widely used for its control. Migration of the BPH have one of its roots in tropical Vietnam in the Mekong River Delta and the insecticide resistance status of BPH populations from Vietnam is thus important for East Asia. In the present investigation, we evaluate the susceptibility of BPH populations from nine provinces from the Red River Delta, the Central Coastal region and the Mekong River Delta of eight insecticides during 2015-17. BPH field populations of Vietnam have developed a low to moderate level of resistance to the neonicotinoids dinotefuran, nitenpyram and imidacloprid, the pyrethroid etofenprox, the anticholinesterase fenobucarb, as well as fipronil and pymetrozine, and the growth regulator buprofezin. There was a correlation of in toxicology of fipronil, dinotefuran, etofenprox, buprofezin, which represents four different modes of action. The neonicotinoid nitenpyram, pymetrozine and fenobucarb did not show correlation in toxicology to any of the investigated insecticides. For most insecticides, a gradient of susceptibility was established from the Red River Delta in the north, through the Central Coastal region and to the Mekong River Delta in the south of Vietnam. The most susceptible populations were from the north. Insecticide resistance of the BPH populations in Vietnam is not at an alarming level and they are not the direct origin of high insecticide resistance found in East Asia. The cross-resistance pattern of BPH populations in Vietnam, where insecticides with different modes of action correlated, indicate that insecticides should be used with caution. There could be a buildup of a general metabolic resistance, which alone or in combination with the emergence of target-site resistance mutations will cause control problems. The results will be beneficial for development of resistance management strategies to prevent and delay development of insecticide resistance in BPH not only for Vietnam, but also for more northern Asian regions due the migration of BPH from tropical Vietnam.