Widespread resistance of the apple aphid Aphis spiraecola to pyrethroids in China.

The apple aphid, Aphis spiraecola, is an important pest in apple orchards. Pyrethroids had been widely used for the control of apple aphid. To verify the resistance level of A. spiraecola against pyrethroid insecticides, 35 field populations from main apple-producing areas of China were collected, and the susceptibility to type I and type II pyrethroids were assessed. Bioassays showed that six populations were highly resistant to bifenthrin (RR = 40.97-93.88 fold), and 12 populations showed the extremely high resistance against permethrin (RR = 161.17-349.27-fold). Moreover, all field populations developed high to extremely high resistance against lambda-cyhalothrin, fenvalerate and deltamethrin, except the HNLY and SXYL populations. Toxicity assays indicated that A. spiraecola field populations have developed serious resistance to multiple pyrethroids, and the relative highest resistance occurred in SXQX population. Pre-exposure to different synergists did not significantly increased the toxicity of pyrethroids to SXQX population, indicating a minor role of metabolic resistance in the resistant A. spiraecola. Sequencing of voltage-gated sodium channel (VGSC) genes revealed two homozygous mutations (918V/V and 1014F/F) and six heterozygous mutations (918M/V, 1014L/F, 918M/V+1014L/F, 918V/V+1014L/F, 918M/V+1014F/F and 918M/L+1014L/F) in A. spiraecola. Moreover, 1014L/F and 918M/V+1014L/F mutations were the dominant genotypes with frequencies of 26.67 % and 33.33 % in field populations, respectively. These results indicate that A. spiraecola in China has developed resistance against pyrethroids, and the resistance are mainly caused by mutations in VGSC. Pyrethroid insecticides should not be employed in these fields-evolved resistance, where the practical resistance management is urgently needed.