The frequency and intensity of Myzus persicae (Sulz.) resistance were studied using bioassays and biochemical tests in two potato crops in Ibicoara, Bahia. Both areas received several insecticide applications which failed to control M. persicae. By the end of the crop season, one of the areas received two applications of pirimicarb and the other was not sprayed. Six samples of 700 aphids each were randomly collected before and after the pirimicarb applications, with an interval of four days. Specimens of M. persicae were also collected from isolated potato plants. At the laboratory, the samples were characterized by the CL50 based on insecticide bioassays and by total esterase activity using colorimetric assays. After the pirimicarb applications, the susceptible (S), the moderate resistant (R1) and mixed S/R1populations presented decreasing resistance frequencies (from 36.6 to 9.9; 12,0 to 7.5 and 11.4 to 5.9%, respectively). On the other hand, high resistant individuals (R2), extremely resistant (R3), and mixed populations of R1/R2 had increasing frequencies (from 17.7 to 36.7%; 2.3 to 9.1%, and 20.0 to 30.8%, respectively). The survival of S individuals was probably due to their wandering behavior on the plants to avoid sprayed areas. All changes in frequency were reflected in the LC50 and in the resistance ratio (rr). In the pirimicarb untreated area, high frequencies of R2, R3 and R1/R2 were observed. High frequency and resistance intensity of M. persicae in areas under intensive insecticide applications can be related to the selection of resistant populations and due to the entrance of winged migrants from spontaneous plants, where the frequency of R2+R3 was 81.4%.
Insecta; pirimicarb resistance; resistance frequency; total esterases; colorimetric assay; green peach aphid
