Spontaneous loss and reselection of resistance in extremely resistant Myzus persicae (Sulzer)

Modification of an immunoassay for detection of esterase-4 (E4), the enzyme conferring insecticide resistance in Myzus persicae, enabled further discrimination of extremely high levels of insecticide resistance (R3) from the very high levels (R2) previously characterised by this technique. The modified immunoassay was used to study the rate and pattern of loss of resistance in 10 R3 clones (all possessing an A 1,3 chromosomal translocation) isolated from the field and maintained in population cages. Clones lost resistance at varying rates and one remained stable at a level intermediate between R2 and R3. Loss was catastrophic; high frequencies of susceptible individuals were produced within a clone in the space of one generation without the formation of stable intermediates. However, this was not associated with loss of the chromosomal translocation. Of a number of other R3 clones isolated from the field only one stabilized at an intermediate level of E4 production equivalent to R1. Reselection of resistance in reverted susceptible subclones, by increasing concentrations of insecticide, was due to gradual selection for existing variation in E4 levels which was not apparent in clones of normal karyotype.