BACKGROUND: A wide range of methods has been used for monitoring resistance in Cydia pomonella L. However, the results from specific tests are not always concordant and they generate doubts over the extent to which the data represent the field situation. Therefore, the variability in the expression of resistance mechanisms and in resistance ratios between developmental stages and their reliability in a field situation were studied. RESULTS: Neonate larvae showed the highest insecticide resistance, except to spinosad. Resistance ratios were from 2 to 9.5, depending on the insecticide and the strain tested, when compared with the susceptible strain. Spinosad exhibited the highest resistance ratio for diapausing larvae (4.4, 12.2 and 4.4 for Raz, Rdfb and RA strains, respectively). Enzymatic ratios were higher in adults than in neonates or diapausing larvae, and the highest enzymatic ratios were observed for mixed-function oxidases (7.3-36.7 for adults of resistant strains). CONCLUSIONS: The sensitivity of different instars depends on the insecticides used, and there is a differential expression of resistance mechanisms between stages. For the population considered in this study, tests on neonates provided the most consistent results to the field situation, but adult response to chlorpyrifos-ethyl and azinphos-methyl in the laboratory was also consistent with the field results. (C) 2008 Society of Chemical Industry.