Abstract: The olive fruit fly, Bactrocera oleae (Diptera: Tephritidae) is the most importantpest of olive crops, causing severe losses in the olive production. Regional management of thepest relies on control of adults via application of baited insecticides. Within the frame of theresistance monitoring scheme, the evolution of resistance to pyrethroids was investigatedusing a diverse range of approaches including bioassays, field and semi-field trials as well asmolecular techniques. A dramatic increase of alpha cypermethrin resistance was observedafter 2013 (RF up to 129 in 2016; mortalities at diagnostic dose < 50%), especially inpopulations from Crete. The phenotype has an apparent impact on control efficacy underoperational conditions, as the mortality of a pyrethroid resistant field population wasdramatically reduced compared to a respective low resistant population in real field – cagetrials. This finding was further supported by a significant association among the percentagemortality in small-cage experiments and percentage mortality in baited laboratory bioassays.Analysis of the IIS4-IIS6 region indicated that resistance is not associated with target siteresistance mutations. Based on the microarray analysis, the ‘electron carrier activity’ wasfound to be enriched among GO terms in the pyrethroid resistant insects, possibly reflecting aP450-mediated resistance mechanism. The contigs 00326 and contig 02103, encoding forgenes with putative monooxygenase activity, were significantly over represented in all testedresistant insects. Pyrethroid resistance seems to be an emerging problem with potentialadverse projections in the actual crop protection efficacy of the baited applications.