Spinosad resistance development in wild olive fruit fly Bactrocera oleae (Diptera: Tephritidae) populations in California.

Background: Among target pests of the insecticide spinosad is the olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae). In Cyprus, spinosad has been sporadically used since its registration in 2002, whereas in Greece its use has been very limited since its registration in 2004, particularly in biological olive cultivars in Crete. By contrast, in California it has been the only insecticide used against the olive fruit fly since its registration in 2004. This study aimed at examining the resistance status of the olive fruit fly to spinosad.
Results: Populations from California, Greece and Cyprus, plus a laboratory population, were tested. Bioassays were performed by oral or topical application of different concentrations of the insecticide. Cypriot populations demonstrated no resistance as compared with that of the laboratory population. Among the Greek populations, only one from Crete demonstrated a fourfold increase in resistance, whereas five populations from California demonstrated a 9-13-fold increase.
Conclusion: The observed resistance increase was associated with spinosad applications in the respective areas. These values are relatively low and do not yet pose a serious control problem in the field. However, the observed variation documents that spinosad tolerance has increased in areas where the insecticide has been more extensively used.