EFFECTS OF COMMERCIAL INSECTICIDE TREATMENTS TO WINTER OILSEED RAPE ON PARASITISM OF CEUTORHYNCHUS-ASSIMILIS PAYKULL (COLEOPTERA, CURCULIONIDAE) BY TRICHOMALUS-PERFECTUS (WALKER) (HYMENOPTERA, PTEROMALIDAE)

Trichomalus perfectus (Walker) is the most widespread and abundant par asitoid attacking Ceutorhynchus assimilis Paykull, a major pest of win ter oilseed rape, in Europe. In a 2-year study, the effects on T. perf ectus of commercial applications of alphacypermethrin and of triazopho s to control C. assimilis were investigated. Triazophos had a detrimen tal effect on T. perfectus. It was applied at the time that parasitoid s were searching for C. assimilis larvae on which to oviposit, and it killed both adult T. perfectus and their host larvae. Alphacypermethri n was less harmful than triazophos, probably because it was applied be fore the main migration of T. perfectus into the crop. The need to con sider natural biological control by parasitoids in pest management of oilseed rape pests is discussed. To encourage T. perfectus populations , growers are recommended to use insecticides only when the numbers of C. assimilis exceed the treatment threshold and then to use a pyrethr oid during flowering, rather than triazophos post-flowering, to contro l C. assimilis. (C) 1997 Elsevier Science Ltd.