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 is the most widespread and abundant parasitoid attacking Ceutorhynchus assimilis, a major pest of winter oilseed rape, in Europe. In a 2-year study (1993-94) conducted in the UK, the effects on T. perfectus of commercial applications of alpha-cypermethrin and of triazophos to control C. assimilis were investigated. Triazophos had a detrimental effect on T. perfectus. It was applied at the time that parasitoids were searching for larvae of C. assimilis on which to oviposit, and it killed both adult T. perfectus and their host larvae. Alpha-cypermethrin was less harmful than triazophos, probably because it was applied before the main migration of T. perfectus into the crop. The need to consider 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 pyrethroid during flowering, rather than triazophos post-flowering, to control C. assimilis.