A large field trial to assess the short-term and long-term effects of 4 insecticides used to control the pollen beetle on parasitic hymenoptera in oilseed rape
Abstract: A large-scale field trial was performed in spring 2013 to assess the effects of Plenum (pymetrozine), Mavrik 2F (tau-fluvalinate), Biscaya (thiacloprid) and Pyrinex (Chlorpyriphos-ethyl) used to control the pollen beetle Meligethes aeneus (F.) (Col.; Nitidulidae) on the populations of pests and beneficial arthropods in winter oilseed rape. The insecticides were applied at their commercial rate just before flowering on large strips of oilseed rape (30 m x 200 m), divided into four plots of 50 m x 30 m. A strip was left untreated as control. Insects were sampled by plant beating methods and sweep netting the day after treatment and thereafter weekly up to 50 days after product application. The direct effects of the products were assessed on adult pollen beetle (target pest), adult cabbage seed weevil Ceutorrhynchus obstrictus (Marsham) (Col.; Curculionidae) (secondary pest) and adult parasitic hymenoptera associated with these insects (Tersilochinae and Pteromalidae). In the context of IPM, long term effects were assessed on pollen beetle larvae to determine their number, the parasitism rate and to estimate the balance of parasitic hymenoptera/pollen beetle that could be produced by the field for the next season.Biscaya, Pyrinex and Mavrik 2F were effective in controlling the adult pollen beetle population and also had activity on cabbage seed weevil, despite the fact that this pest only occurred 2 or 3 weeks after the application of product. Plenum was only effective in controlling the pollen beetle population 1 day after treatment and had no significant impact on cabbage seed weevil.All the insecticides tested had a significant impact on the population of adult parasitic hymenoptera compared to the control. Plenum had limited effects while the other insecticides reduced by 59-72% the numbers captured in the sweep net samples. The main effects were observed on adults of the Tersilochinae family that are mainly specialised in the parasitism of pollen beetle larvae. Biscaya also had a significant impact on numbers of Pteromalidae caught, a family containing species specialised in the parasitism of weevils, despite the 4-week delay between the day of the treatments and the first arrival of these hymenoptera in the crop.Biscaya significantly reduced the parasitism rate of the pollen beetle larvae, which was reduced to less than 15% compared to 43.2% in the control. Pyrinex also decreased the parasitism rate with only 23% larval parasitism, but the difference was not significant. Biscaya and, to a lesser extend Pyrinex, reduced the balance of parasitic hymenoptera: pollen beetle, while Plenum and Mavrik 2F led to the same ratio as the untreated control. These results suggest that the regular use of Biscaya and/or Pyrinex on a large scale before flowering is favourable to the long term development of pollen beetle populations by negatively impacting the populations of their parasitoids, despite their good short-term efficacy to control this pest.