A large field trial to assess the short-term and long-term effects of 5 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), Boravi WG (Phosmet) and Pyrinex (Chlorpyriphos-ethyl) used to control the pollen beetle Meligethes aeneus (F.) (Col.; Nitidulidae) on pest and beneficial arthropods’ populations in winter oilseed rape. The insecticides were applied at their commercial rate soon 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 weekly sampled with the help of beating methods and sweep net from one day 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 of parasitic hymenoptera related to these insects (Tersilochinae and Pteromalidae). In the context of IPM, long term effects were assess 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, Boravi WG, Pyrinex and Mavrik 2F were effective to control the adult pollen beetle population and had also an 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 to control 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 adult parasitic hymenoptera population compared to the control. Plenum had limited effects while the other insecticides reduced of 59% to 72% the captures in the sweep net samplings. The main effects were observed on adult of the Tersilochinae family that are mainly specialised in the parasitism of pollen beetle larvae. Biscaya had also a significant impact on Pteromalidae captures, 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, that dropped to less than 15% compared to 43.2% in the control. Pyrinex also decreased the parasitism rate with only 23% of larval parasitism, but the differences were not significant. Biscaya and, to a lesser extend Pyrinex, reduced the balance of parasitic hymenoptera/pollen beetle while Plenum, Boravi WG and Mavrik 2F led to the end of the growing season to the same ratio than 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.