Possible synergistic effects of fungicide-insecticide mixtures on beneficial arthropods

Abstract: A large scale field trial was performed in spring 2015 to assess the effects ofseveral insecticides used to control the pollen beetle on parasitic hymenoptera in oilseed rape.The tested products were Avaunt 150EC, Steward 30WG (both containing indoxacarb),Plenum (pymetrozine), Mavrik 2F (tau-fluvalinate), Biscaya (thiacloprid) and Cyren 4E(Chlorpyriphos-ethyl). The insecticides were applied at their commercial rates soon beforeflowering on large strips of oilseed rape. A strip was left untreated as control. Insects weresampled with the help of beating methods and sweep net sampling from DBT1 (five daybefore treatment) to DAT43. The direct effects of the products were assessed on adult pollenbeetle (target pest) and adult parasitic hymenoptera (Tersilochinae and Pteromalidae). In thecontext of IPM, long-term effects were assessed on pollen beetle larvae to determine theirparasitism rate and estimate the balance of parasitic hymenoptera/pollen beetle that could beextrapolated for the next seasons.All products were effective to control adult pollen beetle population at least three daysafter application. Avaunt 150EC, Steward 30WG and Cyren 4E were the most effectiveproducts with significant reduction of pollen beetle populations 7 days after treatment and fora longer period than the other products.Significant reductions were observed for Biscaya and Cyren 4E for adults of theTersilochinae subfamily, the main subfamily parasitizing pollen beetle larvae, with 65% and62% less insects compared to the control, respectively. No effects were observed with theother insecticides. No insecticides had significant effects on adult Pteromalidae, the mainparasitic hymenoptera attacking weevil larvae. The other families were only collected in fewnumbers. The parasitism of the pollen beetle larvae were significantly reduced by Biscaya andCyren 4E, compare to control and all the other insecticides tested. The balance between theparasitic wasp and the adult pollen beetle for the next seasons, extrapolated from the analysisof the larval parasitism gave similar results, with Biscaya and Cyren 4E leading to a reductionof the balance in favour of the pollen beetle, with 70% and 62% less parasitic hymenopteraexpected for the next year, respectively. The other insecticides did not impact negatively thisbalance and were considered as least as neutral for the parasitic wasps.In the context of IPM, Avaunt 150EC, Steward 30WG, Plenum and Mavrik Fdemonstrated efficacy in term of adult pollen beetle control and did not present a negativeimpact on the balance parasitic wasp/pollen beetle that could be obtained at the nextgeneration. Therefore, the use of these insecticides can be recommended in oilseed rape tocontrol pollen beetle populations in IPM, with an absence of negative short-term and longtermeffects.