Susceptibility of Danish pollen beetle populations against λ-cyhalothrin and thiacloprid

Abstract

Winter oilseed rape is an important crop in agriculture, where it plays an essential role in cereal-based crop rotations. The main pest in this crop is the pollen beetle which feeds on developing buds. Since 1999, pyrethroid resistance in the pollen beetle has spread throughout Europe. Denmark and Sweden are of interest due to the early detection of pyrethroid-resistant pollen beetle and because of the presence of both target-site and metabolic resistance in the populations. For almost ten years, neonicotinoid insecticides have been used in oilseed rape as well. In this study, the current resistance situation in Danish pollen beetle populations was conducted. The susceptibility of pollen beetle to the synthetic pyrethroid λ-cyhalothrin as well as the neonicotinoid thiacloprid was investigated. Sixty-eight Danish, as well as 6 Swedish and 10 German populations collected between 2014 and 2016 were tested with λ-cyhalothrin. Two-thirds of the Danish populations collected were classified as pyrethroid resistant, whereas Swedish populations were mostly susceptible and all German populations tested were resistant, mostly at higher levels than those detected in Danish strains. Toxicological parameters were calculated by a two-parameter log-logistic model and showed up to 58-fold variation at LC50 for λ-cyhalothrin and up to 17-fold variation at LC50 for thiacloprid. However, almost all Danish populations were susceptible against thiacloprid and the German populations showed up to sevenfold variation in thiacloprid bioassay. In conclusion, pyrethroid resistance is widespread in Danish pollen beetle populations with regional differences, whereas Danish populations are susceptible to thiacloprid in all regions.