Bumblebee colony response to Closer and Amistar pesticides in semi-field study

This study investigates the sublethal effects of the systemic insecticide Closer (sulfoxaflor) and fungicide Amistar (azoxystrobin) on bumblebee (*Bombus terrestris*) foraging behaviour, colony growth, and pollination services under semi-field conditions. Forty enclosures were used, with pesticides applied to *Phacelia tanacetifolia* at label rates—Closer before flowering (BBCH 55–59) and Amistar at full bloom (BBCH 63–65). Results show significant reductions in foraging activity, with both pesticides impairing individual foraging performance and flower visitation rates. Colony weight gain decreased by 11.1% under Closer exposure, and pollen deposition on stigmas and styles dropped by 26.0% and 32.0%, respectively, under Amistar. These effects were likely exacerbated by limited floral resources in the enclosures. No significant interaction between the two pesticides was found, but combined exposure still impaired pollination and colony health. The findings highlight risks to bumblebee populations from sublethal pesticide exposure, especially under resource-scarce conditions, and call for improved regulatory assessments of pesticide mixtures and more ecologically realistic study designs.