Still not one cause though. Maybe a proximate cause but the bees have less forage due to monoculture and climate change screwing with plant cycles. This results in malnutrition and increased susceptibility to viruses. The anti mite drugs allowed them to limp along while feeding them sugar water till they got to orchards but obviously that's not going to cut it. Competitive pollinator services racing to the bottom unsustainably leading to a mass die off as soon as more stress is introduced is the systemic issue at play here