Prospective Large-Scale Field Study Generates Predictive Model Identifying Major Contributors to Colony Losses

Over the last decade, unusually high losses of colonies have been reported by beekeepers across the USA. Multiple factors such as Varroa destructor, bee viruses, Nosema ceranae, weather, beekeeping practices, nutrition, and pesticides have been shown to contribute to colony losses. Here we describe a large-scale controlled trial, in which different bee pathogens, bee population, and weather conditions across winter were monitored at three locations across the USA. In order to minimize influence of various known contributing factors and their interaction, the hives in the study were not treated with antibiotics or miticides. Additionally, the hives were kept at one location and were not exposed to potential stress factors associated with migration. Our results show that a linear association between load of viruses (DWV or IAPV) in Varroa and bees is present at high Varroa infestation levels (>3 mites per 100 bees). The collection of comprehensive data allowed us to draw a predictive model of colony losses and to show that Varroa destructor, along with bee viruses, mainly DWV replication, contributes to approximately 70% of colony losses. This correlation further supports the claim that insufficient control of the virus-vectoring Varroa mite would result in increased hive loss. The predictive model also indicates that a single factor may not be sufficient to trigger colony losses, whereas a combination of stressors appears to impact hive health.Author Summary: Roughly one third of the food supply relies on pollinating insects. The number of colony losses of the domesticated Honey Bee (Apis mellifera) has grown significantly in the past eight years, endangering pollination of crops like almonds. Recent research indicates that colony losses are influenced by a combination of several factors. We conducted an extensive and controlled study that allowed us to look at the contribution of different factors to colony loss. Results helped us build a predictive model showing that a single factor is often insufficient to trigger colony loss. Combination of stressors has shown to have greater impact on hive health; replication of the Deformed Wing Virus, stressful weather conditions, and Varroa destructor comprise the primary identified causes.