| ||In the spring of 2009, Alberta beekeepers once again found high winterkill losses in wintered bee colonies. To determine the extent and possible causes of the winterkill, Alberta Agriculture and Rural Development conducted a survey of 95 beekeepers that have 400 or more colonies. The response rate was 82%. Survey results show that 28% of Alberta bees were killed in the 2008/2009 winter, which is similar to the unusually high winterkill found in the previous two years. Of the surviving colonies, 13% were weak with less than three frames covered with bees in comparison to 8 frames of bees in average strength colonies. Throughout the entire province, 54% of the beekeepers reported losing one-third or more of their productive colonies in 2009 due to winterkill and weak colonies. However, 14% of the beekeepers were able to effectively control Varroa and Nosema and achieve substantially low winterkill and weak colonies (14%).
Overwinter losses in Alberta during 2008/09 may be attributed to a combination of several causes. The most important possible cause for reported high winterkill was increased infestation of Varroa mites and failure of chemical control products. Varroa has become resistant to Apistan and Checkmite+. In 2008 beekeepers used one or a combination of several available treatment measures to control mites but the efficacy of available treatments appeared poor. When Apivar received an emergency registration and became available, 41% of the beekeepers switched to Apivar to achieve an effective treatment to protect winter bees from further damages caused by Varroa mites. Survey participants commented that additional effective control products are desperately needed.
The prolonged winter coupled with a cold, late spring aggravated the winterkill problem in Alberta. Outdoor wintered colonies experienced higher numbers of winterkill and weak colonies than indoor wintered colonies in the same region. Honey bees that wintered outdoors suffered more from Nosema, leading to high percentages of colonies killed or weakened by the end of winter. Participant beekeepers in the survey ranked winter weather similar to Varroa mites’ rank as the most important factor causing reported high winterkill in 2008.
Most beekeepers reported high rates of visible Nosema-like symptoms in wintered bee colonies. Despite beekeepers fed fumagillin medicated sugar syrup in the fall to control Nosema, the chemotherapy did not work effectively. Assuming these symptoms were caused by Nosema, the percentage of bee colonies with Nosema like symptoms significantly correlated with winterkilled and weak colonies. To improve Nosema control measures, beekeepers have started to monitor nosema levels in their colonies and improve treatment methods.
Overall the combination of weather, Varroa infestation and Nosema are most likely the cause of high winterkill in Alberta. These factors are currently under study to improve honey bee pest surveillance and management.