| | In the spring of 2008, Alberta beekeepers once again found high winterkill losses in overwintered bee colonies. To determine the extent and possible causes of the winterkill, Alberta Agriculture and Rural Development conducted a survey of 112 beekeepers, who have 400 or more colonies. Survey results show that 30% of Alberta bees were killed in the 2007/2008 winter, which is similar to the unusually high winterkill found the previous year. This recent high rate of winterkill is twice the long-term average in Alberta. Of the surviving colonies, 14% were weak with less than three frames covered with bees. The survivorship and production of these weak colonies in 2008 is uncertain. The winterkill plus weak colony percentages was highest in the Peace River area, followed by central regions and was lowest in southern Alberta. Throughout the entire province, 50% of the beekeepers reported losing one-third or more of their productive colonies in 2008 due to dead and weak colonies.
Overwinter losses in Alberta during 2007/08 may be attributed to a combination of several causes. Most beekeepers are now finding varroa mite infestations in the spring, despite more fall treatments. Most beekeepers are reporting varroa resistance to Check-Mite or Apistan. Because of this, they are changing to alternative treatments to combat the mites. However, the available alternative treatments (formic acid and oxalic acid) do not appear effective. Higher numbers of dead and weak colonies are associated with the presence of varroa mites in the spring and with resistance. The late discovery of varroa mite resistance has made it difficult to treat varroa in an acceptable time with alternative products to protect winter bees. Late and multiple treatments increase the stress on colonies, making them more vulnerable to winterkill factors. Survey participants ranked varroa mites as the most important factor contributing to winterkill, and many 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 dead and weak colonies than indoor wintered colonies in the same region. Honey bees wintered out doors were not able to defecate in early spring. Consequently, high percentages of colonies died or were weakened by high levels of Nosema. Participants ranked winter weather as the second most important factor behind varroa mites, and cold spring weather was the most common comment provided that effected colony build up.
In the spring of 2008, most beekeepers reported Nosema-like symptoms despite fall treatment with fumagillin. Though beekeepers fed this medicated sugar syrup in the fall in order to control Nosema, the chemotherapy did not work effectively. Assuming these symptoms were caused by Nosema, the percentage infected positively correlates to the dead and dead plus weak colonies.
The combination of fall weather conditions during treatment time and labor availability affected some beekeepers and the development of winter bees in their colonies. The number of dead and weak colonies was substantially higher in this group.
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