So you’ve got the bees all fed and ready to be wrapped for winter — but just suppose, there’s a spell of warmer weather, and you see a chance to feed a bit more. You’ve even got a few pails of sugar syrup leftover on hand… but the Fumagilin B has already been added to the syrup.

The question, then, is:
What is the life expectancy of Fumagilin B in sugar syrup?

Should you mix up another batch, or will the leftover medicated syrup still protect your bees from Nosema through the winter?

According to the people at Medivet, in a recent email exchange with inquiring beekeepers, it’s not possible to give an authoritative answer to this question because so much depends on the storage conditions for the syrup.

In general, however, it is not recommended to make up your medicated syrup very much ahead of time, or to store it for long, because the Fumagilin B is less stable in sugar syrup than it is once it’s been processed by the bees.

In other words, medicated syrup can be stored by bees in the hive — but the beekeeper can’t do it without the Fumagilin B losing much of its power to combat Nosema.

A “best guess” estimate is that syrup may lose about 20% - 30% of its potency in sugar syrup stored for 4 weeks, and another 50% if stored for much longer than that. The bees are likely to get “some benefit” from the old syrup, but it would be best to ensure that the bees have been well fed with freshly-mixed medicated syrup to ensure that the Fumagilin B will be effective.