NBU Alert dated 8th May 2014:
"Many of our Bee Inspectors have reported a high population of Varroa mites in colonies across England and Wales. We believe these high levels are largely due to many colonies continually rearing brood throughout the mild winter. Continuous brood rearing dramatically reduces the efficacy of winter Varroa treatments such as oxalic acid, which do not kill mites sealed in brood cells."
"We urge beekeepers to monitor colonies and check either the natural mite drop from a sticky insert/ open mesh floor or by uncapping drone brood. From May to August, a natural mite drop should be monitored over a week. The number of mites then counted over this week should be multiplied by 30 to give you a rough population of Varroa in your colonies. A figure of 1000 mites or more is considered to be a high infestation. If uncapping drone brood, then only 5 Varroa mites out of 100 uncapped pupae need to be found to be considered a high infestation."
"Should you discover that your colonies have a high amount of Varroa then a range of options are available from biotechnical methods such as drone brood removal, to authorised varroacides. NB Varroacides used will be weather and temperature dependant. If you have supers on your colonies then thymol treatments should not be used due to tainting of the honey. "
More information is available in the NBU leaflet ‘Managing Varroa’, which is available to download from their website.