The True Significance of Winter Brood Rearing

The BeeHolder, October 2011

The recent scientific confirmation of winter brood rearing in the honeybee colony has real significance for the survival of over-wintering colonies in the present circumstances. Despite the fact that there are many beekeepers who either never questioned the accepted wisdom of a hiatus in rearing brood in the bee colony in the dormant period or who have rejected the concept out of hand; winter brood rearing in the presence of the Varroa mite takes on a whole new dimension.

The jury is still out on the causes of the massive bee colony losses world wide but there is increasing focus on the neuro-toxic pesticides such as Imidacloprid, Clothiniadin, Fipronil etc, in the areas where crops like maize and OSR are grown and where the seed and development of these crops is treated with these suspect substances. There is little doubt that the nicotinoid pesticides are deeply implicated in colony losses, however the mystery of the losses deepens when despite the argument that pesticides are the culprit, heavy colony losses are also being incurred in areas well away from intensive agriculture. The issue of winter brood rearing becomes a critical factor when these late winter/early spring losses are addressed.

Recent information coming from Germany advocates that the infestation level in any colony infested with Varroa should not exceed 50 mites at the end of December. A drop of 1 mite/2 days on the floor insert at this time seems to be a good indicator that a mite population of between 35-50 has been reached. If a fall greater than this is registered the colony MUST be treated immediately. Even in Germany many beekeepers, up until the present time at least, still hold the popular belief that there is a hiatus to brood rearing in winter (personal correspondence), however notwithstanding it has been noted that colonies entering winter with low mite infestations have a greater survival rate than otherwise.

A simple calculation might drive home the critical importance of low mite numbers in colonies in early winter:

  • Every larva produced as a result of winter breeding will be a target for a female mite, which will live for around two months.
  • The mite average reproduction rate is reckoned to be some 1:1 new mites per generation
  • Consider a colony entering winter with a mite burden of 50; every 18 days the mite population will double; using mid December as a start date – best case scenario is that by mid January there will be 105 mites, by early February there will be 220, by the end of February 460, by late March 968 and by mid April 1800.
  • The adult bee population is of course meantime being parasitised and debilitated as well. Believe it or not that is the good news.
  • Now consider a colony with a mite population level of just 200, which is quite low by the current accepted standards of the ’winter breeding hiatus’ beekeeper advocate:
  • Initially each developing winter larva will be ‘multiple parasitised’, every 18 days the mite population will increase dramatically and to boot each emerging bee will be a total loss to the colony:
  • Using mid December as a start date – best case scenario is that by mid January there could be 500 mites, by early February there could be 1250, by the end of February 3125, by late 189 March 7860 and by mid April – best case scenario 19,440. The figures postulated could be questioned but the order of increasing magnitude is indisputable. By late April this hypothetical colony could be in real crisis or may have already succumbed.

It is unsurprising that many colonies entering the winter with mite burdens of over 200, especially in the possession of beekeepers who do not carry out their anti-mite treatments diligently or correctly, fail to survive past late winter or early spring. The phenomenon of winter brood rearing in the honeybee colony will be ignored at beekeeper peril. If age-old dogma can be cast aside, who knows, we in Scotland at least could really begin to take control of our colony health and winter survival. By ensuring that the mite has as few potential hosts on which to do her wicked work and really getting to grips with the necessary work of winter mite control – which entails any treatment method, applied at the correct time, that kills mites in the brood cells. Formic acid is, to date, the only substance which does just that. Applied correctly around early April this treatment method could just be the tipping point to get your colonies through to summer to become an effective honey gathering force.

The effectiveness of any late winter treatment will have to be closely monitored to ensure that the mite burden is as low or ideally lower than recommended. Thymol or oxalic acid treatment used correctly will achieve this aim.

Eric McArthur, Scottish BKA
Reproduced courtesy of eBees

I resisted the temptation to re-title this article “How the mitey have fallen”
Ed