Heavy Winter Losses?
There are mixed reports regarding the level of winter losses over the 06/07 winter. The weather across the county was mild with few frosts, however this is not necessarily a good thing, as it is generally better for our bees if it is cold. Low temperatures keep the cluster together, reduce both food consumption and the risk of colonies starving in early spring.
Some beekeepers report low or no losses whilst others are reporting 50%, and in a few cases even more. So why this difference? Is there a pattern which can be identified; and what can we do about it? We cannot ask the bees, so it is necessary to turn detective and try to deduce the cause from whatever evidence we can gather.
What was different about the way in which bees were managed last winter, and in the preparation for winter carried out in the autumn? One area which deserves a closer look is the methods used to treat for Varroa. With the threat of pyrethroid resistant mites now being present in this area, and the continual advice to use different treatments from both the BBKA and the NBU many beekeepers in the county decided to try using Oxalic acid last winter. Based on my informal survey of local beekeepers the majority who suffered heavy losses were in this group. Their colonies were OK at New Year but subsequently dwindled and died out. Brood frames were still full of honey clearly demonstrating that the colonies had not starved. On the other hand, some who used Oxalic acid report no loss of colonies, with rapid expansion in the Spring assisted by the unusually warm April. Similarly, the majority who used the traditional varroacide strips report no problems with their colonies surviving through the winter. How can we explain these differences? The following is my theory.
The advice we have been given with regard to the use of Oxalic acid is not to apply it when there is brood in the colony, as there is a risk of damage to larvae in open cells. In order to achieve this requirement, we are advised to apply the acid in the depth of winter, i.e. December or January. Many beekeepers decided to try this (including me!). They did not treat with strips in the autumn but waited until the year end before applying Oxalic acid. There is no doubt that the acid treatment is effective in removing mites from a colony. Using it with a floor insert in place results in a heavy mite drop within 24 hours of treatment and the drop continues for around a week. Does the acid cause damage to the bees and eventual colony loss? I don’t think so. Oxalic acid has been used in many parts of the world for several years to rid colonies of mites. If it caused the levels of loss we have seen recently then this would already be known and its use would not have been so widespread. There is also the recent local evidence of colonies which have flourished since treatment. So what else could it be?
It is a fact that colonies infected with Varroa typically die out due to the effect of viruses, rather than the presence of the Varroa themselves. Varroa mites act as vectors for virus particles, spreading the infection between bees as they pierce the inter-segment membranes of the bee’s abdomen to feed on the haemolymph (blood). Colonies which were left until December before being treated with Oxalic acid are likely to have had a large number of mites present for the period from August to December, when compared with colonies treated with strips in August. The presence of these extra mites increases the risk to the bees in the colony of being infected with any virus which may be present. The extra mites also increase the pressure on the developing brood as the mites reproduce in brood cells. In the autumn there is a reducing quantity of brood, and an increasing number of mites trying enter the cells to reproduce. This cannot be good for the developing larvae, and these larvae are destined to be the winter bees in the colony. These bees, the last to be reared in the season, will be weakened by this increased level of infestation, and by any viruses carried by the mites. Bees weakened in this way may survive through to early spring but may not survive long enough to start rearing the early season brood. The result being that colonies die out in the early season, having failed to rear brood as there are insufficient bees to support such activity. This fits the pattern that has been observed and also provides a possible answer as to why colonies which have died out have left combs full of honey. Colonies with a reduced number of bees which are not strong enough to rear brood do not need to consume stores.
Why did some colonies treated with oxalic acid survive without difficulty? Could it be that there was little or no virus present in the colony? Research has shown that colonies can survive with very heavy Varroa infections, provided that the virus levels are low. And what of the colonies which were treated successfully with Apistan or Bayvarol strips? Well, perhaps the beekeepers were lucky and either the mites in their colonies are not resistant to the pyrethroid treatments, or alternatively they have pyrethroid resistant mites, but the virus levels are low which enabled the colonies to survive. Either way, they may not get away with it next year!
How can we avoid the heavy losses next winter? It is vital that we reduce the number of mites in our colonies. Current advice is that we need to keep the number below 1000 mites. The NBU produces an excellent booklet “Managing Varroa” on the problems associated with Varroa and how to deal with it. If you don’t have a copy ask your bee inspector, or if you have internet access download a copy from the Bee Unit web site http://beebase.csl.gov.uk. This describes many different methods which can be used to reduce the number of mites in colonies. The key recommendations are:
- Monitor the levels of infestation
- Share your experiences with other beekeepers
- Use Integrated Pest Management techniques
- Only treat when necessary
- Learn how to test for resistant mites
- Be flexible with your treatments
- Keep up to date with new developments
- National Honey Show 2013
- BBKA David Aston Letter
- 2012 Wirral Convention
- Honeys and their origins
- Bees And Horses
- The Bee Dating Agency 2013
- Adopt a Bee Hive
- Heavy Winter Losses
- Queen Breeding V Importing
- Bees By Boat
- Bee Smoker Causes Blaze
- Forest Bees
- First Find the Queen
- Integrated Pest Management 2008
- Myanmar - Oldest Bee Fossil
- Nosema Ceranae
- Preparing For Winter
- Shared Experience
- The BBKA and Pesticides
- Varroa Research
- 2012 Spring Convention
- 2011 Autumn Convention
- Bob Parsonage 1934 - 2009
- Obituary Bob Parsonage
- 2011 Spring Convention
- 2010 Autumn Convention
- 2010 Spring Convention
- Around The Country
- Around The World
- Weather Warning - Check for Storm Damage To Hives
- Greater Wax Moth found in Cheshire
- Drone Brood Removal — just do it
- Drones and Varroa
- The Sugar Roll
- A Three Queen Summer
- Bee Research News: More on the Waggle dance
- Microscopy Day