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Gray, Alison and Peterson, M. and Teale, A. (2008) Varroa and losses of bee colonies in Scotland.
Bee Craft. ISSN 0005-7703
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Gray, Alison and Peterson, M. and Teale, A. (2008) Varroa and losses of bee colonies in Scotland.
Bee Craft . ISSN 0005-7703l
http://strathprints.strath.ac.uk/15261/
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THE APRIL and May 2007 editions of Bee Craft have
carried articles on Colony Collapse Disorder (CCD) in the
USA and elsewhere. Equally, there has been
considerable recent press interest in CCD and its
possible causes, amid speculation as to whether CCD is
now appearing in the UK
In relation to Scotland, some interesting findings on
unexplained colony losses and a possible link to Varroa
infestation of bee colonies arise from a survey of members
of the Scottish Beekeepers’ Association (SBA) carried out in
May 2006. This survey covered the period April 2004 to
March 2006 and was undertaken largely as a response to
reports of apparently newly emerging problems with queen
rearing in parts of southern England and concern over the
effects of the arrival of Varroa destructor in Scotland in
1996 and its subsequent wide spread across the country.
ALL PARTS OF SCOTLAND
The survey was stratified across the four SBA Areas, namely
Aberdeen, East, North, and West. In all, 100 members were
approached, 30 from each of the North, East and West
areas and 10 from the Aberdeen area, to give each area an
appropriate weight according to its size and so that about
10% of the total SBA membership was invited to
participate.
The members were selected personally by the Local
Association Secretaries or Area Representatives rather than
randomly. This method of selection is unlikely to give a truly
representative sample but was the only method feasible
without the consent of SBA members to use Association
records of their contact details. What it did provide was an
exceptionally high response rate of 77%. Participants
completed a questionnaire on a variety of topics relating to
beekeeping practice and experience.
The typical respondent to this survey is a small-scale
beekeeper, with the typical number of stocks kept by
respondents being eight or less for the period surveyed.
However, some larger enterprises were also included, the
largest one keeping 325 stocks in 20 apiaries in the West of
Scotland area.
RESULTS
A short summary of overall findings from initial analysis of
the data appeared in the December 2006 issue of The
Scottish Beekeeper, and the full report of the analysis is
available on the SBA website at
www.scottishbeekeepers.org.uk. Some findings relevant to
the impact of varroa and the incidence of unexplained
colony losses are as follows.
Varroa
The question on the year in which each respondent first
found varroa revealed that the mite is still not present
throughout Scotland. All respondents in the East area had
found it and most had seen it for at least two years. Only
two in the West had not yet found it, but in the Aberdeen
and North areas it was significantly less widespread. In fact
no one in the North had seen it before 2004 and more than
half the respondents in that area had not yet found it.
Swarming
Participants were asked about swarming, as it was thought
that this could well be affected by varroa. Numbers of
swarms observed and taken per season were generally low
(typically one) though, as would be expected, some
respondents on lists of beekeepers held by Local Councils
or Police dealt with many more (up to 16).
Slightly over 50% of respondents used bait hives to try to
attract swarms in both 2004 and 2005, the practice being
more common in the Aberdeen and North areas. Of those
using this technique, 58% were successful in attracting
swarms in 2004 and 62% in 2005. Success in attracting
swarms may prove a useful measure in future of the impact
of varroa in reducing the number of feral honey bee
colonies and hence of the number of stray swarms.
Varroa and Losses of Bee Colonies in
Scotland
Alison Gray, Magnus Peterson and Alan Teale
A survey in Scotland indicates that Varroa is probably a factor in unexplained colony collapse
Some beekeepers reported finding colonies that went into
winter in good shape were small and weak in spring
Unexplained losses
Regarding unexplained losses of colonies, there was an
overall rate of loss per colony kept of just under 5%
amongst respondents for winter 2004 05, about 2% during
summer 2005 and about 7.5% over winter 2005 06.
Small-scale beekeepers appeared to experience
proportionately higher loss rates than large-scale
beekeepers.
Sudden collapse
The sudden collapse of apparently thriving colonies with
hives unexpectedly found abandoned was also investigated
(referred to in the SBA article and survey report as the
‘Marie Celeste’ phenomenon, rather than Colony Collapse
Disorder). About 45% of reported losses were of this type
over winter 2004 05, about 81% over summer 2005 and
about 42% over winter 2005 06.
Most interestingly, a highly statistically significant link was
found between the odds of a respondent reporting this
phenomenon occurring at all and the longer the period of
time that varroa was known to be established in the
respondent’s bees. This provides conclusive scientific
evidence of varroa infestation, directly or indirectly, being a
contributory factor in the risk of CCD. This may, for example,
be associated with activation of associated virus infections.
(The statistical analysis estimated that the odds of CCD
occurring increased by a factor of about 1.5 for every year of
known infestation with varroa.)
Inbreeding
An attempt was made to estimate the risks of inbreeding
among respondents’ bees, on the assumption that soon
varroa will kill off all feral honey-bee colonies, apart from
re-colonisation of old sites by escaped swarms from
beekeepers. Large apiaries with many related queens were
supposed to be at particular risk but due to poor response
to questions on these topics it is hard to draw firm
conclusions.
Queen rearing
An attempt was also made to assess the number of
attempts at queen rearing and mating which were made
and the proportion of these that was successful. Large-scale
beekeepers made many more attempts and were in general
more successful. The success rate was generally over 80%.
There was some suggestion that 2005 was a more difficult
year than 2006, though the difference was not statistically
significant.
Respondents were invited to report any unusual numbers of
supersedures or any deformities among queens being
reared, as well as unusual behaviour of the bees. High
numbers of supersedures were reported only by 3 (4%) and
queen deformities by 5 (6%). However, unusual behaviour
was reported by 23 (30%), who provided useful comments
which are all included in an Appendix to the full report.
One of the respondents, who actually witnessed the
collapse of a hive, described this in detail and also
mentions both previously treating for varroa and possible
stress from the uniting of colonies. The description is as
follows:
‘On a warm sunny day in March 2005 almost all the bees
crawled out of hive, into the grass and up plant stems. They
were normal size and shape but had uncoupled wings and
were trembling. These are text book symptoms of chronic
(slow?) paralysis virus. The queen and about 100 bees
were left in the hive. All were dead in a few days. There was
no brood present but there were sufficient stores and the
combs were clean. The bees in the grass all died and all
traces were gone in a few days. There was very little activity
for a few days before. Varroa was first seen in summer
2004 and treated in September. Perhaps stress was a
factor as uniting of colonies was rather clumsily handled.’
Queenless in spring
Another interesting comment relates to colonies emerging
queenless in spring:
‘As a beekeeper for many years, I have observed an
increase in the number of colonies emerging queenless in
spring over the past three years. The number of colonies
surviving winter but appearing in a weakened state has
also increased over this period of time. In every case the
colonies went into winter in good shape and had an
abundance of stores in spring’. z
[Alison Gray and Magnus Peterson are at the Department of
Statistics and Modelling Science, University of Strathclyde,
Livingstone Tower, 26 Richmond Street, Glasgow G1 1XH.
Alan Teale is Bee Diseases Convenor for the Scottish
Beekeepers’ Association.]
Varroa and associated
viruses seem to be
implicated in unexplained
colony deaths in Scotland
