Denmark is often given as an example of a successful varroa control programme that
doesn’t use synthetic chemicals (such as fluvalinate, flumethrin, amitraz, coumpahos)
that are prevalent elsewhere in Europe. Bayvarol (flumethrin) is available for mite control
in Denmark on veterinary prescription, but according to a survey conducted in the country
in 2000, 86% of the beekeepers (who keep 78% of the country’s total hives) use
‘biological’ methods (organic chemicals and biotechnical controls).
Denmark has 4,600 beekeepers and 155,000 hives. Average production is similar to New
Zealand’s, at about 35 kg per hive. Varroa was first discovered in Denmark in 1984 and is
now spread throughout the country, including on most off-shore islands.
In 1984, the Danish Institute of Agricultural Sciences (Ministry of Food) proposed a
varroa control strategy that did not rely on veterinary drugs. The proposal was later
approved by the Danish Beekeepers’ Association (DBF). Both organisations have spent
considerable time and money since developing the use of organic miticides (such as
formic acid, lactic acid and oxalic acid), as well as biotechnical controls (mostly drone
brood removal, comb trapping and heat treatment). Their objective is to maintain
pesticide use at a minimum and keep chemical residues out of honey and beeswax. The
approach follows on from their commitment not to use antibiotics to control American
foulbrood, a similarity they have with New Zealand beekeepers.
CONTROL OF VARROA: CONTROL METHODS USED OVERSEAS
Denmark is an example of successful varroa control using
organic chemicals and biotechnical methods.
The Danish system can be summarised as follows:
• Drone brood removal/trapping during the late spring/early summer build-up period.
• Short and/or long-term formic acid treatment immediately following the honey
• A late treatment once brood rearing has ceased, using either lactic acid or oxalic
• Monitoring natural mite fall, and mite fall after treatment, especially in the
summer and after lactic acid treatment in autumn.
The system also includes variations for early and late honey production, since some
beekeepers produce ling heather honey at the end of summer or beginning of autumn.
Monitoring is usually done with mesh screens on the bottom board of each hive. Daily
natural mite fall (for colonies with the equivalent of at least one frame of brood) is
multiplied by 120 to give the total number of varroa in the colony. If total mite numbers
rise above 1000, control is carried out as soon as possible.
An interesting drone brood trap used in Denmark consists of a frame divided vertically
into three sections. The frame (without foundation) is inserted into the centre of the brood
nest once the colony and conditions are good enough for comb drawing. When the bees
finish drawing out the frame with drone comb, two sections are removed, and then a week
later one of the rebuilt sections is removed again. This provides by the third week a single
comb with three different stages of drone brood development. Once a section of brood is
capped, it is removed and destroyed (usually up to the beginning of the honey flow).
Nucleus colonies are made to increase numbers and for swarm control, and drone brood
trapping is used in these circumstances to take advantage of broodless situations and the
attraction that a frame of larvae has to mites looking to reproduce.
When the honey flow is over, the honey is removed and hives are treated with formic acid
in a fibre-board square sealed into a heavy-duty plastic bag. A 16 mm hole is cut in each
side of the bag. The board is left in the hive for either one or two weeks.
The colony is then given a feed of sugar syrup for over-wintering, and a second formic
acid treatment is applied for another week (provided mite fall during the previous
treatment was more than 50-100 mites). Mite fall is assessed during both treatments to
ensure temperature conditions are sufficient for the formic acid to evaporate properly and
kill mites in the hive. In some areas, beekeepers also try to co-ordinate their formic acid
applications to improve reduction of mite populations in all colonies in the vicinity.
However, re-invasion of mites is sometimes noticed, and so once the hives have become
broodless in late autumn colonies are often treated with a 15% dilution of lactic acid
sprayed on to each frame side with a garden sprayer. Each side receives 5 ml, and
treatments are repeated until fewer than 50-100 mites fall from the last treatment. If
mite numbers are low, sometimes the late lactic acid treatment is not carried out.
Oxalic acid has so far not been approved for use in the European Union, but is
nevertheless in use in countries such as Denmark. The best recommendation is to use the
substance mixed in sugar syrup. This is trickled on the bees between the combs.
Temperatures at the time of application need to be above 0oC. Rubber gloves and goggles
should be worn. Repeated treatments can cause damage to the bees, and the presence of
sealed brood will reduce effectiveness (since the material does not kill mites in the cells).
CONTROL OF VARROA: CONTROL METHODS USED OVERSEAS
Oxalic acid in sugar syrup trickled over the top bars is now
being used in Europe as a varroa control in the autumn.
For late honey flows, the formic acid treatment is either not done or is done later in the
autumn. One treatment of lactic acid or oxalic acid is sometimes substituted for this
formic acid treatment. A repeat with lactic acid only is carried out if more than 50-100
mites fall as a result of the previous treatment.
A survey of Danish honey and beeswax showed the organic chemical treatment
programme was effective in keeping miticide residues out of bee products. No miticide
residues were present in 43 samples of Danish honey. One sample was found with a
residue of a miticide that had not been in use in Denmark for 10 years. The suggestion
was that the residue, like several wax samples showing fluvalinate contamination, came
from imported beeswax.
Read this useful document from the Ministry of Primary Industries herehttp://www.biosecurity.govt.nz/files/pests/varroa/guidelines/control-of-varroa-guide.pdf