Harmful mite population threshold
There is no clear harmful threshold at which a mite population suddenly causes harm. A mite
population that causes no obvious damage to one colony may prove very damaging to another. This
can be due in part to differences in the levels and types of bee viruses and other pathogens present
in the colonies and the bees’ natural ability to tolerate varroa, as well as environmental factors.
However, in the UK researchers agree that it is wise to aim to keep the varroa population below
about 1000 mites; above this level the risk of damage from the mites, associated pathogens and
the effect of feeding on the bees can quickly become very significant. In Europe and parts of the
United States higher threshold levels of around 4000-5000 mites are generally used.
Varroa population increase
Varroa populations in infested colonies increase naturally through two processes - the
reproduction of mites in brood cells, and the influx of new mites into the colony through
invasion.
Figures 11 to 13 illustrate these two processes. To keep them simple they are based on the
assumption that mite populations double through mite reproduction approximately every four
weeks - although in reality the situation is more complicated as many factors (such as the
amount of worker and drone brood present) influence the rate of mite reproduction.
Figure 11 illustrates the increase in the mite population for colonies infested with differing
numbers of mites at the start of the season (and without any mite invasion from outside). During
the 180 days shown, mite populations build up steadily. Where only very few mites are initially
present the mite population remains well below the harmful threshold of 1000 mites for the
entire period shown. However, colonies starting with larger numbers of mites build up to harmful
levels much faster. It is essential to ensure mite populations are as low as possible at the
beginning of the active rearing season.
Figure 12 illustrates the effect of mite invasion. Where a small number of mites are present at the
start of the season and no mite invasion occurs the mite population remains below 1000 during
the whole period shown. However, mite invasion early in the season causes the mite population
to reach harmful levels much more quickly - depending on how many mites invade the colony.
Figure 13 shows the increase in the mite population that occurs following treatment (for example
with a varroacide). The fall in the mite population when treatment is applied depends on the
treatment’s efficacy. The mite population takes much longer to return to a harmful level when a
very effective treatment is applied than when less effective treatments are used. Understanding
these principles is essential for you to successfully use the principles of varroa monitoring and
control described later in this leaflet.
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