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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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