IPM 3 Fighting Varroa 3: Strategy – Understanding Varroa Population Dynamics
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I’ve sweated blood to try to summarize what we know about seasonal growth of varroa into a simple graph (Figure 2). Varroa population growth is largely dependent upon drone brood being present. Therefore, I’ve tried to make the graph universal for temperate climates by keying the growth curve to the colored brood status bars at the bottom. The only consistent date in temperate areas is that the curve usually peaks the first week of September. You can make the graph even more universal by ignoring the word “summer” and substituting “pollen/nectar flow.” Note that the mite population increases (other than from immigration from robbing) mainly during the pollen/nectar flow when bees are rearing drones; most of the rest of the year, the population is decreasing. The more intense the buildup period, the steeper the mite growth curve.
If you live in California, where we have early bloom, and a very dry summer, the curve shifts to the left. If you live in where it is chilly in spring, and have fall flows, the curve will shift to the right. If you live in a very warm area where broodrearing drops off in the summer heat, and picks up with late summer rains, your growth curve may shift far right and look more like Figure 3. “Luckily” for those of you in subtropical areas, if you don’t already have African bees, you likely will, and they control varroa by themselves, largely by grooming (Arechavaleta-Velasco & Guzman-Novoa 2001).
As a point of interest regarding length of broodrearing period, colonies in northern latitudes may experience the same degree of mite increase as colonies at lower latitudes having a much longer period of broodrearing! Korpela (in Fries 1991) states “the growth rate of varroa populations observed in south-eastern Finland is comparable to, or even exceeds, the values observed in Germany, despite the clearly shorter brood rearing period and the higher mite winter mortality suggested by this study.” This is an area that could use further study.