I recently received some queries related to mites. Here’s an excerpt from the VI Beekeepers Facebook group, where someone asked:
Answers to this question varied quite considerably. Of course, we all know what happens when you ask a two beekeepers an opinion… they provide you with at least 5! Some echoed Marc’s sentiment in their own colonies:
There were also some speculative answers:
I doubt all beekeepers collectively changed their habits and suddenly we can just happily be beekeepers again, rather than also being mite keepers. But stranger things have happened – we’ve all had to adapt to COVID for almost two years now! And it’s true, robbing is a way that supports the spread of disease and pests, but that doesn’t explain the low mite counts, Roblyn.
However, here is the more truthful assessment of why we may be experiencing low mite counts:
Yes, the heat! Do you remember that?! Geez… my solar wax melter was almost on fire, producing golden bars like a factory! The blackberry honey that I left in over that period registered a 13% moisture content on the refractometer – almost taffy-like! In case you missed it, here’s what this year’s (2021) daily temperatures looked like compared to an average of the last five years (2016-2020) (reported by NAV Canada at the Victoria International Airport/YYJ):
The gray zone indicates the average range between 2016-2020, with the red line showing the maximum daytime temperature for 2021 during May-Jul. We experienced some warm days – close to 30°C – at the beginning of June. But this doesn’t even compare with the period between June 26-28 when temperatures reached a maximum of 39.4°C. Why are these 35+°C temperatures important? Consider the following excerpt from an online article:
…studies have shown that the optimum temperature for the development of mites lies between 32.5 and 33.4°C (90.5-92.1°F). The reproductive capability of female mites significantly decreases at temperatures above 36.5°C (98°F), and above 38°C (100.4°F) mites die without engaging in reproduction (Le Conte et al. 1990).V Bičík, J Vagera & H Sádovská (2016), The effectiveness of thermotherapy in the elimination of Varroa destructor, Acta Musei Silesiae Scientiae Naturales, 65(3), 263-269, DOI: 10.1515/cszma-2016-0032
1. For a large group of colonies and over the course of several years, it was found that heating of the brood combs is an equally successful alternative to chemical treatment. Fifty colonies treated in this way thrived for three years without chemical treatment. Colonies treated with thermotherapy not only produced high-quality honey, but were also vital and capable of generating divides.
2. Bee brood can withstand higher temperatures than mites, and that heating kills the mites, but not bee larvae.
3. It is possible to effectively heat the sealed brood.
4. Mites do not survive at a temperature of 45°C, which represents a rise in the temperature of the brood nest by a mere 10°C above normal.
5. Ideal temperatures for the development of the mite are below 35°C, and that even the upper limit of the normal temperature range for the brood is not suitable for the parasites.
6. Even colonies completely cured in summer can be reinfested in late summer due to reinvasion.
The takeaway is provided in the final two points, showing that 35°C is already problematic for mites and reduces reproduction capabilities. Yay! But beware – reinfestation will occur, so you still have to be vigilant!