Beekeeping Basics
Screened Bottom Boards, Honey Bees, and Mites
The cost of a screened bottom board is not that different from that of an assembled and painted solid one, though the Ultimate Screened Bottom Board1 (well worth the money) is a bit more, so the real question is, which one is better for my bees, and will it help keep mite numbers low? We hear both sides of the story, that screened bottom boards help reduce mite numbers, and also that they don’t really work so we shouldn’t bother with them — so which is it? Let’s look at some facts, not just what makes sense to us, but at the research that tells the real story.
Screened bottom boards are supposed to keep mites that fall off of bees from being able to get back into the adult bee population, by either letting the mites fall through to the ground, or holding them on a sticky board. Still other people tell us that the mites that fall are dead already, so what’s the big deal? One study2 found that 39-50% of fallen mites were still alive, so keeping that many mites from recovering and reproducing again could be valuable.
Some surprises regarding the efficacy of screened bottom boards
Many studies used only a small number of test colonies, or didn’t take into account environmental factors. One study3 that used a lot of colonies — 184 in the first test, and 234 in the second — learned not only about the efficacy of bottom boards, but also how the local environment affected it. In their first test in 2000, they found that when screened bottom boards were open to the ground, mite loads were much worse in drier and windier areas, with an average of 29.2% more varroa than control colonies with solid bottom boards; some colonies had 119% more mites! Areas that were in full sun and protected from wind had 66% fewer mites in colonies with screened bottoms, even when open to the ground.
In 2001, the same scientists tested again, but this time with a “mite drawer” (like a sticky board, but closing in the air gap more effectively). This raised the relative humidity and temperature of the broodnest. Mite proliferation was reduced by 37% on average in these tests. The scientists concluded that not only was a lower temperature more conducive for foundress mites laying eggs, but also that since it takes longer for bees to pupate at lower temperatures, even more reproductively mature mites emerge with the bees.
Effects of temperature and humidity on mite reproduction
This theory, that warmer temperatures are detrimental to mites, has been explored. We can look to bees that suffer less from mite depredation than our European honey bees: Africanized bees in the tropical climate of South America, and Apis cerana, the Asian bee that hosted Varroa destructor before it came to our shores. LeConte et al.4 found that Apis cerana keep broodnests warmer, at 99.5 to 101.3 degrees Fahrenheit, while, in the same apiary, Apis mellifera kept their broodnests at a much cooler 93.2 to 96.8 F. At 101.3 degrees, 100% mortality of female mites was recorded, while no bee losses occurred.
Obviously, screened bottom boards that are open to the ground will lead to cooler broodnest temperatures …
