A stack of mite boards coated with salad oil and with screens held on by elastics. Tabs made of tape identify the hive tested. We test about 10% of the hives at random, and only test further if we see problems. Click to enlarge.
Take a test. How many adult varroa do you see in this picture? Click to enlarge. Write me with your answer, if you like.
We picked up the varroa mite drop boards this morning and looked them over. The first batch of five sticky boards showed 10, 6, 2, 0 & 2 mites over three days of natural mite drop, and the second batch showed 0, 0, 1, 1 & 3. The third showed 0, 0, 1, 1, & 4. The boards had been in for three days and, therefore, each result should be divided by three then multiplied by 100 (the estimated average varroa lifespan ) to estimate total mite loads. Thus the worst hive could be assumed to have a 333 mite load. That is nowhere near serious. 350 mites as a total load, is very light, in fact. A visit to the varroa calculator gives a different perspective, but it also assume drone brood, which we no longer have at this time of year.
Counting is always difficult when there are so few mites, and we are always tempted to count the immatures -- the occasional mites we see in the board that were almost fully developed in a cell when the host bee emerged, but which die immediately of exposure, when deprived of the special conditions inside a cell. Such mites have the shape, and often the size of mature mites, but are pale and translucent. We know they did not live even one day after emerging, and were never part of the adult, reproducing, population. Since their lifespan is zero days, the 100 day estimated average lifespan multiplier does not apply, and I figure immatures should not be counted.
By this time of year, there should be very little brood in which mites can hide, so most varroa should be phoretic at this point. When the mites are phoretic -- on bees, and not hidden in brood -- they are at their most vulnerable, and have the highest mortality rates, so, even using a multiplier of 100, which could be high, we are seeing very low infestation rates. Over winter, the mites will be under even greater pressure, as they occasionally fall off bees and are exposed to the cold conditions at the hive floor, unless they are able to grab back onto another bee.
For some reason, varroa is not giving us much trouble. For the past several years, the only treatment we have used is a single Apistan™ strip placed in the centre of the cluster in the early spring and left for 42 days. Our tests always show very low levels of mites, much lower than when we used two strips in the fall plus several formic treatments. Granted, we had a dry year in 2002, and we spilt heavily this year, and both these factors tend to reduce varroa loads, but, nonetheless, we did not split all hives, and when everything is considered we still are seeing lower levels that we would expect and lower levels than we saw in the past.
The drop boards we use are just Coroplast™ sheets cut to roughly the size of a sheet of foundation, with a tab of Duct tape added, and with a piece of 6 mesh hardware cloth sitting on top to keep bees out. The screen has been bent to be a bit dish-shaped so that it sits up above the board about 1/4" so the bees will not contact the salad oil/Vaseline™ mix which is smeared onto each board before it is placed under the hive, to catch and drown falling mites.
To insert the boards, we just hold the hive floor down with one foot, if necessary, and tip the hive back a bit. It does not matter if some frames touch the screen when the hive is lowered. As for catching all the mites, we centre the boards under the cluster and figure if we miss one of two mites, that this will not grossly affect our estimate. This is not rocket science. We're just looking for a rough indication whether to expect trouble, or not worry for another six months.
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