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Measuring Mite Populations
The total number of varroa mites in a colony of bees is determined by estimating the number of mites on adult bees and the number of adult mites within the capped brood cells. Our procedures for these measurements are as follows:
Mites on adult bees
Weigh the entire hive, equipment and bees. We screen the colony's entrance during the night before weighing so that bees cannot leave during weighing.
Weigh the hive equipment without the bees. We brush all of the bees from the hive body and combs into an empty box before re-weighing the empty hive equipment. Do not shake the combs if they are to be examined for mite reproduction because immature progeny mites can be killed in a brood cell when the pupa is rattled against them.
The difference in these two weights is the weight of the adult bees in the colony.
Scoop ca. 1,000 bees from the box (after mixing) and put them into a pre-weighed jar. The jar is re-weighed, and the difference gives the weight of bees within the sample.
Wash each sample of bees with 75% ethanol (Figure 1). The bees are washed over a sieve that allows varroa mites to filter through the mesh while retaining the adult bees. We rinse each sample until we get two consecutive washes that contain no mites. This procedure gives us an estimate of the number of mites per gram of bees. For example, if 30 mites were counted from 150 grams of bees, the estimate is 0.2 mites per gram of bees.
Figure 1. A sample of adult bees is washed from each colony to determine the mite load on the adult bee population. The sample of bees is shaken in a jar of alcohol and poured over a wire mesh. The wire screen retains the bees while dislodged mites flow to the pan below.
The total number of mites found on all adults bees is found by multiplying the total weight of bees (3) by the mites per gram estimate (5). For example, a colony with 3,000 grams of bees containing 0.2 mites per gram would have a total of 600 mites on all adult bees.
Mites in capped brood cells
Estimate the total area of capped worker brood cells in the colony. We use a 1 x 1 inch wire grid placed over the brood comb to estimate the total square inches of brood for each side of the comb (Figure 2). We only measure worker brood because we do not allow drone brood within our test colonies.
Convert square inches of capped brood into number of cells of capped brood. There are 23.6 worker-sized brood cells per square inch of capped brood. Multiply the total brood area by 23.6 to convert the area to number of brood cells. For example, 185 square inches of worker brood equals 4,366 cells.
Estimate infestation rate of capped brood. We select two brood combs from each colony to estimate the number of mites per 100 capped cells. We choose one comb containing young capped brood (prepupae and white-eyed pupae) and the second comb containing older brood (purple-eyed and tan pupae). We open 50 brood cells from each side of a brood comb and count the number of varroa mites within each cell (Figure 3). Brood cells are opened along a straight horizontal line that bisects the brood patch along its middle. Only foundress (or adult females that entered the cell) mites (and not progeny) are counted in this estimate. If 56 foundress mites are found in a total of 200 cells, we report 28 mites per 100 brood cells or 0.28 mites per brood cell.
Total mites in the capped brood is found by multiplying the total number of brood cells (2) by the infestation rate (3). For our current example, the total mites in the capped brood cells is (4,366 cells) x (0.28 mites per cell), or 1222 mites.
Total mites = (mites on bees) + (mites in brood)
= 600 + 1222 = 1822 mites
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