Is it necessary to sample all beehives for mites?
Answer: Actually no, systematic sampling is just as effective.
Every 10 years—including 2010—the United States Constitution mandates that the federal government census its citizenry. In doing so, they are obligated to count every man, woman, and child in the country. These are important numbers, of course, since they determine voting districts, funding priorities, and service provisions. Every decade, however, there is always a certain percentage of people who fail to be counted, either because they refuse or because it is difficult to find them. To make the official count more accurate, census officials are able to estimate the actual number from the partial estimate obtained.
Surveys and sampling are also very important to the beekeeper. Take varroa mites, for example. We all know of its importance for bee health, and most are aware of the need for regular monitoring to estimate the number of mites in a colony (lest those numbers get too high and the bees are negatively impacted). Many beekeepers use the 'sticky board', a white cardboard with adhesive on the surface to trap varroa mites that fall onto it. When slid beneath the nest on the bottom board, it collects dead mites over time. Those mites can be counted to get an estimate of how many are in the entire colony.
But if you've ever counted a sticky board with a lot of mites on it, or have dozens and dozens of boards to count at once, the monotony and tedium make you start to wonder if there's a better way. Here's where sampling can help. Studies have shown that if you count every third square on the board, then multiply that number by three, on average it will be just as accurate as if you counted the number of mites on the entire board. That can save time as well as your eyesight!
A recent study out of the University of Minnesota took this sample approach to sampling to the next level. At issue is whether or not we beekeepers need to sample each and every hive (after all, each colony is bound to have different numbers of mites) or if sampling among hives is just as effective. Their findings show the latter: beekeepers can take some "short cuts" in measuring mite prevalence in their hives. For within hive estimates, beekeepers can take a single sample of 300 bees, dislodge the mites from the bees (either with powdered sugar or ethanol, depending whether or not you wish to save the bees or have a more accurate count), the double the count to calculate the percentage of infested adults and brood within the colony. On average, this was just as accurate as taking three separate samples from the same colony from different areas of the brood nest (although the variance was lower than that for a single sample). For among hive estimates sampling whole apiaries, one need only estimate mite density in eight colonies—regardless of how many hives are in the apiary—in order to get an accurate picture of the average prevalence of mites in the whole yard. This can be a big time saver, particularly for commercial beekeepers or those who have a large number of hives (the study population of the investigation).
In the end, however, it comes down to regularly and systematically measuring mite levels. Only by staying on top of these pesky parasites will we be able to sustain colonies in their presence. At least we don't have to go knocking on doors or send out mailers to do so.
Lee, K. V., R. D. Moon, E. C. Burkness, W. D. Hutchison, and M. Spivak. (2010). Practical sampling plans for Varroa destructor (Acari: Varroidae) in Apis mellifera (Hymenoptera: Apidae) colonies and apiaries. Journal of Economic Entomology, 103: 1039-1050.