A. Mites in brood
All the methods except the visual inspection of brood only provide information about the number of mites on adult bees. When a hive is in full brood production, it is estimated that only about 15% of mites are on adult bees. Thus, the number of mites on adult bees in the sample has to be multiplied by a correction factor of 6 to estimate the likely total number of mites in the hive. At other times during the production season when brood is present, use a correction factor of three. When no brood is present, no correction factor is needed.
B. Jars of bees
When using an adult bee survey technique that doesn’t involve a miticide and samples only a portion of the bees (i.e., ether roll, sugar shake, soapy water wash), a rule of thumb would be to divide the number of bees in the hive (15,000 in a full Langstroth super) by an estimate of the number of bees in the sample.
This will give a figure that can be multiplied by the number of mites in the sample and a mites-in-brood multiplier to determine the likely number of mites in the hive.
As an example, if the hive has one full box of bees (15,000) and the sample has 300 bees:
15,000/300 = 50
If the number of mites in the sample is 2 and the hive is in full brood production (see A. above), then the total number of mites in the hive is:
50 x 2 x 6 = 600
C. Brood sampling
To estimate the number of mites in a colony from a sample of brood, British researchers recommend different multipliers for drone brood (10) and worker brood (1.8). To begin, a percentage of infestation is determined by dividing the number of cells found to be infested by the total number of cells examined. Then an estimate is made of the total amount of sealed brood. Use a figure of about 1000 cells for one side of a good (60% covered) frame of capped brood. This is then multiplied by the correction factor to determine the total mite population in the hive. The estimate should only be made in the summer.
D. Natural mite fall
According to British researchers, the daily natural mite fall on a screened bottom board with a sticky board in winter can be multiplied by 400 to get the total number of mites in the hive, while in the summer the multiplication number is 30. In the early spring when brood is expanding rapidly, and in the autumn when brood amount decreases, they say mite fall is unreliable but estimate a multiplication factor of 100. All of these
multiplication factors include mites on brood. estimated bees in hive
bees in sample x mites in sample x brood multiplier = mites in hive
Danish researchers, on the other hand, suggest multiplying daily mite fall by 120 to give total varroa in a colony during the production season.
E. Whole hive sampling
If Apistan, Bayvarol or formic acid is used, assume 85% of the mites on adult bees were killed during a 24 hour survey. So divide the total number of mites on the board by 0.85 to get the total number of mites on adult bees in the hive. Also multiply the total number by 6 if there is substantial brood in order to determine the total number of mites in the hive (see A. above).
F. Counting mites on sticky boards
Since varroa mites are small, a magnifying glass is recommended for mite counting. Before beginning to count boards, it is also important to do a quick refresher on the size and shape of varroa, and how it compares with the melittiphis mite. Retaining a sample of both mites (e.g., laminated on a card) can be especially useful.
The method used to count mite numbers on sticky boards depends on the reason the mites are being counted and the number of mites on the board. In most cases all that is necessary is to estimate the number of mites. This can be done by just making a quick count of a measured area and then multiplying it by the total area of the board. However, if a more accurate count is needed, two different methods can be used:
• Low mite numbers – When mite numbers are low, it can be difficult to scan a board
to determine if there is a varroa on it without missing some areas. To make the count more uniform, draw parallel lines 2.5 cm apart over a sheet of clear Perspex that has been cut to the same size as the sticky board. The Perspex can then be placed over the board with each pair of lines used as a guide to ensure the whole board is assessed.
• High mite numbers – High mite numbers can also be difficult to count. The best
way to proceed is to also use a sheet of Perspex. Draw a 2.5 cm x 2.5 cm grid over the sheet. Randomly select 25% of the squares and mark their boundaries with another colour, or erase the lines marking the squares that are not selected. The number of mites in the selected squares is then counted and multiplied by 4 to estimate the total number of mites on the board. If the distribution of mites does not appear to be reasonably even over most of the board, use the method for low mite numbers above and count all the mites.
CONTROL OF VARROA: APPENDIX 1. ESTIMATING MITE POPULATIONS IN HIVES