4.2. Subjective mode
This section and the next describe the subjective mode of measuring colony strength and are thus best suited for collecting response variables while the field experiment is in progress. The gist of the method is the use of human observers who visually estimate the surface area of a comb covered by a target; bees, brood, honey, pollen, etc., and if necessary convert comb surface to target-appropriate units, i.e. bees, cm2, or cells. The syntheses in sections 4 and 5 draw from the work of Burgett and Burikam (1985) and derivative papers from North America (Skinner et al., 2001, Delaplane et al., 2005, 2010), Imdorf et al. (1987) and Imdorf and Gerig (2001) from Europe, and Gris (2002) and Guzman-Novoa et al. (2011) from Central America.
1. Visual estimates of bees on combs will vary according to time of day and bee foraging activity. For this reason it is important to control for this effect – either by limiting observations to a narrow time window on successive days, randomly assigning time of inspection such that day effect is equitably and randomly distributed across treatments, or closing hive entrances in the early morning until bees are counted.
2. Estimates should be carried out by no fewer than two human observers, preferably each with a dedicated secretary who writes down numbers, or each fitted with an audio recorder.
3. A colony is opened and combs of bees sequentially removed. Each observer looks at one side of a comb, visually estimates the percentage of the comb surface covered by bees, and records the number with the secretary or audio recorder. It is convenient to label frames 1-X, with each side indicated A or B. For beginners it is advisable to “calibrate the individual” with estimates made by an experienced observer. Observers describe the process as a kind of mental “resorting” the bees, such that the bees are imaginatively moved into a contiguous mass on the comb surface, at which point the reader estimates the percentage surface of the comb they cover. It is important to visually sort the bees into a contiguous mass that approximates their density if the frame were fully covered because the bee densities given in Table 2 (1.23 – 1.77 bees per cm2) apply to combs at full carrying capacity.
4. Investigators can use the values in Table 2 or calculate the comb side surface area unique to their equipment. Fig. 8 is a screenshot of an Excel datasheet demonstrating the conversion of raw data from two observers into colony bee population. There are two fictional colonies, each with 5 North American deep frames, each with two sides. Columns D and E show the respective visual estimates of two observers for percentage comb surface covered by bees, and column F is the mean of the two. Column G converts the mean percentage surface covered by bees into area (cm2) covered by bees, using the surface area for one side of a North American deep frame from Table 2 (880 cm2). Column H converts cm2 bees to number of bees with the appropriate bee density (1.38 bees / cm2). Finally, rows 12 and 23 sum the bees of each frame and side to yield colony bee population.
5. If investigators use colonies with different sized
supers and frames it will be necessary to adjust calculations for the one-side
surface area unique to each comb type. Bee density at full carrying capacity is
consistent within North America or Europe, so it should be adequate to pick a
density value from Table 2 that best fits one’s local situation.