2.2.3. Testing for biotic (honey bee) pollination – multiple visits
Although single-visit fruit-set is a standardized measure of pollination efficiency and independent of pollinator foraging density (Spears, 1983; Sampson and Cane, 2000; Dedej and Delaplane, 2003) in flowers bearing many ovules (ex. apple, pear, melon, pumpkin and kiwi) a single honey bee visit is usually not enough to deposit all the pollen grains needed to set the fruit or to fertilize most of its ovules.
- Choose a given number of flower buds prior to anthesis.
- Protect these buds with pollination bags (section 2.1.1.) and identify with weather-resistant tags.
- Randomly designate each flower as a recipient of 1, 2, 3, or 4 (or more depending on plant species) honey bee visits, remove bags after flowers open, and observe each flower for its assigned number of flower visits.
- After the assigned number of flower visits is achieved, rebag the flower until it is no longer receptive, after which the bag is removed.
- A few days later, check whether fruit has developed from the visited flowers and compare fruit-setting results with those from bagged and hand-self, geitonogamy, cross pollination, and open control treatments to know the importance of multiple honey bee flower visits to that particular plant species. The treatment which produces the closest fruit- or seed-set to the best hand-pollinated (or open-pollinated) treatments determines how many honey bee visits are necessary to set acceptable yields.
These can be tiring and time consuming experiments because although one can have many marked flowers within one’s visual field, it is usually not possible to observe all flowers at the same time, and bees may take a long time to visit those particular unbagged flowers, especially when there are other flowers around. Some investigators get around this problem by offering freshly-cut female flowers on long extender poles to bees visiting nearby flowers in the patch (Thomson, 1981; Pérez-Balam et al., 2012). This method takes some skill to avoid disturbing the natural foraging behaviour of bees and is obviously only good for destructive measures such as pollen deposition on stigmas (see section 3.2.), but it can greatly speed acquisition of data. In any case, observations should be done at roughly the same period of each day to avoid diurnal variations in flower receptivity. Also, one must not allow a different flower visitor to land on the flower while waiting for specifically honey bee visits; otherwise that flower must be discarded and all work invested on it is lost.
An alternative approach is to use a video camera that follows groups of flowers as they open. A quantitative analysis of the recording will reveal relationships between the number of visits each flower receives and its subsequent seed set. Because flowers are not enclosed, buildup of pollen and nectar reflects natural rates. As the relationship between number of bee visits and seed set can only be determined if there is less than full set (once seed set is maximized additional visits are superfluous), it may be necessary to bag flowers (section 2.1.1.) after they have been videoed for an appropriate length of time to prevent full pollination. This method has the advantage that the number of visits required for full pollination can be measured directly rather than estimated as it may be when just measuring the effect of single bee visits.