2.2.2. Testing for biotic (honey bee) pollination – single visits

With this test, one will be able to check the role of biotic pollinators, in our case the honey bee, in fruit- or seed-setting of a particular plant species. In nature, fruit-set usually happens after repeated flower visits by one or more species of pollinator, but when evaluating different candidate pollinators, it is best to compare fruit-set on the basis of single flower visits; this is the most equitable way to compare innate pollen vectoring capacity among flower visitors. The investigator will bag unopened flowers, un-bag them after they open, observe a single visitor, re-bag the flower, then follow the flower’s development for subsequent fruit or seed. The flower now has a history, and the efficacy of the specific agent can be compared with others (Vaissière et al. 1996). It is good practice to have a second flower open at the same time which can be rebagged without being visited to act as a control for bag effects as well as a set of non-manipulated and labelled flowers as open-pollinated controls. Depending on the flower species, the standing stock of nectar or pollen may build up in the bagged flower to the extent that it may influence behaviour of bees visiting newly exposed flowers. To check whether this is affecting forager behaviour, the behaviour of bees visiting previously bagged flowers can be compared to visitors to flowers that have not been bagged.

  1. Choose a number of flower buds prior to anthesis.
  2. Protect these buds with pollination bags (section 2.1.1.) and identify with weather-resistant tags.
  3. After the flower opens remove the bags and watch for the first visit of a honey bee. Rebag the flower after the bee leaves it. The bag should remain on the flower while it is still receptive to avoid undesired visits and should be removed afterwards. Limit observations to the same time each day and to weather conditions that are suitable for insect flight.
  4. The following measures may be taken at the time of bee observation and retained for possible use as explanatory covariates: length (sec) of visit, whether the bee is collecting nectar or pollen, ambient temperature, wind speed, and relative humidity.
  5. At harvest, check whether fruit has developed from the visited flowers and compare fruit-setting results with those from bagged controls, hand-selfing, geitonogamy, cross pollination, and open-pollinated treatments to know the contribution of a single honey bee visit to the pollination needs of that species.
  6. A modification of this method employs a direct measure of Pollinator Effectiveness after Spears (1983):
    where Pi = mean number of seeds set per flower resulting from a single visit from pollinator i, Z = mean number of seeds set per flower receiving no visitation, and U = mean number of seeds set per flower resulting from unlimited visitation.