5.2. Direct tests of whole fields to find the required number of colonies per hectare
A less common approach is to stock fields with different numbers of colonies and establish whether the rates used have an effect on pollination (Palmer-Jones and Clinch, 1974; Vaissière, 1991; Brault et al., 1995). Direct comparisons of different colony densities are rare because it is difficult to obtain acceptably large numbers of replicate fields for each treatment. Past research usually involved a few fields (e.g. Eischen and Underwood, 1991), or a few fields repeated over a few years with treatments rotated among fields (e.g. Stern et al., 2004).
If multiple fields are available for testing, they should be:
- As similar as possible regarding cultural practices (e.g., irrigation, drainage, fertilization, pest and weed control), available pollinisers, soil type and surrounding habitat
- Far enough apart (ideally
>3 km) to isolate bee populations
If multiple fields are available, similar fields should be paired and honey bee colonies introduced into half of the fields while the other fields serve as controls without supplemental bees. A recent recommendation (Vassière et al., 2011) is to use ≥5 fields per treatment, with bees introduced at the onset of effective flowering (i.e., at the time of first bloom that would lead to a product).
Considerations when collecting data about pollination outcome include the following:
- Use units of yield per field, plot, plant or flower as appropriate for the crop. Yield may include fruit and seed quantity and quality (see section 6).
- Alternatively, use pollen deposition (see section 3.2.), or fruit- or seed-set (see section 2). It is useful to measure pollination outcomes prior to harvest to prevent losing fruit to events (e.g. natural herbivory, violent weather) that can confound treatment effects. Note, however, that pre-harvest measures of immature fruits do not reflect outcomes typical of agricultural commerce.
- It is advisable to estimate realized densities of bee foragers in fields resulting from the different numbers of colonies (see section 5.1.).