Is more better?

Dr. Margriet Dogterom

How many bees is enough?

How many bees does it take to pollinate a crop so that yield is high?

We are speaking about the pollination dilemma that growers of berry crops face every year. If a grower wants good pollination he wants good fruit set and large fruit. More yield usually means more returns. But how does a grower ensure good pollination of his crop if weather is such an integral part of pollination, fruit set, and fruit development and cannot be controlled. And is small fruit size necessarily an indicator of poor pollination? In 1999 the Blueberry Council of BC asked the question if small fruit size production in the Matsqui area over the previous four years was caused by poor pollination. It is a simple, albeit time consuming, task to examine the pollen loads on the female stigmas of blueberry flowers. My earlier findings indicate that fruit size does not increase if more than 125 pollen grains are found on the female part of the flower -a stigma. Thus, 125 pollen grains can be used as a quantitative measure of good pollination (Dogterom and Winston, American Journal of Botany,1999). Nine plots were randomly chosen from 42, 2-3 acre plots of 'Bluecrop' owned or managed by four growers, and located in Abbotsford and the Matsqui Region. Flowers (539) were collected and pollen grains counted. Pollen loading of flower stigmas was well below what is required for maximal fruit size. In all fields, only 5-15 % of flowers collected had adequate pollen loading of 125 pollen grains or more. In other words, 80-95% of flowers were inadequately pollinated for fruit set and fruit size. Thus, in 1999 pollen deposition was inadequate in the Matsqui/ Abbotsford Region, and likely resulted in a significant loss in yield. The next obvious question is whether the 125 pollen loading level can be reached at all. Pollen deposition depends on bees visiting flowers, but is also affected by flower density, percent bloom, honey bee density, competing bloom, and weather conditions. Not all factors can be controlled. However, honey bee density can be increased by increasing colony number, increasing colony size, and modifying colony placement within the field. If fields were chosen with a range of honey bee colony densities, would any of these fields have adequate pollination levels? In 2000, we examined pollen loading of flowers (360) adjacent to high and low number of honey bee colonies. We examined fields in Matsqui, Richmond, Delta and Coquitlam of the Fraser Valley Region.

Results pointed to the fact that overall pollination is inadequate in the majority of fields tested. In only one field the level of 125 pollen grains was reached in all flowers collected in both early and late bloom. Only 3 fields had more than 50% adequately pollinated flowers at early and late bloom. In six out of 24 samples pollination was less than 15%. Thus, overall pollination remains inadequate although growers believe their stocking rates with honey bee colonies are adequate.

In summary, we found that adequate pollination levels can be reached in fields of highbush blueberry, although overall, fields are inadequately pollinated. Only two of the fields sampled reached the desired goal of adequate pollination. It is recommended that growers increase their colony density until adequate pollen loading is reached and that fruit size will improve in size. Our results suggest that adequate pollination can be reached with an average colony density of 3 colonies per acre when honey bee colony consist of 2 boxes of bees, brood and stores.

Acknowledgements: blueberry growers, Amy Mukai, Alice Miro, Andrea Lonon and Doug Talling, BC Blueberry Council Investment Agriculture Foundation Program, Agriculture and Agri-Food Canada and Canadian Adaptation and Rural Development Fund.

 

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