Biotic and non-biotic factors contribute to cranberry pollination

Background/Question/Methods

Two-thirds of crop plants require or benefit from insect pollination.  As managed and native bee populations continue to decline, farmers face possible crop failures due to insufficient pollination.  Crops, however, vary in the degree to which they depend on pollinators, suggesting that some crops may not be as susceptible to pollinator decline as others.  Cranberry (Vaccinium macrocarpon) is generally considered to be dependent on insect pollinators, although dependency estimates range from 30-100%.  Several studies suggest that other, non-biotic factors may be important for cranberry pollination, but these have generally been ignored by the literature.  Therefore, it is unclear to what extent individual biotic and non-biotic factors contribute to cranberry pollination.  The objective of this study was to assess to what extent biotic (i.e., bees) and non-biotic (i.e., wind, agitation) factors contribute to cranberry pollination.  We did field and greenhouse experiments to compare pollination success under biotic and non-biotic treatments.  In the greenhouse, plants were hand pollinated, manually agitated, or left undisturbed.  In the field, plants were left open to bee pollination (“open”), caged to exclude insects but allow wind (“wind”), caged to exclude insects and wind (“closed”), or caged to exclude insects and wind but manually agitated during bloom (“agitation”).   

Results/Conclusions

We found that in the absence of bees, cranberry plants produced viable fruit in both field and greenhouse conditions.  In the greenhouse, yield estimates were highest in the hand-pollinated treatment, followed by the manual-agitation treatment and the undisturbed treatment.  The number of berries per upright followed the same pattern as yield, but the weight per berry was the same across all treatments.  In the field, yield estimates were highest in the “open” treatment, followed by equal yields for the “wind” and “agitation” treatments, and a lower yield for the “closed” treatment.  The number of berries per upright did not vary among “open”, “wind”, and “closed” treatments, but was lower in the “agitation” treatment, possibly due to damaged flowers caused by the agitation.  The weight per berry was greatest for “open”, followed by “agitation”, “wind”, and “closed”.  The difference between biotic and non-biotic treatments was much smaller in the field conditions than in the greenhouse, suggesting that additional factors such as plant density may play an important role in pollination.  These results demonstrate that both biotic and non-biotic factors contribute significantly to cranberry pollination suggesting that cranberry is not as dependent on pollinators as previously thought.