The effects of agricultural practices on native bee community structure and highbush blueberry crop production

Insect pollinators are essential for facilitating cross pollination and reproduction in many crops and wild plants. Both managed honeybees and wild bees provide great monetary value through their role in the production of food crops via cultivated plants. These pollinators are threatened globally, with populations diminishing as natural habitats are destroyed and agricultural intensification increases. Demands for insect pollinated crops continue to rise, and with honeybee colonies continually on the decline due to Colony Collapse Disorder, exploring the factors affecting native bee communities is essential for ensuring sustainable pollination services in the future. Here, I studied the effects of farming practices (conventional vs. organic) on native bee communities and crop yield by collecting native bee and blueberry samples from both conventional and organic 'Duke' variety blueberry farms in lowland NW Washington State. I sampled bee communities using pan traps and netting at nine study sites (five conventional, four organic), from early May to mid-June, 2012. During this same period, I collected random clusters of berries at each farm site to compare yields between farm types, and to assess potential correlations between berry production and bee community metrics. I also performed a pollinator exclusion experiment at one of the four organic sites, to determine the importance of pollination services to berry production. Farming practices had no effect on indices of native bee diversity and richness, but native bee abundance was significantly higher on organic farms compared to conventional farms. Furthermore, farming practices influenced native bee community structure, with a suite of bumblebee species being more common in organic fields. Combining the results of this study with published surveys of bees in various agricultural and natural habitats in the Pacific Northwest, it appears that in general, bee communities on agricultural lands in this region exhibit relatively few native bee individuals and low species richness. Perhaps for this reason, I found that crop yield did not differ between farm types, nor was berry production correlated with measures of bee abundance and diversity, despite the fact that the pollinator exclusion experiments showed that 'Duke' variety blueberries are highly pollinator dependent. Specifically, berries exposed to pollinators had significantly larger mass, diameter, and seed counts, and lower abortion rates. Overall, the results of this study indicate that organic farming is favorable for native bee populations in general, and bumblebees in particular, but that neither conventional nor organic farms support many native bees. Future studies should focus on determining if aspects of organic farming that enhance bee populations can be adopted on conventional farms, as well as determining management strategies to improve the condition of native bee communities across agricultural landscapes.