Masters Thesis

The role of native bees on organic farms in Humboldt County, California

A growing body of literature supports the position that both natural habitat surrounding farms and high flowering crop diversity helps insure farms against pollination shortfalls and an overreliance on European honeybees. I assessed the importance of native pollinators for the ostensibly bee-rich farm habitat of Humboldt County, California (USA). I also evaluated the management potential of an indigenous bee, Osmia lignaria, for orchard crop pollination. I found native bees were less diverse and abundant than expected based on predictions from surveys in other regions on similar crops. The importance of native bees compared to honeybees was especially low on spring crops, and became a little more balanced on summer crops. However, the only two crops where native bees were the main pollinators, squash and tomato, were visited almost exclusively by a single species of bee, and therefore still exhibited an extremely low diversity of pollinators. Studies of O. lignaria showed promising management potential on coastal farms, where their flight season overlapped strongly with orchard bloom and females regularly collected apple pollen. Overall, my study demonstrates that crop pollination may still depend disproportionately on the non-native honeybee, even in areas with high semi-natural habitat in close proximity to farms. Furthermore, the variable importance of native bees across crops and time of season demonstrates that full-season multi-crop studies should be conducted when assessing the role of native bee communities within a region. To safeguard against potential declines in honeybee populations, nesting structures should be deployed on farms to manage local populations of native bee species.

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