"As honeybees become more scarce, it becomes more important to have better pollinators," said Sarah Greenleaf, a postdoctoral researcher at UC Davis and first author on the study.

As a graduate student at Princeton University, Greenleaf carried out a two-year study of honeybees used to pollinate sunflower crops on farms in Yolo County, Calif., near UC Davis.

Compared to honeybees, wild bees did not contribute much directly to crop pollination. But on farms where wild bees were abundant, honeybees were much more effective in pollinating flowers and generating seeds, Greenleaf found.

There appear to be two reasons for that. Male wild bees, probably looking for mates, will latch onto worker honeybees, which are sterile females, causing them to move from one flower to another. Secondly, female wild bees appear to "dive bomb" honeybees, forcing them to move. Frequent movement between flowers spreads pollen around more effectively.

Greenleaf and her co-author Claire Kremen, now a professor at UC Berkeley, calculated that wild bees contributed about $10 million of value to the $26-million sunflower industry alone.

All the fields in the study were conventionally farmed, but varied in their proximity to natural habitat, Greenleaf said.

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