Bees don't always work for free. Many crops rely on pollination by honey bees and, as a result, there’s a market for the services of professional beekeepers and their bees. And the cost of those services has been on the rise. What’s driven the increase in pollination fees over the past 20 years? A new study from North Carolina State University shows that honey prices, invasive mites and the cost of diesel fuel are key factors.