Bee contribution to cranberry yield varies with local farm management

Cranberry (Vaccinium macrocarpon), a perennial fruit crop native to North America, is generally considered to be greatly dependent on insect pollinators.  As a result, individual cranberry growers spend thousands of dollars each year renting honey bee hives to ensure crop success.  Native bees have also been shown to be efficient pollinators of cranberry and may provide additional pollination services to the growers.  It is possible, however, that environmental factors including the surrounding landscape and local management may have a strong influence on cranberry yield and the contribution of bees to yield.  The objective of this study was to determine which factors best predict cranberry yield.  We selected 30 cranberry marshes that fell along a landscape gradient from highly wooded to highly agricultural where we pan trapped bees and measured cranberry yield in the presence and absence of bees.  At a subset of sites, we also measured the density of flowering uprights.  Cranberry yield was best predicted by local factors including honey bee hive density and cranberry floral density.  We found no correlation between landscape and yield.  Native bees were strongly influenced by the surrounding landscape but we did not find evidence of a correlation between native bee abundance or species richness and cranberry yield.  The contribution of bees to cranberry yield increased with an increasing density of flowering uprights.  This suggests that local farm management practices that increase the density of flowering uprights will also increase the effectiveness of the bees.