Native bees are great pollinators on their own- they have been shown to be more efficient (on a per bee basis) for a variety of crops globally (there is a wonderful paper in the journal Science by L. Garibaldi et al. describing this). Studies are also showing that native bees help make honey bees better pollinators.
The first to examine this phenomena was a paper from my PI Claire Kremen and her former graduate student Sara Greenleaf. They found that behavioral interactions between native bees and honey bees in sunflower increased honey bee efficiency by 6 times. This was because of two major reasons. Typically multiple honey bees will forage on a single sunflower. They can bump into one another and still stay there working their way around the blooms on a head. However, when a native bee and honey bee meet on while pollinating sunflower, they both flew to a new sunflower. This helps move more pollen between different sunflower individuals, which is required for cross-pollination. The second way honey bees increase between flower movement is when they are scared by male native bees who accidentally try to mate with them while zipping up and down rows looking for females of their own species (Greenleaf and Kremen, 2006, PNAS).
A more recent study has found a similar interaction between native bees and honey bees in almond, the most economically important crop in California. Native bees spur honey bees to move between rows of the orchard crop- increasing the amount of pollen from different trees honey bees carry on their bodies. When the honey bees later deposit this pollen on the stigmas, having more pollen from different individuals increases the chances of pollen tube formation (a grain of pollen extending down and fertilizing the ovary- which results in fruit set). Brittain et al. (the authors of this 2013 study published in Proc. R. B) found that this increased fruit set, which translated into more almonds in farms with higher numbers of native bees present. The mechanism causing honey bees to move more in almonds is still unknown- the authors suggest it could be because native bees deplete pollen at flowers (they tend to forage at earlier times of day than honey bees), forcing them to move larger distances, or they could be leaving scent traces on flowers that honey bees avoid. Nevertheless, the positive outcome for farmers remains
These studies makes me wonder if these indirect interactions are going on in every crop, even those that haven’t yet been studied. It seems like our hard-working native bees are doing their part to help honey bees and, in turn, boost yields. Thanks team native bee!