While there has been a great deal of attention placed on the phenomenon of colony collapse disorder, the bees that are affected by CCD are commercial honeybees, with known populations that have numbers that can be easily quantified. However, the pollination provided by wild bees is also important to the growth cycle of crops, and supplements the job done by commercial honeybees. But as their hives aren’t monitored by beekeepers, a loss in their numbers aren’t as immediately noticed.

A new study conducted by the University of Vermont has mapped out the relative abundance of wild bees across the U.S., and has found that their numbers are thinner in regions where their services are most needed. Overall, wild bee populations had declined 23 percent from 2008 to 2013. Areas affected with low populations include the Upper Great Plains, the lower Mississippi River, parts of northwest Texas, California, Arizona and Washington.

The study finds that the areas that are hardest hit tend to be agricultural regions where only a single type of crop is favored, such almond cultivation in California’s Central Valley.

“I think the reason why the Central Valley lights up so much is largely almonds," says study co-author Taylor Ricketts. "It’s a hugely valuable crop that’s been expanding like crazy, and it’s entirely dependent on pollinators.”

This study was conducted in response to a call by the White House to address the decline of bee populations across the country, and a USDA initiative for growers to diversify their pollinator populations. Data on regional bee populations is key to achieving some of the goals set out by these initiatives, such as adding additional habitats for pollinators.