Solitary Ground-Nesting Bees: Opportunities and Challenges for Crop Pollination Management: Short title: Managing ground-nesting bees.

The majority of bee species worldwide are solitary and nest in soil or earthen substrate (i.e., ground-nesting). Among these, several are effective crop pollinators. Despite their abundance, diversity, and ubiquity, only one species of solitary, ground-nesting bee (Nomia melanderi) has been commercially deployed in the field for crop pollination. Compared with bees that nest aboveground in cavities, ground-nesting bees pose unique challenges to management, but offer several advantages as well. Soil is a relatively cheap and abundant nesting material, but laborious and difficult to move, and the soil preferences of many ground-nesting bee species are unknown. Additionally, farm management, including tillage or pesticide applications, when done directly to the soil, can significantly limit the successful establishment and population growth of ground-nesting bees. Despite these challenges, a number of ground-nesting bees show potential to be managed in the field or greenhouse and could be valuable supplements to the current over-reliance on a few social and/or cavity-nesting bees. In this review, we highlight past successes at managing ground-nesting bees for crop pollination and describe the bee traits that have led to success (i.e., Nomia melanderi). We also present several species with potential for further development and management on-farm or in greenhouses owing to their ability to persist in or near crop fields, their willingness to use provided nesting substrates such as soil mounds or adobe blocks, and our growing understanding of their nest preferences.