At the Xerces Society’s page on Building a Better Mulch pile they tell us that thirty percent of our native bees are cavity nesters while 70 percent nest in the ground. The USDA Agroforestry Notes --Enhancing Nesting Sites for Native Bee Pollinators --has tips to make your yard part of the movement to protect these insects – very few of whom are social and therefore defensive. Elderberry, boxelder, and raspberry and blackberry canes support the cavity nesters while bare or very lightly mulched areas of ground can serve as areas where ground nesters can lay eggs. Ground wood mulch, popular because it lasts so long, is a poor choice for a pollinator friendly yard. Try to let your mulch mimic the leaf litter you find in forests – mulch your fallen leaves and save space in landfills.