Solitary ground-dwelling bees are active now, and you may find areas of your lawn with dozens of these small mounds popping up in March and April.  These bees are valuable pollinators, are not aggressive, and don't pose a threat to people.  In addition, they don't degrade turf; they simply take advantage of sites where the turf is already thin or weak for other reasons (such as heavy compaction, poor fertility, or other problems).  Enjoy the presence of these mounds as a healthy sign of nature at work, and avoid any insecticide applications where the mounds are present.