On another forum, someone posted this resource for identifying beetle grubs, and it looks so straightforward that I think it's worth sharing. Although it comes from MI State and is specifically geared towards the Great Lakes states, they show all the adult beetles as well, and just about all of them are present in New England. I figure it will be useful in deciding if I can try Milky Spore to get rid of them or if I need to use nematodes or something else as long as I can get over the ick factor of looking at grubs that closely.