I have tons and tons of them and have only had problems with a few of my seedlings. I just use a systemic insecticide like the Bayer Tree and Shrub (can be used on things other than trees & shrubs) granules or something similar and it takes care of the larvae. The benefits of the alfalfa outweigh the few seedling losses. And I've only lost 2 mature plants to rot and it was because of the weather.
One way to avoid having them ( or as bad) is to let the soil dry out before watering again ( or the top layer of it). That helps keep them at bay and find someplace that's wet to lay the eggs. We water ever other day plus we have really high humidity so everything stays damp around here. When things get hot and dry I don't see them nearly as much.
