It looks like it may be the larva of the red lily beetle. The larva kind of resemble 1/4 to 1/3 inch slugs and they cover themselves in their own excrement to deflect predators. You probably did not have them in your garden before as they are fairly recent to the US (arrived in Massachusetts in 1992 and have been slowly moving outward from there). Unfortunately, they are extremely destructive and attack primarily lilies and Fritillaria. Here is a link with more information including treatment:
http://extension.umass.edu/lan...
Again
to ATP.
