Garden Pests and Diseases: Something is eating my swiss chard, 1 by riverhamill

riverhamill wrote:
Hi friends, I have a greenhouse here in Taos New Mexico where I grow hardy greens all winter long. I am able to keep the temperature just above freezing on cold nights, with a little heat. Something is eating my swiss chard, and ONLY my swiss chard. I do not see frass. I spray with Bt with no effect. I go out at night with a flashlight, and do not see any pests. Yet the damage continues. Any ideas??