For those of you growing winter greens in a high tunnel, be wary of aphids infesting your leafy greens, especially spinach and lettuce. These carryover pests from the summer crops can build up populations fairly rapidly in the confines of a tunnel, in the absence of predators. Plus these past few days have been sunny, which increases the temperatures in the tunnel, which increases growing degree days, which increase the aphid populations!
Control will probably need to be strongly considered. The Midwest Vegetable Production Guide for Commercial Growers lists several alternatives, including organic as well as conventional.  
http://www.btny.purdue.edu/Pubs/ID/ID-56/ID-56.pdf