On an unusually warm day amid the encroaching winter, I came across a large number of these false chinch bugs (Hemiptera: Lygaeidae: Nysius sp.). There was a mix of both adults and nymphs at different stages of development congregating around the foundation of a barn. These bugs often exhibit gregarious behavior in their pursuit of food and winter shelter.
In the spring these bugs feed predominantly on wild plants in the mustard family (Brassicaceae), using their piercing-sucking mouthparts to siphon the sap. As their preferred food is depleted they move to other plants. This often brings them to gardens, orchards, and crop fields in large numbers, and under certain conditions they can become serious pests.
Since their migration tends to bring them to human cultivation, it also tends to bring them to human buildings. As with boxelder bugs and lady beetles the winter shelter is apparently appealing to them. These bugs overwinter as both nymphs and adults, as seen with these individuals.