As the name implies, squash bugs attack squash-family plants, especially winter squash and pumpkins. The adults are dark brown or black, almond shaped, and about 1/2 inch long. They suck plant juices, causing leaves to wilt; they also feed on fruit.
In spring, adults lay masses of hard brown eggs crowded together on the undersides of leaves; look for these and destroy any you find. Adults spend nights under flat objects, so lay out boards in the evening, then turn them over early in the morning and kill any bugs you find (they emit an unpleasant odor when crushed).
For chemical control of adult bugs, use carbaryl.