What could be ruining my nearly ripened tomatoes by causing many hard, yellowish spots in and on the fruits?
Your description sounds classic for results which come from the feeding of the stinkbug on young, green fruits. An enzyme secreted during feeding prevents normal color development and ripening in affected spots; however, these are not evident until ripening is pretty far along. A puncture wound in the skin can generally be found in the center of the yellow spots. A good insect control program will prevent this fruit blemish.