Well this is a tough one since hostas are rarely a problem. And, since I can't see the plants in question, it's difficult to be sure. I would make sure your soil drains well. Hostas, can't sit in waterlogged soil and thrive. It also sounds like you may be overwatering. Bascially, once your plants have been in the ground for a week or so, you don't really need to water much unless it's during a drought. Mulching also helps maintain soil moisture. Again, I can't be sure, but yellowing leaves are often a sign of too much water. Slug and grasshopper damage is evident by holes or cuts in the leaves. If you don't see this damage you don't need to treat. Hostas are usually healthier when you ignore them a bit. Make sure the soil is rich, organic and well drained and mulch the plants.