Answer: It's unusual for hydrangeas to have insect problems but yours might be host to caterpillars. You'll need to find the pests in order to use the proper treatment. As you have discovered, not all insecticides are safe to use on all plants. Sevin was obviously too harsh for your hydrangea and affected the flowers. Most hydrangeas grow best in morning sun and shade from the hot afternoon sun. If they are exposed to hot afternoon sunshine they will wilt but usually recover overnight. If this is happening with your hydrangea you may want to move it to a more sheltered location this fall or early next spring. Visit your plant several times during the day to see if you can find the pests that are making holes in the leaves. When you find them, place a couple in a plastic baggie and take them to your cooperative extension office or well staffed nursery for identification. Helpful folks there can suggest control remedies that will be safe for your hydrangea. If you don't find any pests, they have done their damage and moved on. There's not much you can do to repair the damage except to prune your shrub back to encourage healthy new growth. Best wishes with your hydrangea!
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