Sounds like you may on generation #2 this growing season. I used Orthene when the problem was first detected (May?) and again the following year (a couple of weeks before the problem started). They never returned. Here is more info with pictures so you can confirm that what you are seeing really are mealybugs:

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/mealybugs/mealybugs-outdoors.aspx

They are "cousins" of scale and somewhat resemble them too so watch out.

please post an image of your hydrangea's infested buds.

Would also like to see photos - mealy bugs are not common pests of hydrangeas. But scale can be and cottony scale looks a lot like mealies but requires different control methods.

So it rained this morning so
hard to see the little creatures but here are some photos of what I found

Those look much smaller than the mealybugs that I had. Actually, they are very hard to pick out on the pictures so I am wondering if you can get a close up of just one of them? In case they are something else like scale, etc.