For starters - the grubs in the lawn didn't appear because there's horse manure in the amended soil. The grubs were/are simply attracted to optimum conditions - like lawns everywhere with good soil.
There are MANY methods out there for controlling grubs - both chemical & organic. It all boils down to how much of a "Lawn Nazi" you want to be. For an organic solution, you may want to look into applying nematodes.
We have lawn grubs here (Brown & Green June Beetles mostly), but frankly really don't care all that much. They don't do a lot of damage, & the lawn really isn't all that important to us. The birds & skunks have a field day with them, & since we don't discourage their foraging, maybe that's why we don't have a big grub problem.
My body is a temple. Unfortunately, it's a fixer-upper.