Ash trees can be eaten by Japanese Beetles but so are many other plants. If you want to treat for grubs, that is best done by applying Bonide Annual Grub Control in July followed immediately by 1″ of water. If the grub control isn’t watered in with an inch of water right away the chemical will break down and not work. Don’t count on rain or a light spraying with a hose to do the job. Set up a lawn sprinkler and put a small tuna fish or cat food can in the area. When the can is full, you have applied enough water. Here’s a link all about that.
In the meantime, an application of MoleMax to the area will keep the skunks and any other rodents away from the area.