No bugs in your tree roots = no reason for half-shell possum to dig in your tree roots.

I treat twice a year - once in the spring, and then again in the fall.

If your area is badly infested with japanese beetles it would be worth researching Milky Spore treatment.
The commercially available pheremone traps are very effective for getting rid of mass quantities of JBs before they can reproduce.
Toss the piddly collection bags that come with the trap and funnel the vermin into 50 gallon drums or big plastic garbage cans.

If you've just got junebugs and other native beetle species look at nematode treatment for an environmentally-friendly approach to grub control.