Never say too late! Find a good grub control solution and water it in *well* for fast results if you don't mind (and can use) pesticides in your soil. Scott's and Bayer Advanced (to name two) make pesticides that control oriental beetle grubs. The main drawback here is that you're applying poisons to your lawn and killing a lot of beneficial things along with the bugs you're trying to get rid of. You'll need to weigh the advantages/disadvantages against the desired outcome and make your choices accordingly.
The ones that are flying around now will do nothing more than eat and reproduce - meaning there'll be even more of them next cycle. "Next cycle" starts now - they usually reproduce in late June/July.
Turf management isn't an avid interest of mine, so you're going to be a lot better off talking to a pro, or at least a well-informed salesperson.
That said, a couple of notes about all those grubs:
The oriental beetle grubs are a bit larger than the japanese beetle grubs. Both will eat your lawn out by the roots.
If you notice ground mole (some call them ground voles. I guess it depends on the local vernacular) burrows in your yard, *RESIST* the temptation to smash them flat or otherwise get rid of them. They move in when there is a feast in your yard - the grubs! For this particular instance, I guess you could call the ground moles a beneficial pest. When there aren't any more grubs, they usually move on to somewhere there's more food.
The areas infested now can potentially be cleared and reseeded, but you're going to see what you see now well into fall, if not next spring.
If you can't / don't want to use pesticides, a couple of options are:
Beneficial nematodes can help control grubs fairly fast. How long they'll stay around is a question I don't know the answer to. Pesticides kill them. Which nematode you want depends on where you are - average soil temperature determines what will work best.
A better long-term control is milky spore (milky disease) - *conditionally*. Milky spore is a long-term, non-toxic method of control that is enviromentally friendly. The drawback is that it takes as much as 5 years to establish. Once established in the soil, it can persist for 15-20 years. B. popilliae is the specific strain that affects *ONLY* japanese beetles, and is the one most readily commercially available, but other strains have been created that affect a range of scarab beetle grubs. To use it successfully, you would need to make certain that the strain you're getting affects oriental beetle slugs as well as japanese beetles... that's the conditional part.
Short of all that, there's always the infamous "daggers of death". They're sold either as shoes or a strap-on attachment with spikes coming out of the bottom (think golf or turf shoes on steroids and you'll probably get a good picture of them). You walk around the yard and *literally* skewer the grubs you're walking over... while drawing stares from the neighbors and giving yourself a good workout pulling the spikes back out of the dirt with every step. Seriously. I've seen them.
Hope this helps.
