Here is a post from Jay Goughnour of Racoon Valley Golf Course in Jefferson, Ia. While grub damage on Iowa golf courses is not unusual, the problem is generally from Masked Chaferes and occurs in August to October. The problem below happened in mid July. The grubs on the site are much larger than chafer grubs and appear to be June bugs. June bug larvae feed earlier than chafers, usually mid July, which coincides with this damage. June bug damage is fairly rare, however, and I have seen damage only a few times over my 31 years here in central Iowa from this species, whereas I can find chafers every year. I hope this isn't a trend. If anyone else has seen early grub damage from large grubs (1 inch in length), let me know.
Because of their size, it does not take as many June bug grubs as chafers to damage turf. While we normally look for 20 to 30 chafers per square foot to damage turf, June bugs can damage turf with populations of 5 or 6 per square foot.
Racoons love to dig up turf for these grubs. I would be worried if my golf course was named Racoon Valley.
July 20th. Fairway on the right was treated with imidacloprid for grubs. The rough on the left is untreated. Pretty dramatic difference. Looks like the larvae from June bugs instead of the european masked chafers I'm used to seeing.
Jay can be reached at firstname.lastname@example.org.