Steve, If you have a grub issue the grass should pull up from the soil very easily, almost like pulling up carpet. Grubs are active from about june thru october, so it could very likley be that. I would pull up a 1ft by 1ft section and dig around to see if you find any grubs.

If you have more than 12 of these bad boys in 1 square foot of grass, then they are the culprit. Normally lawns can tolerate some grub feeding without showing damage, but when you get above the critical number of about a dozen, then you are going to see the damage. Good and bad news is the damage is done. They are finished feeding for the season. If you kill them now you are closing the barn door after the horses are gone. The time to stop them was last July (possibly even before you became aware of the damage). If you spend the money to kill them now, it will just be spiteful...which is not a bad reason now that I think about it. Do you remember last June whether your porch lights were covered with beetles? That is one indicator that you are about to have a grub problem. If it happened like that to me, I would spend my money on making the lawn look better rather than killing the sleeping grubs. This is for you and you along in the California high desert: I believe the very best thing you can do now is to winterize. Without grubs I would suggest winterizing later in the year, but you have this going on. You are between the Labor Day and Thanksgiving holidays when lawn fertilizer seems to work. Since it is this long after Labor Day, I would go ahead and winterize. Winterizer fertilizer is high nitrogen/fast release chemical fertilizer. It causes fast growth at the tops which feeds the roots. One thing you could do at the same time as winterize is to apply an organic fertilizer like alfalfa pellets. Apply at 20 pounds per 1,000 square feet. That will do more for your soil (and lawn) than any combination of other treatments you might consider - especially if you have not used an organic fertilizer in a long time. Is your soil made of the loose rubble that is common in the high desert or did they bring in fill that looks like soil? Have you ever had your soil tested by a professional (not university) soil test lab? My worry in the SoCal high desert is boron levels. The only place to get a good soil test I know of is Logan Labs. Their $20 test is better than most university special order tests costing $100 or more. It will give you boron levels. Boron is a tricky element. All life needs a little bit, but once you get more than a threshold level, nothing lives.

My Master Gardener agent teaches that the best time to treat the lawn for Grubs is in Mid August, when the new crop of eggs have hatched and they are most suseptible to the effects of the grub killer. He says the grubs are too mature in the spring and the grub killer is mostly ineffective if applied then. My lawn seems to agree with him. Here's What Ohio State recommends and they seem to agree with Indiana. "Chemical Controls - Insecticides The grubs are best controlled when they are small and actively feeding near the soil surface, usually late July to mid-August. However, with the development of new grub control chemistry (e.g., imidacloprid [Merit] and halofenozide [MACH2]), applications in June and July have sufficient residual activity to kill the new grub populations as they come to the soil surface in late July through August. Control of grubs in late-fall or early-spring is difficult, at best, because the grubs are large and may not be feeding. Only trichlorfon (Dylox) and carbaryl (Sevin) formulations are available for such rescue treatments. The key to good control is to make an even application and water thoroughly." Taken from The Ohio State University Extension Fact Sheet Here is a link that might be useful: Control of Japanese Beetle Adults and Grubs in Home Lawns

Know your enemy : Identifying grub damage Symptoms of white grub infestation include wilting of grass in patches, eventually turning brown or dying, sod that pulls up easily, in one piece, with the white grubs then visible underneath, feeding on the roots. Increased feeding activity of birds, moles, raccoons,etc. are another sign, you may notice holes in the sod where critters have been digging. The damage is most noticeable in Fall or in dry spells, though serious infestations may be visible in Spring. Dead brown areas can have other causes, such as pet urine, improper use of fertilizer or insecticide, turf diseases or improper mowing, so be sure to identify the problem before trying to treat it! In the case of grubs, if the insects are not easily visible just under the turf, there is no point in treating for them. The usual suspects.... The white grubs you will find feasting on grass roots are the larvae of several members of the beetle family, having an idea WHO you're hosting helps in controlling them. All are fat white C-shaped grubs, with brown heads and six legs, but the larvae of the May or June beetle can easily be 2" in length. This species can remain underground up to four years before becoming a winged adult, so timing of chemical controls is not as crucial, though they must be applied when the ground is warm enough for the grubs to be actively feeding. Japanese beetle larvae are smaller, about 1/2 inch, and have a noticeable V-shaped pattern on the 'raster', the dark bristly area at the end of the abdomen. Japanese beetles complete their life cycle in one year, laying eggs in mid-July which hatch in early August, chemical controls should be applied at that time while larvae are most susceptible. Masked chafer grubs are similar to Japanese beetle larvae in size, but lack the V-pattern on the raster.Unlike Japanese beetles, the adults of this species are short-lived, and do not eat while in the winged state. Their life cycle is similar, with August being the time to apply chemical controls. Dates given above for applications are most applicable for Zone 5, your local County Cooperative Extension Service will be happy to help you with timing treatments in your specific area. Any pesticide you choose MUST be labeled for use on white grubs, and label directions MUST be followed exactly. Preventing grub infestations: biological controls There are a few natural controls available to treat your lawn BEFORE grubs become a problem. Milky spore disease, which is most effective against Japanese beetles, and to a lesser extent against May/June beetles. The bacteria that cause the disease affect only white grubs, and after proper 'inoculation' into a lawn area, remain active in the soil for years. The bacteria need to be present in the soil BEFORE the grubs begin feeding to be most effective, as the disease can take months to kill established grubs. Beneficial Nematodes are microscopic, nonsegmented worms that occur naturally in soil all around the world. Steinernema carpocapsae and Steinernema feltiae prey on ants, termites and the larval and grub stages of various beetles, weevils, armyworms, cutworms, chafers, webworms, borers, maggots, fleas, fungus gnats (sciarid flies). Once they are released, the nematodes seek out host insects and enter their prey through body openings and emit an endo-toxin that results in death for the host insect within 48 hours. The nematodes reproduce and their offspring feed on the insect cadaver and emerge to seek out new hosts. Nematodes are easy to use. They are shipped in a formulation that you mix with water. The solution can be applied using a watering can; hose end, backpack, or pump sprayers; or through irrigation or misting systems. Always release early in the morning or late afternoon when temperatures are cooler. Generally, 1 vial of Beneficial Nematodes will effectively treat approximately 900 sq. ft. of conventional garden rows. Make releases every 3-6 weeks or until infestation subsides. Nematodes can be stored in the refrigerator (do not freeze!) for up to 2 months.