They are grubs and eat roots causing brown spots/patches in the lawn. The bigger problem is that they will attract moles and then your lawn will get lumpy and torn up. Previous posts say not to worry unless there is a substantial amount of grubs. I believe they said 12 per square foot. I don't worry about them until moles move in, then I treat for grubs and trap moles.

This post was edited by pno_nor on Thu, Sep 19, 13 at 7:32

I have also read posts that say "don't be concerned unless there are 12 per sq ft" this is foolish. I would say 1 per sq foot throughout your lawn would be a concern.

The reason 1 per 1,000 is not a concern is they can only eat so much. We all have to live on the same planet. If everyone sprayed their 3,000 square feet with insecticide for every grub they found, the soil microbiology would be thrown way out of balance.

Here is an excerpt from the NRCS Soil Biology Primer

Arthropods can be grouped as shredders, predators, herbivores, and fungal-feeders, based on their functions in soil. Most soil-dwelling arthropods eat fungi, worms, or other arthropods. Root-feeders and dead-plant shredders are less abundant. As they feed, arthropods aerate and mix the soil, regulate the population size of other soil organisms, and shred organic material.

Thus if you kill all the arthropods and micro-arthropods with insecticide, the result will be overpopulation of the soil with fungi and/or other arthropods, and reduce the natural decomposition of other organic materials in the soil. The arthropods will eventually repopulate the soil but only after the food supply for each species reaches a minimum level.