Grubs are not usually a major problem in our organic lawns. Out of 600 lawns we will have 5 or 10 a year where grubs are a problem. For a chronic grub problem we have found it effective to top dress with compost in early July, when the beetles (European chafers, Japanese beetles and others) are laying their eggs. They don’t seem to deposit them in a fresh layer of compost. I think grubs like a sandy soil that they can move through easily. They may also prefer an environment with less biology and organic matter.
If we see grubs or think they may like a particular lawn, we apply beneficial nematodes that are parasitic to the grubs. Nematodes do best when the soil is moist and we try to apply them when we have a few days of rain. If the rain stops, the nematodes will be most effective if watered in for a couple of days. We use a mix of two species, one that stays close to the surface to about 3” and one that goes deeper to about 7”.
Surprisingly, nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms. We see them in our compost and compost tea and they are an integral part of an active healthy soil. There are 20,000 species of nematodes, including bacterial feeders, fungal feeders and some that feed on the roots of plants. The ones we use to feed on grubs last in the soil for only a couple of months