How to Control Grubs
"Controlling all sorts of critters that spend most, if not all, of their lives underground can be a real challenge, especially if you can't see them," says master gardener Paul James. "And that's especially true of grubs, the larvae form of most beetles and in particular the Japanese beetle. The adult stage of these beetles can do a tremendous amount of damage to your plants, especially roses."
To help get ahead of the adult beetles, you can begin by working on the grubs. Most grubs begin life in early summer as eggs laid four inches underground by their mother. Early the following spring, the larvae begin an upward migration seeking food, primarily in the form of various plant roots. The secret to controlling larvae is to know the timetable of their life cycle and to take appropriate steps to control them.
One of the best ways to control them is by using a white powder known as diatomaceous earth, or simply DE. DE is a nontoxic, organic concoction made of the fossilized remains of tiny one-celled marine animals called diatoms. Chemically, it's almost pure silicone dioxide. To humans, DE feels really soft to the touch, but to grubs it's like razor wire that literally shreds them as they touch it.
Sprinkle DE on the soil at the rate of about 1/2 cup of powder per square foot and mix it into the soil to a depth of about six inches. DE can cause respiratory problems, so wear a mask and work with it on a nonwindy day. DE can also harm beneficial insects, such as earthworms, so use only where grubs are a problem.
"You should apply diatomaceous earth each spring in the soil to control grubs," Paul recommends, "and you can also sprinkle it around the base of plants during the summer months to control just about anything that crawls — from Mexican bean beetles to squash bugs." But make sure to buy the horticultural grade; there's another kind available for swimming pool filters, but it has been treated and won't work on grubs.