You may notice grubs only when you start seeing their damage to your lawn. To prevent this damage easily, apply grub control at the best time, usually early to mid-summer.
By preventing the next generation from forming, you avoid damage that would occur the following season. This is effective as typical insecticides don't work during the larva stage.
If you already have a grub problem, apply insecticides in early spring to kill grubs as the larvae grow. However, to use such products, you may need to hire professional services.