If you’re using nematodes, apply 2 to 3 times per year. Preventive insecticides, on the other hand, are usually applied once a year, in late spring/early summer. (Some homeowners skip this application if they haven’t had recent infestations.) For active infestations, apply a curative grub killer either in very early spring (before the grubs pupate in early May) or in late summer/early fall, when they are actively feeding. Curative treatments can take days or weeks to work, and they’re less effective as grubs move deeper into the soil for winter. Generally, preventive treatments are less toxic and better at reducing grub populations. But if you’re trying to save your lawn from a serious infestation, hit it with a curative insecticide and then follow up in the spring with a preventive.