You could spend the money to buy those nematodes and they would find other ways to survive, but since there are no grubs to be found, why? For grubs to damage a lawn there needs to be a fairtly large population, 10 per square foot. Also, in May any grubs that might be in the soil will be pupating, preparing to morph into adults to emerge as those adults in June. A better time to apply nematodes, or the Milky Spore Disease, or any other means of contol of grubs, is early to mid August or about when the grubs hatch from eggs and are small enough for environmentally safer products would be more effective at control.