“Grubs have to eat milky spore for it to infect them, so the best time to apply milky spore is in early fall when grubs feed most actively. You can apply milky spore in spring and summer, as well, because grubs are present pretty much any time the ground isn’t frozen.
The ideal soil temperature for milky spore is between 60 and 70 degrees. This is another reason why fall, when temperatures are cooler, is the ideal time for milky spore applications. “