Read our Privacy noticeWhat does it mean when brown patches appear on an otherwise healthy lawn? Yet many homeowners automatically place the blame on lawn grubs. The difficulty is that this common assumption is frequently incorrect, and even when grubs are present, minor populations are not actually problematic. Because grubs are most vulnerable to pest-control measures when they are young, those measures will only be effective if applied in late summer. The nematodes work by infecting grubs with a bacterium that can kill them in as little as 48 hours.