I'm not sure if the large, non-biting insects I see every summer
around this time are crane flys or gnats. They look like big
mosquitoes but they don't bite.

If they are crane flys, then what can I put on my lawn either right
now, or later in the fall, or maybe in the spring to kill the
leatherjacket larvae that will eat the roots? Will grub killer also
kill these buggers to?