European crane flies lay their eggs in grass in the summer or early fall. The eggs hatch into larvae or maggots, known as 'leatherjackets' because of their gray-brown leathery skin. European crane fly larvae live in the soil and feed voraciously on the roots, shoots and crowns of grass plants during the fall and following spring. Normally, this is not a problem unless, of course, leatherjackets happen to be in dense populations in your lawn, pasture or grass seed field. This dieback is especially noticeable in the spring.
Damage can also occur when birds come in and scratch at the lawn surface looking for larvae to eat.
Spring is the best time, as damaged areas show up the best. Dig a patch of lawn out, about 12 inches square and about two inches deep. Then count the number of leatherjacket larvae you see in this patch. If there are more than 25 of the leatherjacket larvae, you might consider treatment or lawn renovation if your lawn has severe damage. If there are less than 25 per square foot, then your lawn grass can probably outgrow the munching maggots.