Leatherjackets are the greyish coloured larvae of crane flies, a tough, grey, legless larva without a clearly visible head. Only the mouth parts are visible. Unlike scarab grubs, with which they're often confused, leatherjackets do not have legs.
Leatherjackets have five different larva stages, and spend most of the day in a shallow vertical gallery, emerging at night to eat those parts of the plants which are above ground. Leatherjackets therefore do not eat the grass roots but instead need the green parts of the grass as food. Bare patches develop in the grass around a leatherjacket's gallery. They sometimes also cut off the grass and pull it down into their galleries. A piece of grass will initially be seen to poke out of the hole, and will disappear later as it is eaten from down below.
There are five different species of leatherjackets living in the grass, the most important of which is Tipula paludosa.
Tipula paludosa has one generation per year, and these crane flies fly from early September to early October.
Tipula oleracea is less common, though they can be prolific in certain years. This species has two generations per year, the first in May and the second from mid-August to mid-September. The leatherjackets of the first generation in particular can do great damage in the summer (golf greens!).
Once the crane flies fly out, the empty pupa can be seen sticking out above a short mown lawn or the green.
Populations of 200 to 300 per m² can cause serious damage to lawns, while plants adjoining the lawn can also be affected.
The leatherjackets themselves are only partially responsible for the damage; they are a favourite source of food for birds and moles, who hunt them by digging up the grass.