Leatherjackets are the larvae of crane flies or as most know them, daddy-longlegs. They can be very damaging to your lawn if left untreated as they eat the roots of the grass.
Adults appear for a few weeks at the end of August but predominantly in the first few weeks of September where they will mate and lay eggs within lawns near to the soils surface. The eggs hatch within 10 – 14 days and are highly dependent on soil moisture. The larvae will overwinter below ground and feed on the roots of the grass, larger larvae can sometimes be seen in thatch eating the crowns or leaves. During April and May damage to your lawn can be seen as the larvae feed on the roots and so kill the grass. They stop feeding by mid June and borrow deeper in to the soil where they will pupate in late August and not too long after emerge to the surface as adults. The lifecycle begins again.
Lawns develop patches where the grass turns yellowish brown and often dies. This can be distinguished from similar effects caused by lawn diseases or adverse growing conditions by lifting the affected turf and finding leatherjackets in the surface of the soil.
They are a good source of food for birds, mainly crows and magpies, but many others, that will dig at the lawn to find them.
Unfortunately there is no current legal chemical control for leatherjackets in lawns.
Biological controlis available for controlling leatherjackets in lawns. This is a nematode, Steinernema feltiae, which is watered into the turf. The nematodes enter the bodies of leatherjackets and infect them with a bacterial disease. To be effective, the nematode requires soil that is well drained but moist and with a minimum temperature of 12°C (54°F). Treatments should be applied according to the manufacturers instructions but usually during September – early October and then again as a stronger dose during April – May.
Another way to control is to heavily water the lawn and then cover at night with black plastic. The, first thing in the morning, take off the plastic and remove any leatherjackets that have come to the surface.
Biological controls can be applied to prevent leatherjackets.
Good lawn maintenance will lessen any impact that leatherjackets may have. Seasonal treatments and aeration will help to produce a very strong grass plant and should help if there is an attack, not only before but also the recovery period afterwards.