Leatherjackets

What are they?
Leatherjacket grubs are the larvae of daddy long legs or crane flies, large insects commonly seen flying in summer and autumn, often attracted to electric lights. The grubs are legless and grey/brown without a distinct head and usually grow to a maximum length of 30.4 mm. The number of leatherjackets varies from year to year but they are usually more common after an autumn when there have been more adult flies around, as in autumn 2005.

In late summer, each adult female fly lays up to 300 eggs in grassy areas, where they soon hatch into leatherjackets. They stay underground over winter and cause most damage to the turf the following spring before hatching and flying away as new crane flies. They then mate and produce more eggs.

When will I find them?
If leatherjacket grubs are already present in newly delivered turf they may sometimes be spotted falling out of the turf if it is left on a paved area for a few hours before laying. They also occur in turf after it has been laid, sometimes inhabiting the thin space between the new turf and the soil beneath, particularly if the turf bed has been overcompacted. They will not be found unless the turf is taken up to expose the roots. Crane flies are just as likely to choose a newly laid lawn to lay their eggs as any other established turfed area.

What damage do leatherjackets do to the turf?
The insect larvae feed mainly on the roots of the turf, reducing its ability to take up nutrients and thus stunting growth. Damage is also caused by animals and birds scratching up the turf in their search for grubs.

Can I control them with an insecticide?
Bayer’s Provado Lawn Grub Killer (active ingredient imidacloprid) is now available in garden centres. It is most effective when the larvae are small and soft in the autumn. Only use insecticides if really necessary, as they can affect the health of bees and birds.

Is there a biological control?
‘Nemasys Leatherjacket Killer’ contains a parasitic nematode called Steinernema feltiae. It is watered onto the lawn in autumn when the larvae are small and the nematodes can enter their skin and kill the leatherjackets, provided conditions are suitable.

Summary
Leatherjackets are commonly found in lawns and other turf areas. You can only legitimately complain about leatherjackets to your turf supplier if they were in the turf when it arrived, as they commonly develop from eggs which have been laid after the turf was supplied. However, most turf suppliers will advise on leatherjacket control, even though the problem is not their direct responsibility. In order to treat them effectively, it is best to treat the young soft larvae in the autumn, rather than the large, tough leatherjackets in the spring.

Produced for the TGA by independent agronomist Robert Laycock, member of RIPTA