Plants affected
Mainly a lawn pest, but vegetable seedlings and small ornamental plants in flower borders or pots can also be damaged.
Symptoms
Lawns develop yellow patches where the grass has died as a result of leatherjackets eating the roots. Dead patches in lawns can also be caused by other pests, diseases or cultural problems. In late summer large numbers of the adult daddy-longlegs or crane flies may be seen in the garden. Brown pupal cases may be seen protruding from the lawn where adult flies have emerged.
Biology
Adult crane flies of the species that damage lawns mainly emerge in late summer and early autumn. The females deposit eggs in the turf and these hatch in the autumn and begin feeding on grass roots. In cold winters little feeding or development takes place and so signs of damage may not be seen until the summer. However, mild winters can allow the grubs to develop overwinter and sometimes cause damage in late winter or early spring. The larvae are greyish-brown and up to 45mm long when fully grown. The larvae have no legs or obvious head and they have a tough, leathery outer skin. Heavy rain and waterlogged soil conditions can drive large numbers of leatherjackets out of the soil and on to paths and patios, where they may cause alarm. Leatherjackets complete their feeding during the summer and pupate in the soil. Before the adult fly emerges, the pupa wriggles half out of the soil, so the brown pupal case is left sticking out of the turf.
Control
Damaging infestations are more likely to occur after a wet autumn, as damp conditions increase the survival of eggs and young larvae. A dry autumn may result in few leatherjackets, even if large numbers of adult flies were seen in the garden. Leatherjackets and adult crane flies have many natural enemies, such as starlings and other birds, spiders and parasitic insects, that help to limit their numbers.
A lawn pesticide, Provado Lawn Grub Killer, which contains imidacloprid, can be used against this pest. It should be applied with a watering can fitted with a rose or dribble bar in late summer to early autumn in order to control the newly hatched larvae. Mow the lawn before application.
An alternative treatment is to water a pathogenic nematode, Steinernema feltiae into the turf. This is sold from chilled cabinets in some garden centres or it can be obtained by mail order. The microscopic nematodes enter the bodies of leatherjackets and infect them with a fatal bacterial disease. If it is to be effective, the nematode needs to be applied while the soil is moist and the soil temperature is in the range of 12-20ºC (54-68ºF). The ideal time is in early autumn before the weather has turned cold.
On small lawns, leatherjackets can be encouraged to come up to the surface where they can be collected. This is done by covering the turf with black polythene or some other light-impermeable material. This should be done when the soil is moist and leatherjackets are feeding close to the surface. Leave the cover on overnight, then in the morning peel it back in stages and collect any leatherjackets that are exposed. Do not uncover too big an area as leatherjackets will soon crawl back into the turf when exposed to daylight.