Leatherjackets and their control
Most people are familiar with crane flies or "Daddy Long Legs" - an insect that flies around on warm Autumn nights. It is often attracted into houses by the warm and light, and can be seen dancing around lampshades or making shadows on walls. Not so many people are aware that the larvae of the crane fly is a grey-brown cigar-shaped grub called a "leatherjacket". These grubs develop under the grass in late August/September. They overwinter there until they stop feeding in mid-May to pupate, emerging as the leggy adults we know in August.
From August until mid-May they are eating the roots of your lawn, resulting in brown tufty patches, resembling Chafer damage, but looking its worst in March/April. Often lawn damage is made worse by the foxes, magpies, and other birds which grub up the turf looking for a tasty meal. Small infestations are not troublesome, but more than 20-25 grubs per sq.ft. becomes a problem.
In tunnels and glasshouses leatherjackets can be a problem if the ground has been left uncultivated for a while. In our glasshouses in Sussex we are plagued by leatherjackets every spring, and they eat up the roots of peppers and other tender transplants. On the left you will see a picture if a handful of the grubs that we found when weeding in February 2011.
Steinernema feltiae (nematodes)
These are microscopic beneficial nematodes (eel worms) which are watered into the lawn with a watering can or hose feeder. They enter the grub and poison it so that it will die within a few days.
Apply when the soil temperatures are above 10°C at least 3 hours of the day and keep the turf reasonably damp while the nematodes establish themselves. The best time to apply leatherjacket killer is at the end August-October before the grubs move deeper into the ground to overwinter. Treatment can also be applied in the Spring but it is not quite so effective as the overwintered Leatherjacket grubs have much tougher skins.
The grass should be moist before application and should be irrigated directly afterwards to wash the nematodes into the soil. It should be kept from drying out over the next 2-3 weeks, or the nematodes will dry out too. They do not harm any humans, birds or animals.
The nematodes which target leatherjackets Steinernema feltiae are also useful in controlling sciarid fly larvae and will tackle thrips pupae found on the soil