Garden foe - Leatherjackets
There seems to be something of a plague in Marden gardens right now. The symptom is devastated lawn turf and the culprit: leatherjackets. To find out what is going on and what to do about it, we need to look at the life cycle of the Daddy Long Legs, aka the Crane Fly.
We are usually only aware of Crane Flies for a few weeks in September when they are flying about, sometimes inside our houses. What they are doing during this brief period is mating and laying eggs, as many as 300 per female. They lay their eggs on or near the soil surface where there is grass or a cereal crop because that is what the larvae feed on. The eggs hatch into greyish black or brown larvae (grubs), known as leatherjackets, after a few weeks and feed on underground stems and roots during the day and plants above the ground at night. During the winter they move into deeper soil but as the soil warms up in the spring, they move up into the root zone again to feed. This period, when they are growing bigger, is when they are causing most damage. Damage to turf can be compounded by birds pecking at the soil to eat the leatherjackets but at least the birds are reducing numbers and are a warning sign of an infestation. By mid-summer, the larvae are ready to pupate and they emerge as adult Crane Flies in late August/September to start the whole cycle again.
If you are not sure if you have leatherjackets in your lawn, you can find out by thoroughly watering a small area and covering it overnight with black plastic. In the morning, many grubs in the soil will have come to the surface. This can be a method of eliminating the leatherjackets if you are prepared to brush them all up and destroy them. But be aware that even a square meter can yield as many as 200 grubs.
So what else can be done to get rid of this lawn pest? At the present time there are no chemical controls available to the amateur gardener but if you have a contract with a lawn care company, you could consult them. What is available for home use is nematodes, specifically Steinernema Feltiae, which are microscopic worms that enter the grubs and infect them with a bacteria which kills them. Temperature, soil moisture and the age of the larvae are all critical if the treatment with nematodes is to be successful. Two periods of application are recommended: autumn and spring.
The ideal time to water nematodes into the lawn is when the larvae have hatched in the autumn and the soil is warm and moist. When you start to see the adult Crane Flies in your garden (towards the end of August), you will know that in a few days they will be laying eggs. These will hatch quickly and you should aim to apply the nematodes two weeks after you see the adults. Your grass needs to be cut short and the lawn thoroughly irrigated before watering in the nematodes during dull conditions or in the evening. Immediately after application, it is very important that you thoroughly irrigate the area again to wash the nematodes into the soil so they get down to where the leatherjackets are feeding. Ensure that the area is kept moist for at least two weeks after application.
If we have a dry September, many of the eggs will dry out and die before they have a chance to hatch into leatherjackets. So, if your lawn is looking parched in the heat, resist the temptation to water it until you are ready to apply the nematodes.
A spring application of nematodes isn’t as effective as one in autumn as the leatherjackets are bigger and the soil temperature likely to be more variable. In the spring therefore, a double strength application is recommended if you have discovered an infestation. The soil temperature should be between 10°C and 30°C during application and for at least two weeks after.
Follow a spring double strength application with a regular autumn strength application. More information about nematodes and how to use them is on the link below.
Resist the temptation to reseed or re-turf your lawn until you are sure you have eliminated the leatherjackets, otherwise your work will be in vain.