Insect grubs in my soil and lawn

Background

Insect larvae are occasionally found in the soil layer of freshly delivered turf.

Turf is one of the most natural products you can buy. Turf soils contain a vast array of living organisms, ranging from larger creatures such as larvae of beetles and flies, spiders, millipedes, earthworms and insects, to tiny microorganisms. The presence of these creatures is a sign of a healthy, living soil.

Most insect larvae cause no problems. Occasionally, though, the larvae of two insects can damage turf if they occur in high enough numbers: crane fly (daddy long legs) and chafer beetle.

Chemical control no longer possible

Before a chemical can be used by turf growers, farmers and homeowners it has to be approved by the Government. Recent concerns have led to the withdrawal of those pesticides used for the control of leatherjackets and chafer larvae.

Under these circumstances, it is more likely than before that those insect larvae will be present in turf leaving the turf production fields.

In one way, this is a good thing because it means that fewer pesticides have been used in producing the turf - which is good for the environment and, in particular, good for other beneficial insects found around the turf farm.

Where have the larvae come from?

Insects of many types are common in gardens and the insect larvae which may be found in rolls of turf could equally be found naturally in an established lawn or elsewhere in the garden.

Unless the insect larvae are found at the time the turf is being laid, it is sometimes difficult to decide whether they arrived with the turf or were already present in the soil on site. Once turf has been laid and has become a lawn, it can become the home to all of the pests and diseases which may occur naturally on an established lawn. The insects and their larvae that may be found in rolls of turf are common throughout the British Isles and could develop within a few months from eggs laid by adult insects after the turf was installed.

If turf is being laid on the site of an existing lawn it is possible that the larvae are in the soil already - they may have been the cause of problems with the previous lawn.

Identifying insect larvae in turf

The most commonly found insect larvae are those of crane flies (daddy long legs) and chafer beetles.

Leatherjackets, the larvae of crane flies, can be up to 30 mm long depending on their age and have a grey-brown leathery skin and no legs or distinct head.

Chafer grubs are the larvae of chafer beetles. They are whitish with a distinctly shiny brown head and six legs. They can be up to 15 mm long and are usually found distinctively curled in their tunnel. There are several species of chafer beetles in the UK, the normal production cycle of turf growing ensuring that only the smaller of these are found in turf rolls. The most commonly found species is the garden chafer. It is most unlikely that larvae of the largest and most damaging chafer beetle, the cock chafer, will be found in turf rolls because its life-cycle is much longer than the time taken to grow the turf crop.

What damage do insect larvae cause to turf?

Insect larvae feed on the roots of grass and leatherjackets sometimes come above ground to eat the grass leaves at night. Any damage caused to the turf is temporary and will cease when the larvae metamorphose into adults and fly away.

Poor growth of an established lawn is sometimes due to insect larvae living in the soil and feeding on the turf roots. Very often the first signs of insect larvae (grubs) in a lawn are when birds begin pecking in the grass.

A healthy, well fed lawn is most able to withstand the activities of insect larvae and this is a good reason to always use a pre-turfing fertiliser before laying turf.

Control of insect pests in turf

If you can see leatherjackets or chafer larvae on the turf when it’s delivered just pick them off and release them somewhere else.

Biological control can also be used in the form of nematodes. These are microscopic parasitic worms which only feed on specific insects. Because they are very small they have to be used at the correct time of year and in exactly the correct conditions of soil moisture and temperature. Their use is economical when the lawn is small but would be impractical on a field scale. Nematodes used to control insect pests are known by their Latin names. Those used to control leatherjackets are normally Steinernema feltiae while those used to control chafer grubs are normally Heterorhabditis megidis or H. bacteriofora. Sources of nematodes can easily be found on the internet.

Produced for the TGA by independent agronomist Robert Laycock, member of RIPTA.www.robertlaycock.co.uk