Plant Health by Fiona Burnett and Davy McCracken (SRUC) for the Farm Advisory Service

Leatherjackets, the grubs of the crane fly or 'daddy-long-legs' can decimate spring arable crops and seriously affect the productivity of grassland.

The loss of chlorpyrifos for control means that avoiding seriously infested field or adding additional cultivations are the only practical means of reducing damage and yield loss. Testing fields well enough in advance to change spring field management decisions is more important than ever.

The risk to spring-sown crops applies throughout Scotland, while the risk of yield loss and damage to grassland is particularly high in the west. In SRUC survey data, levels have been creeping up over the decades, even before the removal of chlorpyrifos and population peaks in the last 15 years exceed peaks seen between the mid-1970s and late 1990s.

High populations are also more common now with almost half of recent seasons being regarded as high risk.

Assessing grub levels in grassland or grassland planned for spring cropping between November and February allows the scale of the infestation to be judged before the grubs are large enough to do lot of damage. Numbers below 0.6m grubs/ha should not be damaging to spring crops following grass, so no further action is needed. Levels above this represent a risk of damage to spring crops and so early testing before grass is ploughed up allows for additional cultivations of the soil which will kill a greater proportion of the grubs.

This clearly adds to the expense of the operations, but will reduce the risk of damage to spring crops unless the initial grub population was very high. For very high levels of infestation, the decision might well be to select another lower risk field for spring cropping and avoid the expense of establishing what will probably be a very low yielding crop.

It is also useful to test grass remaining as grass. At the lower end of the scale (grub numbers between 1-2m per ha) then grass may be able to grow away from damage. But at higher grub levels, the productivity of grass will be much reduced and there is little that can be done.

Rolling affected fields when the grubs are small may help limit grub movement until such time as the grass can grow away from the damage but any possible short-term benefit from that would have to be balanced against the likely soil compaction issues that would arise from rolling fields in mid-winter.

An alternative approach is to accept that grass yields will be lower in such affected fields and to concentrate the necessary forage and fodder production for the farm on alternative fields for the year.

SRUC Auchincruive’s annual survey of leatherjacket densities is currently in progress. It is part supported from the Scottish Government’s Monitoring and Reporting to Aid Adaptation to Climate Change programme which keeps the cost for each field down.

If you want grassland fields surveyed then get in touch directly with Billy Harrison ( as soon as possible to ensure you can take advantage of the reduced rate before the end of the survey.