Leatherjacket risk high for second year in a row

Large bare patches in grass are characteristic indications of leatherjacket damage

Leatherjacket is the name given to the grubs of the crane fly or “daddy-long-legs”. They live just below soil level and each year eat the roots of grasses and other plants from August through to June. Scotland’s Rural College has conducted an annual survey in central and south-west Scotland since the mid 1970s. Grub densities fluctuate from year to year but over the past 20 years they have consistently risen higher, linked, it is believed, to climate change and wetter, milder autumns.

Bad weather coupled with frozen fields delayed the survey this winter so results are just available and indicate that grub populations in grasslands are at similar high levels to those observed last spring, with the average density recorded across the fields in the survey being 1.1 million grubs per ha. The fact that two-thirds of the fields contained grub densities of over 0.6 million per ha means there is a very high risk of damage to any spring crops planted after grass as this is the threshold for economic damage. And with over half of the fields sampled containing over 1 million grubs per ha (the damage threshold for grass), then many grassland fields are also likely to see grass yield losses and damaged areas becoming evident over the coming months.

The risk to spring sown crops applies across all central and south-west Scotland, while the risk of yield loss and damage to grassland is particularly high in Ayrshire, Bute, Lanarkshire, Renfrewshire and Dunbartonshire.

The lack of any ‘quick fix’ through chemical control now that chlorpyrifos is no longer approved for use means that the focus has switched to damage limitation. SRUC’s advice continues to be to survey key fields to know what densities are present so the results can inform grassland management decisions and spring crop choices.

Identifying which grassland fields have high grubs numbers, well before they start to cause damage, is key before time, effort and money is wasted applying fertiliser to fields where the forage yields will be much lower or crops are planted into a field where they will be subsequently decimated by the grubs still present after ploughing and cultivation. Similarly knowledge on densities will inform choices on spring cropping in subsequent years.

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