Scientists from Scotland’s Rural College have warning that this spring there is not only a high risk of damage to spring cereals from leatherjacket grubs but also concerns that infestation will damage fields remaining in permanent grassland.
With control methods now more limited they are urging farmers to prepare their plans now. The risk to spring sown crops applies across all of the Scottish regions surveyed by SRUC, while the risk of yield loss and damage to grassland is particularly high in Argyll, Ayrshire, Bute, Renfrewshire and Dunbartonshire.
SRUS has conducted an annual survey since the mid 1970s. These latest results prove the value of such long term surveillance and the data generated. 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. According to Professor Davy McCracken, who undertook the survey: “The loss of reliable chemical controls last year and the high degree of risk this year means that it is even more important that farmers start planning now if they want to limit leatherjacket damage to crops and grassland on their farms. SRUC’s advice remains the same as we highlighted at the end of last summer – survey key fields now to know what densities are present so the results can inform spring management decisions.”
Leatherjacket populations in many of Scotland’s fields have, as predicted, continued to build from the low levels observed two years ago and the medium levels seen last year. According to the latest annual survey, undertaken in west and central Scotland between November 2016 and February 2017, the average density recorded across the fields in the survey was 1.6 million grubs per ha.
“The likely impact on production will depend on the population density of the grubs, the proposed use of the field and equally importantly at what point a farmer realises that a damaging level of grubs is present,” comments Davy. “Although chemical control is no longer feasible, farmers still have options if they establish the risk present in each field early enough.”
“It is worthwhile emphasising that grassland fields at very high risk were recorded in every region in the survey” explains the Professor. “So if high levels of grubs are found within a particular grassland field the farmer can then decide if it is worth continuing to use that land for spring crop or silage production, knowing that yields will be affected and damage likely to occur, or instead decide to concentrate efforts in another field with lower grub densities.”
“The lack of any ‘quick fix’ through chemical control now means that the focus has switched to damage limitation. Therefore identifying which grassland fields have high grubs numbers, well before they start to cause damage, will be key before time, effort and money is wasted applying fertiliser to fields where the forage yields will be much lower or spring crops are planted into a field where they will be subsequently decimated by the grubs still present after ploughing and cultivation.”
Assessment of leatherjacket infestations in individual fields can be carried out for farmers as a chargeable service by staff from SAC Consulting, a Division of SRUC. Farmers wishing to use the service should contact their nearest SAC Consulting office.
SRUC acknowledges the support of Scottish Government for the survey.