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SAC Warns Leatherjackets Numbers High – Hard Winter Has Not Stopped March Of The Daddy Longlegs

SAC News Release Ref. No: 10N23
Published: 07 Apr 2010

Leatherjacket sampling (opens in new window)

Leatherjacket populations in Scotland are exceptionally high this spring and the risk to spring sown cereal crops and grassland is very real.

According to the latest annual survey carried out by SAC in south-west Scotland, the average population of crane fly larvae is 1.24 million grubs per hectare. It is the sixth highest level recorded since the survey started 35 years ago.

Leatherjacket is the name given to the grubs of crane flies or “daddy longlegs” which live just below soil level and eat the roots of grasses and other plants from August through to June each year. According to SAC Ecologist Dr. Davy McCracken, who undertook the survey…  

“Unfortunately, the severe winter weather this year will not have had any impact on reducing grub numbers. Leatherjackets are very resilient to cold and frost and all they do under such conditions is reduce their feeding until the soil warms up again.”

SAC has conducted an annual survey each winter since the mid-1970s and these latest results show once again the value of such long-term surveillance and the data generated. In addition to the risk posed to crops, leatherjackets are an important food resource for farmland birds. Since the mid-1990s SAC’s farmland biodiversity research has focussed on a better understanding of what drives changes in leatherjacket populations. It shows grub densities fluctuate from year to year but in the past 13 years they have consistently been higher, linked, it is believed, to climate change and wetter, milder autumns.

This year the risk is particularly high in the west and south-west including Wigtownshire, Ayrshire, Argyll, Bute, Lanarkshire, Renfrew, Stirling and Perth. Only in the Dumfries, Kirkcudbright area does the risk appear low.

Farmers in the east cannot be complacent either. Survey results around Stirling and Perth are a good indication that likely grub levels in non-surveyed areas such as the NE and south-east of the country Scotland are just as high with damage likely this year in short- and long-term grassland as well as spring cereals.

Two-thirds of all the fields sampled harboured populations greater than 0.6 million grubs per hectare, while over half were well above that at 1 million/ha. It means the damage  and loss of yield could be serious when hungry grubs start feeding as soils warm up. However, Dr. McCracken advises farmers against spraying just for insurance.

“Despite this potential grub threat, it is important to carry out an assessment of leatherjacket infestation in each location before any insecticide treatment. An assessment will identify those fields at risk before any damage and associated loss has occurred and means control measures can be targeted cost-effectively.”

SAC crop protection specialists advise that for high infestations an insecticide should be applied as soon as ground conditions permit. When planning to sow spring crops after grass, insecticide treatment of the grass before ploughing will knock leatherjacket numbers back sufficiently to give the crop a good start. However, where grub densities prior to arable crops are marginal for treatment, current advice is not to take preventive action against the grub straight away but to wait and monitor the emerging crops and treat only if necessary.

“It makes sense, for both economic and environmental reasons, that any insecticide application should be restricted to only those fields where the need has been demonstrated” says Davy McCracken.

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News Release Contacts

SAC Contact

Dr Davy McCracken
Senior Agricultural Ecologist
SAC (Scottish Agricultural College) Work SAC, J F Niven Building, Auchincruive,
Ayr
KA6 5HW
TelWork 01292 525299
MobileWork 07712 001 267
Fax 01292 525333

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News Release Issued By

Mr Ken Rundle
Senior Communications Officer
SAC (Scottish Agricultural College) Work SAC, King's Buildings, West Mains Road,
Edinburgh
EH9 3JG
TelWork 0131 535 4196

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Notes for Editors

Assessment of leatherjacket infestations in individual fields can be undertaken by SAC. Farmers are advised to contact their local SAC Farm Business Services Office to arrange for soil samples to be taken.

1. The survey covers Argyll, Ayrshire, Bute, Dumfriesshire/Kircudbrightshire, Lanarkshire, Renfrewshire, Stirlingshire/Perthshire and Wigtownshire. The survey has been conducted every year since winter 1975/76 and this year is therefore the 35th year of annual data collection.

2. The average population density per ha was 0.68 million/ha in winter 2008/09 and 0.32 million/ha in winter 2007/08. Grub numbers have therefore shown a steady rise in the past two years after the severe crash from the record high average densities of 2.11 million/ha and 2.5 million/ha recorded in the winters of 2006/07 and 2005/06, respectively.

3. Of 242 grass fields sampled between November 2009 and March 2010, 67% contained populations in excess of 0.6 million/ha. Hence, the risk of damage to spring cereals out of grub-infested turf is very high. In addition, over 50% of the fields sampled contained populations in excess 1.0 million per ha and so the risk of damage to fields remaining as permanent grassland is also very high.

4. Mild and wet weather in the autumn enhances the survival of young grubs when they first emerge from eggs laid into the soil in the late summer/early autumn. The extremely wet weather across Scotland in July and August 2009 and associated milder temperature in the autumn were ideal for survival of large numbers of eggs and young grubs. Once the severe weather hit in December 2009, the grubs were big enough and resilient enough to ride out those adverse soil conditions.

5. The long term monitoring of mean leatherjacket density in grassland in south-west Scotland in every winter since 1975 has highlighted that densities are expected to fluctuate from one year to the next. However, it has also shown that grub densities have been more consistently higher over the past 13 years. In particular, high mean leatherjacket densities (i.e. more than 600,000 grubs/ha) have been recorded in 10 of the past 13 years compared to being recorded in only 10 of the previous 22 years. In addition, the three highest recorded mean leatherjacket densities in grassland have all been recorded in the past 11 years. The observed increase in mean leatherjacket densities over the past 13 years is related to climate change and especially the increase in wetter, milder autumns in south-west Scotland.

6. SAC receives financial support from the Scottish Government – their Crop Health Advisory Activity programme and Research Programme were used to provide part funding for the survey this year. Dow AgroSciences UK Limited provided partial sponsorship towards this survey and a proportion of the remaining costs were offset by the farmers involved having their fields sampled at a substantially discounted rate.

7. For further technical information, contact Dr Davy McCracken.