The focus for PestWatch changes from Wheat Bulb fly to Leatherjackets now and starts with a stark warning of very high risk of this pest in Scotland, as indicated by surveys of grassland across the country.
Each autumn Dow AgroSciences commissions ADAS and the SAC Auchincruive to survey leatherjacket numbers in grassland sites across England and Scotland as part of its commitment to the stewardship and product support for users of its insecticides, Dursban WG and Equity. Information can be accessed at www.dowagro.com/uk/cereal/pest/htm. The latest survey indicates a very high risk in Scotland this spring. In England there is an increased risk of damage in 2010 compared to last year, but overall the risk in English regions is low.
Sarah Hurry of Dow AgroSciences explains that the results give an indication of the risk to susceptible crops. "Risk is based on the threshold of 60 leatherjackets/m2 in grassland preceding winter and spring cereals. However, be aware a lower population may cause damage to root crops."
In west and central Scotland a total of 242 sites were sampled between November 2009 and March 2010 for PestWatch. The survey showed that leatherjacket populations have increased steeply from the low levels of the past two years. The overall mean leatherjacket population was 124/m2, with 67% of sites over the cereal damage threshold of 60/m2; 51% were over 100/m2 and 19% were over 200/m2. Fields especially in Ayrshire, Argyll, Bute, Lanarkshire, Renfrew, Stirlingshire, Perthshire and Wigtownshire are especially at high risk. Only in Dumfries/Kirkcudbright does the risk appear to be lower.
Wet conditions in Scotland last July to September favoured egg survival and larvae emergence at a time when they are normally vulnerable to desiccation. This wet weather and associated milder temperatures through the autumn were ideal for survival and growth of the young larvae thereby ensuring a large number of juveniles came through the adverse conditions experienced later in December and January, explains Sarah.
In England a total of 46 grassland sites were sampled during autumn 2009. The overall mean leatherjacket population was 16.3/m2 compared with 7.1/m2 in 2009 and the long term average of 61.0/m2 .
In the Midlands/Western Region incorporating Cheshire, Shropshire, Staffordshire, Warwickshire and Herefordshire, the overall mean was 16.2/m2 (mean 20 sites). In the Northern Region covering North, South and West Yorkshire, the overall mean was 18.0/m2 (mean 20 sites). However in the Midlands/Western Region 10% of fields sampled and in Northern Region 5% of fields sampled had leatherjacket numbers in excess of 60/m2. In the Southern Region of Berkshire, Hampshire and Oxfordshire, the overall mean was 2.0/m2.
The survey suggests there is a low risk of damage from leatherjackets to susceptible crops in England. These numbers are probably a reflection of dry weather during September/October desiccating eggs and young larvae, she says.
The risk of economic damage from leatherjackets in 2010 in Scotland is very high. Because leatherjacket populations will vary from field to field, the risk should be assessed before treatment is made. Leatherjackets are very resilient to cold and frost; under such conditions they react by reducing their feeding activities until the soil warms up again. High infestations should be treated as soon as ground conditions permit and when leatherjackets are feeding.
Whilst in England the risk is low it has not gone away. Fields with a history of leatherjacket damage, particularly going into spring cereals, should be monitored for large numbers of rooks and crows feeding on the larvae. Risk assessments and testing for the presence of leatherjacket larvae will provide a good indication of fields that are most likely to need treatment.
Sarah Hurry advises that if treatment is justified, use Dursban WG at 1.0 kg/ha or 1.5 l/ha of Equity applied in 200 to 1000 litres of water. "Dursban is an effective soil insecticide with a residual activity in the soil of 6-8 weeks. Pre-ploughing treatments are more effective than those applied once damage is underway in the new crop. Growers are warned that control will be reduced if temperatures are below 5oC as the larvae will move deeper into the soil profile."
Dursban WG (75% w/w chlorpyrifos formulated as a Water Dispersible Granule) is recommended for the control of Leatherjackets as well as Wheat Bulb fly, Frit fly and Wheat Blossom Midge. For Leatherjackets, two applications at dose rates of 1 kg/ha maybe applied. Currently Dursban WG is packed in 1 kg packs.
Equity (480 g/l chlorpyrifos formulated as a Emulsifiable Concentrate) is also recommended for the treatment of Leatherjackets but at 1.5 l/ha in 200 to 1000 litres per hectare of water on fields at risk.
