The focus for Pestwatch moves from Wheat Bulb fly to Leatherjackets and starts with a warning of variable risk of this pest in different parts of the country plus advice to assess risk on a local basis.
Each autumn Dow AgroSciences commission 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 http://www.dowagro.com/uk/cereal/pest.htm . The latest survey indicates a medium to low risk in Scotland and low risk in England with the exception of Yorkshire where mean leatherjacket numbers increased by 126% compared with 2009.
Sarah Hurry of Dow AgroSciences explains that the results give an indication of the risk to susceptible crops such as spring barley and sugar beet. "Risk is based on the threshold of 60 leatherjackets/m2 in grassland preceding winter and spring cereals. However, a lower population, more like 30/ m² may cause damage to root crops."
Pestwatch indicates that in Scotland a total of 219 grass fields were sampled between November 2010 and March 2011. The survey showed that leatherjacket populations had decreased from the very high levels seen last year. Nevertheless 39% of fields tested had levels over the 60 leatherjackets/m2, compared with 67% in 2010. The risk is highest in the areas of Argyll, Ayrshire and Bute.
In England a total of 49 grassland sites were sampled during autumn 2010. The overall mean leatherjacket population was 18.1/m2 in 2010 compared with 15.1/m2 in 2009, 7.4/m2 in 2008 and 15.7/m2 in 2007. The long term mean for all sites in the period 1993 to 2010 is of 61.2/m2.
The highest populations were found in Yorkshire where 139 leatherjackets/m² was recorded.
In the Northern Region overall, covering North, South and West Yorkshire, the overall mean was 32.5/m2 (mean 20 sites). In the Midlands/Western Region (Cheshire, Shropshire, Staffordshire, Warwickshire and Herefordshire), the overall mean was 8.9/m2 (mean 20 sites). In the Southern Region of Berkshire, Hampshire and Oxfordshire, the overall mean was 6.0/m2.
Sarah Hurry points out that the survey suggests a low to medium risk of damage from leatherjackets to susceptible crops in Scotland and a low risk in England. "But leatherjacket populations vary from field to field and local risks should be assessed before treatment is made. Risk assessments and testing for the presence of leatherjacket larvae will provide a good indication of fields that are most likely to need treatment. 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. High infestations should be treated as soon as ground conditions permit and when leatherjackets are feeding."
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 mineral soil of 6-8 weeks. Growers are warned that control will be reduced if temperatures are below 5oC as the larvae will move deeper into the soil profile."
