Crop Clinic

To-date, a total of 191 grass fields have been sampled in west and central Scotland between November 2008 and January 2009 as part of the SAC annual survey of leatherjacket infestation levels. Table 1 shows how these results from the current survey compare with the previous five years. The population categories in the Table are used by SAC when considering the risk to fields where either the grass is to be ploughed for spring crops or where the field will remain in grass for forage or fodder production (see Table 2).

Although a small number of fields remain to be surveyed in Ayrshire and Dumfrieshire/Kirkudbrightshire, it is clear from the 191 fields sampled to-date that grub populations in grasslands in central and south-west Scotland have increased from the very low levels observed last year. Overall, levels have risen to what would be termed a ‘generally average year’. Hence, there is some risk of damage to fields remaining as permanent grassland this year, with nearly 30% of the fields sampled containing populations in excess 1.0 million/ha. In addition, 20% of the 191 fields sampled contained grub populations in excess of 0.6 million/ha. Hence the risk of damage to spring cereals out of grub-infested turf has increased from last years levels, although the degree of risk to such spring-sown crops varies between SAC Advisory areas. The risk is particularly high in Bute, Lanarkshire, Renfrewshire and Stirlingshire/Perthshire and relatively high in Argyll and Ayrshire. Only in Dumfrieshire/Kirkudbrightshire and Wigtownshire does the risk appear low.

Despite the potential threat to spring cereals out of grass, uninformed insurance spraying is not recommended. Grub populations can vary, not only from area to area but also within an area and from field to field on the same farm. Consequently, even in years of high predicted risk, there will always be fields which contain lower levels of infestation. Hence it is recommended that an assessment of leatherjacket densities is conducted prior to deciding whether any insecticide application is necessary. Conducting such assessments in late winter and early spring identifies fields at risk before any damage has occurred. If leatherjacket infestations are found to be high an insecticide should be applied as soon as ground conditions permit. Pre-ploughing treatments are more effective than those applied once any damage is under way in the spring-sown crop. Assessment of leatherjacket infestations in individual fields can be undertaken by SAC as a chargeable service; details of costs are available from your local SAC Farm Business Services Office.

* 2008/09 interim results as a small number of additional fields have yet to be surveyed in Ayrshire and Dumfrieshire/Kirkudbrightshire

Though not included above, populations much lower than 0.60 million/ha may cause damage to root crops drilled to a stand and to spring direct reseeds.

SAC received financial support from the Scottish Government and Dow AgroSciences UK Limited towards the cost of this survey and a proportion of the remaining costs were offset by the farmers involved having their fields sampled at a substantially discounted rate.