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Stackyard News Feb 08

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    Leatherjacket Population Crashes but Risk Remains

Leatherjacket population levels in Scotland have crashed this winter from the extremely high levels recorded over the past four years. According to the findings of the annual survey carried out by SAC, the average population density of 0.32 million grubs per hectare is the fourth lowest recorded since the survey started over 30 years ago.



The risk of leatherjacket damage to fields remaining as permanent grassland is very low this year across all of the areas surveyed in south-west and central Scotland. However, over one-fifth of the 221 fields’ sampled contained populations greater than 0.6 million grubs per ha. Hence, there still remains a risk of damage to any spring crops sown into grassland fields which are currently harbouring grubs.

The degree of risk to spring-sown crops after grass varies from area to area. The risk is particularly high in Argyll, Bute, Renfrew and Wigtownshire and also relatively high in Ayrshire, Lanarkshire and Stirling/Perth. Only in Dumfries/Kirkcudbright does the risk appear low. Surveyed grub population levels in Stirling/Perth can be taken as a good indication of likely grub levels in non-surveyed areas (such as north-east and south-east Scotland) where ley-grassland is also a more common feature of the farmed landscape. Hence it is likely that the risk of damage to spring-sown crops after grass is also relatively high in the north-east and south-east this year.

The survey has been undertaken by SAC ecologist Dr Davy McCracken who advises farmers against insurance spraying:

“Despite this potential grub threat to spring crops out of grass, it is important that an assessment of leatherjacket infestation is conducted prior to the application of any insecticide treatment. Conducting such assessments now helps identify those fields at risk before any damage, and associated loss of productivity, has occurred. This allows any necessary control measures to be targeted cost-effectively.”

“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”.

SAC crop protection specialists advise that if leatherjacket infestations are found to be high, 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 preventative action against the grub straight away but to wait and monitor the emerging crops and treat only if necessary.

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