Assess leatherjacket risk to target spray effectively

12 February 1999

Assess leatherjacket risk to target spray effectively

FOR the second year running, leatherjacket grub populations are high across most of the West of Scotland, says Davy McCracken of SACs Environmental Division.

But despite the predictions of a serious grub threat, he advises against blanket spraying.

"Even where past history of problems may suggest that fields are at risk from leatherjacket attack, it is important that some form of assessment of leatherjacket infestation is conducted prior to the application of any insecticide treatment" says Dr McCracken.

Conducting such assessments in late winter/early spring 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.

Of 89 grass fields sampled during December 1998 and January 1999, in the SACs annual survey, over 50%, in almost all areas, harboured leatherjacket populations of more than 0.6m/ha, indicating a very substantial and widespread risk of damage to spring cereals. The fact that at least a light infestation of grubs is present in almost all fields also poses a serious threat to root crops drilled to a stand, the SACnotes.

Research suggests leatherjacket infestations are likely to be higher in fields which have a tendency to water-logging and/or which are flat or predominantly east or south facing.

Fields which have been used for silage and have received applications of dung or slurry the previous year also tend to have higher infestation levels, especially if the sward height was relatively high in the late summer/early autumn.

Where leatherjacket infestations are very high a spray should be applied as soon as ground conditions permit.

Past SAC trials have indicated that pre-ploughing treatments are more effective than those applied once damage is under way in the replacement crop. But 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 developing crops and spray only if necessary. &#42

Super-spreader cuts cost of control

SAC research shows that a new "super-spreader" adjuvant and half-rate pesticide combination can cut the cost of controlling leatherjackets in crops following grass.

Research over two years on spring cereals by SAC-Aberdeen agricultural entomologist Collette Coll shows growers can halve the rate of chlorpyrifos need to control leatherjackets by using the adjuvant Silwet L-77.

The adjuvant appears to help the pesticide move more thoroughly within the seed-bed. As a result it is better distributed and in closer contact with more grubs. At the same time Silwet L-77 assists penetration of the pesticide into the leatherjacket.

In the trials, the reduced rate of 1 litre/ha or even 0.75 1itre/ha of chlorpyrifos with L-77 gave the same level of control as the full rate of insecticide on its own. "This makes the combination a good option on spring crops under threat," says Dr Coll.

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