Leatherjackets set for bumper year

UK - February 16, 2006

Cereal growers may need to brush up on distinguishing between leatherjacket and slug damage this spring as daddy-long-legs larval populations are at the highest levels for many years across the UK.  

Leatherjackets feed at night on the soil surface causing shredding and holes in leaves – easy to confuse with the leaf shredding associated with slug damage.  But other symptoms include bare patches with discoloured leaves and seedlings, or tillers that can be easily pulled from the ground having been severed below the surface.   

Dow AgroSciences’ PestWatch Service is advising careful monitoring of late planted winter cereals and all spring planted crops.  

“Our sampling has shown populations in the Midlands and the North of England are at the highest recorded levels in 15 years,” says Dow’s technical services specialist Jim Butchart.  “Of the 47 sites sampled in England 83% contained leatherjacket populations above the advisory threshold, compared to only 34% last year.” 

Leatherjackets numbers in Scotland top the scale, with an average population of 250 grubs/m², the highest ever recorded in 30 years of sampling.  The treatment threshold for an economic response is just 50 leatherjackets/m² equivalent to 500,000/ha for spring cereals; 100/m² for established grassland; and 25/m² for sugar beet and other high value vegetable crops. 

Mr Butchart warns that the greatest damage usually happens in April and May.  “Leatherjackets that hatched in the autumn have continued to graze through the winter, increasing in size.  A common sign is crows and rooks feeding in infested fields.  But don’t rely on the visible crop symptoms before treating as it may be too late.  Dig just below the surface in affected areas and it’s easy to find the larvae.”  

Crops at highest risk are those planted after grassland, or following grassy unsprayed autumn stubbles.  Minimal tillage systems and the Countryside Stewardship option of non-sprayed autumn and winter stubbles to be followed by a spring crop may also be at risk. 

“By far the most effective product for leatherjacket control is Dursban WG.  In very high-risk fields best control results from an application prior to spring cultivations.  All applications should be at 1 kg/ha in 200-1,000 l/ha of water,” advises Mr Butchart. 

“As a general rule of thumb if you can find 15 or more leatherjackets per 30 cm of row in a spring cereal or a ley reseed, control is most likely to be necessary.  Half this number is required to justify treatment in high value vegetables and sugar beet,” says Mr Butchart.    

Extra cultivations pre-drilling can often destroy leatherjackets, rolling a crop will reduce the damage, and an increased dose of early nitrogen, promoting strong and early tillering, can help crops recover.