Leatherjackets set for bumper year in sugar beet

UK - April 24, 2006

With leatherjacket populations at the highest levels for many years, East Anglia’s sugar beet growers must monitor crops closely for the pest, the larvae of crane flies or daddy-long-legs, this spring, advises Dow AgroSciences’ PestWatch Service.  

“Our sampling has shown populations across the UK are at the highest levels recorded in 15 years,” says Dow’s technical services specialist Jim Butchart. 

“The treatment threshold for an economic response is just 25 leatherjackets/m2 equivalent to 250,000/ha for sugar beet. Crops at highest risk are those planted after grassland, or following grassy unsprayed winter stubbles.”

Leatherjackets that hatched last autumn have grazed through the winter, increasing in size.  Crows and rooks feeding is a common sign of infested fields. 

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

However, growers who wait to find visible crop symptoms may be too late for effective treatment.  In affected areas, digging just below the surface will readily reveal active larvae. 

Dr Alan Dewar, from Broom’s Barn Research Station warns East Anglian sugar beet growers to look out for leatherjackets, no matter what the rotation, following unforeseen problems last year with a different species of crane fly. 

“The spotted crane fly (Nephrotoma appendiculata) caused serious damage in 2005 in sugar beet crops following cereals.  So check crops regularly after emergence and tackle leatherjacket damage as soon as it is seen,” advises Dr Dewar.  

He has another sure tip to check for leatherjackets before damage occur.  “Place black plastic sheets on the ground, weighted down in the corners.  Check under the sheet first thing the following morning for leatherjackets, before they disperse.  If they are there, treatment is likely to be worthwhile.” 

Mr Butchart confirms that leatherjackets of the spotted crane fly have already been found in sugar beet crops this year.  “By far the most effective product for leatherjacket control is Dursban WG (chlorpyrifos).  In very high-risk fields the best control results from an application prior to drilling.  All applications should be at 1 kg/ha in 200-1,000 l/ha of water. 

“As a general rule of thumb if you can find 10 or more leatherjackets per 30 cm of row control is most likely to be necessary,” recommends Mr Butchart.