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Prevention and control of mite infestation in rapeseed


HGCA PROJECT REPORT 376

Prevention and control of mite infestation in rapeseed


by

David Armitage
Central Science Laboratory, Sand Hutton, York,  YO41 1NZ

September 2005

Abstract

At the end of July and beginning of August 2003, six loads of rapeseed, varying in mc between 7.3 and 8.3%  moisture content (mc) were received. These comprised three of Recital, two of Pollen and one of Royal. Since the intention in this first year was to assess the risks of current practice - storage at 9% mc (and ventilation using equipment designed for cereals aeration), attempts were made to dampen the grain and this resulted in mcs between 8.9 and 9.8% beneath the surface. This coincided with equilibrium relative humidities (erh) of 73-77% at 25°C as determined by dew-point meter. 

The two driest bins were left untreated as controls; the two dampest were top-dressed with diatomaceous earth at 3g/kg, and the others were treated at 1g/kg. After 12 weeks, the moisture content at the surface of all bins had reached nearly 12% and there were several thousand mites per kg, despite the DE treatments. Beneath the surface, numbers varied between a few hundred per kg in the driest grain to over 10,000/kg in the dampest. 

Despite ventilation, heating of seed to over 35°C occurred in one bin at 9.1-9.2% mc and this had to be remedied by higher-rate ventilation. To achieve the first cooling front, reducing temperatures from 20-25°C to 15°C took 420h, nearly 3x the expected fan hours to achieve the same cooling with cereals, but problems with the automatic cooling regime delayed the second cooling front.

At the beginning of August 2004, three ca 25t loads of rapeseed, varying in mc between 7.1 and 7.3% were received. Each was divided between two bins. The aim was to assess the risks associated with current best practice, ie storing at 7.5% mc and at half depth to account for the higher resistance of rapeseed in comparison with wheat and the consequent effects on cooling speed.  

The two driest bins were left untreated as controls, the two dampest were top-dressed with DE at 3g/kg, and the others were treated at 1g/kg.  Mites reached levels of several thousand per kg at the surface of control bins where the mc exceeded 11% but numbers were below 100/kg below the surface and where top dressing of DE had been applied. Mean temperatures at 1m fell below 15°C by early October after 200h aeration to below 10°C. However, despite the greater aeration rate, temperatures fell little more rapidly than in the first year; the only difference was in the fewer hours of aeration required.

 

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