The effects of conveying on insects and mites infesting grain intended for export: Significance with respect to chemical treatments

Abstract

Published information suggests that conveying results in substantial mortality of insects and the relevance of this is discussed in relation to journey times and the time required for populations to recover to their initial level. However, much previous information does not distinguish between different methods of conveying and many of the most dramatic reductions were observed when a pneumatic conveyer was used. A survey of the eastern region British ports suggested that belt conveyers, a much gentler form of conveyance were normally used and these were not expected to cause such useful reductions in insect numbers.

Six experiments have been carried out to determine the effect of conveying on insects. In 2 of these, an elevator was used to pass the contents of an infested 1 t bin into an empty bin; in the other 2, an auger was used and in the final 2 a pneumatic conveyer was the subject of investigations. There was a small reduction in live adult Oryzaephilus surinamensis using auger or elevator but Sitophilus granarius numbers apparently increased, probably because the disturbance knocked them out of the grain. In contrast, the pneumatic conveyer caused unequivocal reductions in insect number.

Exposing the S. granarius from these experiments to pirimiphos-methyl on treated filter papers did not show a difference between susceptibility of the insects before and after conveying.

A series of experiments to devise a standard process to damage insects was carried out. Shaking insects in jars containing grain had a greater effect than tumbling, with T. castaneum being unaffected and S. granarius being less affected than O. surinamensis. A device, comprising a plastic cylinder, suspended at its centre so it could be rotated through 360/, was used in an attempt to damage insects in a standard fashion. This did not apparently damage them sufficiently to increase their susceptibility to insecticides. A final test showed that repeated impacts caused by shaking insects in jars had no effect on the susceptibility of the 3 species of insects to 3 pesticides.

Overall, it seems that only pneumatic conveying is likely to inflict sufficient insect damage to cause heavy mortality. The relatively gentle forces experienced during conveying prior to export, mainly using belt conveyers, do not appear to cause sub-lethal effects that enhance insecticide treatments sufficiently to allow lower doses to be effective.