An Investigation of the Changes in a Bin of stored Wheat infested by Insects

Abstract

One and a half tons of Plate wheat, of water content 12·9 per cent., infested by insects, especially the small form of C. oryzae, Rhizopertha, and Laemophloeus, were placed in an iron bin in May 1941. By the following November the wheat was heating near the surface and was extremely heavily infested in the 25 cm. surface layer. The carbon dioxide concentration beneath this layer exceeded 10 per cent.During December the hottest depth in the grain was nearer to the surface. The temperature reached 37°C, 27°C. above air temperature. Many weevils came to the surface in early December and walked off the grain. Then in mid December the grain temperature fell rapidly and reached air temperature by the end of the month. A week later many insects were still alive, but by March 1942 very few living insects were found. The carbon dioxide concentration fell rapidly but rose again in March, presumably due to the increased grain water content and the respiration of micro-organisms.In 1942, conditions became suitable for insect activities in April, and by May an infestation had started, this time predominantly C. granaria. Laemophloeus was also present, having apparently survived the winter in the bin, but neither C. oryzae nor Rhizopertha had survived. Insects were confined to the surface layers by the carbon dioxide present deeper in the bin. Consequently heating did not occur in 1942, and when the grain was moved in October 1942, its temperature was falling after reaching a maximum of 27°C. in August.The effect of the presence of insects upon grain is discussed. It is shown that the conditions arising in the bin resemble those in general storage, except that the use of an open topped bin with airtight sides and bottom caused a much greater accumulation of carbon dioxide than is usual.It is suggested that if the grains infested by weevils are buried in the bulk, heating is likely to occur, whilst if the infestation is superficial the grain may not heat. The part played by weevil larvae in the heating of grain is stressed.It is suggested that regular examination of samples of grain from deep in a bulk should be made for insects and that all bulks should be examined soon after being placed in position. Routine temperature and water content readings should be made at points deep in the grain and about 10–20 cm. below the surface.