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Mortality in Oryzaephilus surinamensis following short‐term exposure to conditioned kernels by high‐density culture

The saw-toothed grain beetle, Oryzaephilus surinamensis (L.) is considered to be a secondary pest of stored grain because it cannot attack undamaged grain and can be found in association with primary pests such as Sitophilus sp. (Ciesielska, 1966). Mechanized harvesting of grain that fractures kernels may promote damage by O. surinamensis (Tuff & Telford, 1964; Howe, 1973). Thus if mechanical harvesting is practised, O. surinamensis (Armstrong & Howe, 1963; Flemming, 1988), Tribolium castaneum (Herbst) (White, 1982) and Cryptolestes ferrugineus Stephens (Tuff & Telford, 1964) may be regarded as primary grain pests. In contrast to the above observations, we have found 10 times more progeny produced after 20 weeks by 2000 O. surinamensis in 300 kg of whole wheat than in identical amounts of wheat with various gradients (3 to 15%) of cracked grains. The number of O. surinamensis at the surface of bins with cracked grain was higher than at the surface of the bin with whole grain, possibly because detritus in the space between whole grains prevented adults from moving. However, in ventilated 1 litre jars with initial densities of 250 adults per 0.5 kg of whole grain, populations of O. surinamensis were unable to develop. Our objectives were to examine the effects of the population density on the survival and reproduction of O. surinamensis, and to assess the influence of previously-inhabited 'conditioned' wheat on mortality. The term 'conditioned' is commonly used to qualify the medium in which stored grain insects have lived for some time (Park, 1936, 1937; Crombie, 1942; Ghent, 1963). Conditioned media may contain eggs, larval exuviae, feces and various chemicals resulting from prolonged infestation (Ghent, 1963; Hughes, 1982).