Quinone contamination of dehusked rice by Tribolium castaneum (Herbst) (Coleoptera:Tenebrionidae)

The accumulation of quinone secretions, from the abdominal and thoracic defence glands of adult Tribolium castaneum, was investigated in dehusked rice of four different milling degrees, Three quinones were detected in measurable quantities, 2-methyl-1,4-benzoquinone, 2-ethyl-1,4-benzoquinone and 2-ethylhydroquinone, but these were present at very low concentrations, Even after substantial exposure to adults, equivalent to an adult-week product of 1345 on 80 g of rice, the combined total contamination with these three compounds was only about 1.0 ppm. The accumulation rate of quinones in the samples was estimated to range from 0.0470 to 0.0631 mu g/adult/week and seemed unaffected by the milling degree of the rice, However, the distribution of quinone contamination in the samples between the rice grain and insect frass was influenced by milling degree, In unpolished rife, about 80% of quinone contamination was in the rice itself rather than in the frass, while in rice of 95% milling degree the contamination was more or less evenly distributed. The low quinone contamination rates observed in rice suggest that the accumulation of these chemicals is unlikely to pose a serious health threat when rice is exposed to 'normal' rates of infestation by T. castaneum for typical storage periods, e.g. 6-12 months, Even the highest observed degree of contamination, 1.0 ppm, would in theory take about 32 y to accumulate if adult beetles remained at a relatively high average density of 10 adult T. castaneum/kg. These conclusions require confirmation by direct observation of rice after long-term warehouse storage in the tropics. Copyright (C) 1996 Elsevier Science Ltd