QUINONE CONTAMINATION OF DEHUSKED RICE BY TRIBOLIUM-CASTANEUM (HERBST) (COLEOPTERA, TENEBRIONIDAE)

The accumulation of quinone secretions, from the abdominal and thoraci c defence glands of adult Tribolium castaneum, was investigated in deh usked rice of four different milling degrees, Three quinones were dete cted in measurable quantities, 2-methyl-1,4-benzoquinone, 2-ethyl-1,4- benzoquinone and 2-ethylhydroquinone, but these were present at very l ow concentrations, Even after substantial exposure to adults, equivale nt to an adult-week product of 1345 on 80 g of rice, the combined tota l contamination with these three compounds was only about 1.0 ppm. The accumulation rate of quinones in the samples was estimated to range f rom 0.0470 to 0.0631 mu g/adult/week and seemed unaffected by the mill ing degree of the rice, However, the distribution of quinone contamina tion in the samples between the rice grain and insect frass was influe nced by milling degree, In unpolished rife, about 80% of quinone conta mination was in the rice itself rather than in the frass, while in ric e of 95% milling degree the contamination was more or less evenly dist ributed. 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 b y T. castaneum for typical storage periods, e.g. 6-12 months, Even the highest observed degree of contamination, 1.0 ppm, would in theory ta ke about 32 y to accumulate if adult beetles remained at a relatively high average density of 10 adult T. castaneum/kg. These conclusions re quire confirmation by direct observation of rice after long-term wareh ouse storage in the tropics. Copyright (C) 1996 Elsevier Science Ltd