Effect of diet on phosphine toxicity, rate of development and reproduction of the rice weevil Sitophilus oryzae (Linnaeus)

One of the loci responsible for strong phosphine resistance encodes dihydrolipoamide dehydrogenase (DLD). The strong co-incidence of enzyme complexes that contain DLD, and enzymes that require thiamine as a cofactor, motivated us to test whether the thiamine deficiency of polished white rice could influence the efficacy of phosphine fumigation against insect pests of stored grain. Three strains of Sitophilus oryzae (susceptible, weak and strong resistance) were cultured on white rice (thiamine deficient), brown rice or whole wheat. As thiamine is an essential nutrient, we firstly evaluated the effect of white rice on developmental rate and fecundity and found that both were detrimentally affected by this diet. The mean time to reach adult stage for the three strains ranged from 40 to 43 days on brown rice and 50–52 days on white rice. The mean number of offspring for the three strains ranged from 7.7 to 10.3 per female over a three day period on brown rice and 2.1 to 2.6 on white rice. Growth and reproduction on wheat was similar to that on brown rice except that the strongly resistant strain showed a tendency toward reduced fecundity on wheat. The susceptible strain exhibited a modest increase in tolerance to phosphine on white rice as expected if thiamine deficiency could mimic the effect of the dld resistance mutation at the rph2 locus. The strongly resistant strain did not respond to thiamine deficiency, but this was expected as these insects are already strongly resistant. We failed, however, to observe the expected synergistic increase in resistance due to combining thiamine deficiency with the weakly resistant strain. The lack of interaction between thiamine content of the diet and the resistance genotype in determining the phosphine resistance phenotype suggests that the mode of inhibition of the complexes is a critical determinant of resistance.

Effect of Increasing Low Dose Simplexin Exposure in Cattle consuming Pimelia trychostachya

Pimelea species (or desert riceflower) are small native plants endemic to the drier inland pastoral regions of Australia, which cause a unique syndrome in grazing cattle characterised by submandibular oedema and oedema in the brisket area as a result of right-sided heart failure attributed to the toxin simplexin. Field evidence suggests that poisoning can occur through minor, inadvertent consumption of Pimelea plant material, but the minimum simplexin intake required to induce Pimelea poisoning is not known. In this study, mild Pimelea poisoning was induced at a daily dose of 12.5 mg Pimelea/kg bodyweight per day, equivalent to 2.5 µg simplexin/kg bodyweight per day, demonstrating the high potential toxicity of these plant species. Effects in all animals diminished with prolonged low dose feeding and we postulate that these animals developed mechanisms for detoxifying simplexin, 1, possibly through rumen bacteria adaptation or activation of liver enzymes.