Comparing ecological risks of pesticides: the utility of a Risk Quotient ranking approach across refinements of exposure

Environmental risk assessment of pesticides and other chemicals often uses the Risk Quotient (RQ) method to characterize risk quantitatively. An RQ is calculated by dividing an environmental exposure value by a toxicity end-point value. Tier 1 RQs, which are characterized by highly conservative toxicity and exposure assumptions, are used primarily for screening out negligible risks in regulatory decision making. It has been argued that the tier 1 RQ approach is valuable for making direct comparisons of quantitative risk between pesticides. However, an outstanding question is whether relative risks among pesticides would change if refinements of exposure are incorporated into the RQ calculations. This study tested that hypothesis. Aquatic ecological risk assessments were conducted for 12 herbicide and 12 insecticide active ingredients used on agricultural crops in the USA. The pesticides were chosen because surface-water monitoring data for them were available as part of the United States Geological Survey's National Water-Quality Assessment Program (NAWQA). Ecological receptors and effects evaluated were aquatic non-vascular plants (acute risk), aquatic vertebrates (acute risk) and aquatic invertebrates (acute risk) for the herbicides and aquatic vertebrates (acute and chronic risk) and aquatic invertebrates (acute and chronic risk) for the insecticides. The data indicate that there were significant statistical correlations between numerical rankings of tier 1 RQs and RQs using refined environmental exposures. The results support the hypothesis that numerical ranking of RQs for the purpose of comparing potential ecological risks is a valid approach because the rankings are significantly correlated regardless of the degree of exposure refinement.     

Insecticidal activity of menthol derivatives against mosquitoes

BACKGROUND: The insecticidal activity of essential oil of Mentha piperita L. emend. Huds. against local mosquitoes as disease vectors was recognized and found to be due to the presence of menthol, which is the major aroma compound of the oil. The minor compounds of the oil, i.e. menthone, beta-caryophyllene, menthyl acetate, limonene, alpha-pinene and pulegone, showed either less or no activity against the mosquitoes tested. L-Menthol derivatives were synthesized and their knockdown effect and mortality were evaluated against local mosquitoes of Culex quinquefasciatus Say, Aedes aegypti L. and Anopheles tessellatus Theobald as disease vectors. This is the first report of mosquitocidal activity of menthol and its derivatives against Cx. quinquefasciatus, Ae. aegypti and An. tessellatus. RESULTS: Derivative synthesis followed by structure-activity relationship studies identified several derivatives, i.e. menthyl chloroacetate, menthyl dichloroacetate, menthyl cinnamate, menthone glyceryl acetal, thymol, alpha-terpineol and mugetanol, with enhanced mosquitocidal activity against Cx. quinquefasciatus, Ae. aegypti and An. tessellatus relative to the parent compound L-menthone. CONCLUSION: In ester derivatives of L-menthol the optimum activity is dependent on the size and shape of the ester group and the presence of chlorine atoms in the ester group. In structurally related derivatives of L-menthol the optimum activity is dependent on the aromaticity, the degree of unsaturation, the position of the hydroxy group and the type of functional group.