Probabilistic risk assessment of cotton pyrethroids: I. Distributional analyses of laboratory aquatic toxicity data

This is the first in a series of five papers that assess the risk of the co tton pyrethroids in aquatic ecosystems in a series of steps ranging from th e analysis of effects data through modeling exposures in the landscape. Pyr ethroid insecticides used on cotton have the potential to contaminate aquat ic systems. The objectives of this study were to develop probabilistic esti mates of toxicity distributions, to compare these among the pyrethroids, an d to evaluate cypermethrin as a representative pyrethroid for the purposes of a class risk assessment of the pyrethroids. The distribution of cypermet hrin acute toxicity data gave 10th centile values of 10 ng/L for all organi sms, 6.4 ng/L for arthropods, and 380 ng/L for vertebrates. For bifenthrin, cyfluthrin, lambda-cyhalothrin, and deltamethrin, the 10th centile values for all organisms were 15, 12, 10, and 9 ng/L, respectively, indicating sim ilar or somewhat lower toxicity than cypermethrin. For tralomethrin and fen propathrin, the 10th centiles were <310 and 240 ng/L, respectively. The dis tribution of permethrin toxicity to all organisms, arthropods, and vertebra tes gave 10th centiles of 180, 76, and 1600 ng/L, respectively, whereas tho se for fenvalerate were 37, 8, and 150 ng/L. With the exception of tralomet hrin, the distributions of acute toxicity values had similar slopes, sugges ting that the variation of sensitivity in a range of aquatic nontarget spec ies is similar. The pyrethroids have different recommended field rates of a pplication that are related to their efficacy, and the relationship between field rate and 10th centiles showed a trend. These results support the use of cypermethrin as a reasonable worst-case surrogate for the other pyrethr oids for the purposes of risk assessment of pyrethroids as a class.