The impact of insecticides and herbicides on the biodiversity and productivity of aquatic communities.

Relyea (2005a), asserts that current application rates for the glyphosate formulation Roundup are highly lethal to many amphibian species. We strongly disagree with this conclusion since it is based on inappropriate generalization and extrapolative inference from a single test that is arguably irrelevant to most real-world scenarios. We contend that the application rates and resultant aqueous exposure concentrations used in this experiment are quite atypical of those associated with dominant uses of glyphosate formulations in agriculture, forestry, and industrial sectors. Two specific statements in the Relyea paper serve as focal points for concern: 1) This study represents one of the most extensive experimental investigations of pesticide effects on aquatic communities and offers a comprehensive perspective on the impacts of pesticides when non-target organisms are examined under ecologically relevant conditions. 2) Collectively, the available data indicate that, contrary to conventional wisdom, current application rates of Roundup can be lethal to many species of amphibians. Both statements exemplify a tendency for overgeneralization and excessive extrapolation. We submit that no single experiment represents an extensive investigation of pesticide effects on aquatic organisms. Neither could it offer a comprehensive perspective on such a broad topic area. Bartell et al. (1992), notes that ecological risk estimation is complex and requires, as a minimum, full characterization of concentration–response relationships and determination of the range of real-world exposure concentrations relevant to major use patterns. Resultant risk estimates are based on the probability that significant deleterious effects may be generated under real-world exposure regimes. A comprehensive perspective would require review and critical analysis of hundreds of previously published papers. None of these fundamental requirements are met in this publication. We certainly agree with the author on the general value of mesocosm studies in assessing potential effects of pesticides in aquatic systems. However, as discussed previously (e.g., Touart and Slimak 1989, Thompson 2004), mesocosm testing should be considered as one element in a multi-tiered research program that generates data, which when considered in whole, may be sufficient for effective estimation of ecological risk. The industry and complexity involved in conducting a single mesocosm study does not confer broad spectrum inference potential. Moreover, results must be considered in relation to relevant pre-existing data and with due consideration for the ‘‘weight of scientific evidence’’ principle. These criteria are also not met by the Relyea (2005a) publication. This is demonstrated by the fact that a number of directly relevant papers (e.g., Berrill et al. 1997, Lajmanovich et al. 2003, Chen et al. 2004, Edginton et al. 2004, Howe et al. 2004, Thompson 2004, Thompson et al. 2004, Wojtaszek et al. 2004) comprising a significant proportion of the preexisting body of knowledge on potential effects of glyphosate herbicides to amphibians were neither cited nor discussed in the paper. This omission is particularly disquieting given the author’s explicit statement (Relyea 2005a:625) that ‘‘Prior tests of glyphosate on amphibians have been rare,’’ the fact that several of these papers draw opposing conclusions, and the author’s assertion as to what the ‘‘collective data’’ on this topic indicate. In our view, neither the specific results of this experiment nor the collective data support the conclusion that Roundup is likely to be lethal to many species of amphibians under environmentally realistic use scenarios. This viewpoint is based on the general lack of relevance of experimental results to typical use patterns and application rates, and available data on measured concentrations in lentic systems associated with the predominant use of glyphosate formulations in agriculture, forestry or industrial sectors. In addition, several methodological issues and design flaws result in substantial uncertainty in extrapolation of test results to effects that might be expected in natural aquatic systems. To facilitate cross-comparisons and avoid confusion, all application rates and aqueous concentration values in this letter have been converted to units of glyphosate acid equivalents (a.e.). In the case of formulations containing the isopropylamine (IPA) salt, as for the test material in the Relyea study, the appropriate conversion factor is based on the molecular weight ratio of glyphosate acid to isoproplyamine salt (356/480 1⁄4