Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

parathion was classified as synergistic toxicants, (i.e., more fish died than predicted based on an
additive response). Validation of chemical concentrations with analytical chemistry was not
conducted. Although the lack of raw data makes it difficult to determine exact concentrations
tested and the lack of analytical confirmation precludes determination of precise concentrations,
the study shows that binary combinations containing carbaryl exhibit greater toxicity than
exposure to carbaryl alone. Additionally, the majority of pesticide-containing mixtures tested
resulted in either additive or synergistic responses including mixtures containing carbamate and
OP insecticides.
Additive toxicity of binary combinations of carbamates and OPs at a cellular level was
demonstrated from in vitro experiments with Chinook salmon (Scholz, Truelove et al. 2006).
Carbaryl and carbofuran, in addition to the oxons of diazinon, chlorpyrifos, and malathion
caused additive toxicity as measured by AChE inhibition in salmonid brain tissue (Scholz,
Truelove et al. 2006). Further, the joint toxicity of the mixtures could be accurately predicted
from each insecticide’s toxic potency, simply by adding the two potencies together at a given
concentration. Since the experiments were conducted using in vitro exposures with the oxon
degradates and not with the parent compounds, the authors conducted subsequent experiments to
investigate whether additive toxicity as measured by AChE inhibition also occurred when live,
juvenile coho salmon were exposed for 96 h to the parent compounds, i.e., in vivo exposures.
The results of the second set of experiments were unexpected by the authors (Laetz, Baldwin et
al. 2009). Measured AChE inhibition from some of the binary combinations was significantly
greater than the expected additive toxicity, i.e., synergistic toxic responses were found (Laetz,
Baldwin et al. 2009). As with the in vitro study, brain AChE inhibition in juvenile coho salmon
(O. kisutch) exposed to sublethal concentrations of the carbamates carbaryl and carbofuran, as
well as the OPs chlorpyrifos, diazinon, and malathion, was measured (Laetz, Baldwin et al.
2009). Dose-response data for individual chemicals were normalized to their respective EC50
concentrations (AChE activity compared to control) and collectively fit to a non-linear
regression. The regression line was used to determine whether toxicological responses to binary
mixtures were antagonistic, additive, or synergistic. No binary mixtures resulted in antagonism.
Additivity and synergism were both observed, with a greater degree of synergism at higher
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