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

Additionally, pairings of two OPs produced a greater degree of synergism than mixtures
containing one or two carbamates. This was particularly true for mixtures containing malathion
coupled with either diazinon or chlorpyrifos. At the highest exposure treatment, 1.0 EC50
(malathion at 37.3, chlorpyrifos at 2, diazinon at 72.5 μg/L), binary combinations produced
synergistic toxicity. Many fish species die following high rates of acute brain AChE inhibition,
between 70-90% (Fulton and Key 2001).
Coho salmon exposed to combinations of diazinon and malathion (1.0 and 0.4 EC50) as well as
chlorpyrifos and malathion (1.0 EC50) all died (Laetz, Baldwin et al. 2009). This result was in
contrast to the predicted AChE inhibition from in vitro tests with Chinook salmon. Coho
exposed to these OP mixtures showed toxic symptoms of inhibition of AChE, including loss of
equilibrium, rapid gilling, altered startle response, and increased mucus production before dying.
OP combinations were also synergistic at the lowest concentrations tested. Diazinon and
chlorpyrifos were synergistic when combined at 7.3 μg/L and 0.1 μg/L, respectively. The
pairing of diazinon (7.3 μg/L) with malathion (3.7 μg/L) produced severe (> 90%) AChE
inhibition, including classical signs of poisoning as well as death. We expect that juvenile
salmonids exposed to these effect concentrations in the environment will respond similarly.
Multiple studies indicate compounds that share a common mode of action frequently result in
additive and at times synergistic responses in aquatic organisms (Anderson T. D. and Lydy 2002,
Belden et al. 2007, Bocquene et al. 1995, Jin-Clark et al. 2002, Laetz et al. 2009, Lydy and
Austin 2005, Macek 1975, Monserrat et al. 2001, Scholz et al. 2006). Unfortunately, we are
unable to create a predictive model of synergistic toxicity as dose-response relationships with
multiple ratios of pesticides are not available at the present time and the mechanism of synergism
remains to be determined. That said, we conducted a mixture analysis with carbaryl, carbofuran,
and methomyl based on additive toxicity with the caveat that synergism is likely where
circumstances mirror the experimental conditions of this study, i.e., similar exposure durations
and pesticide concentrations. This is a reasonable approach based on the current state of the
science. We used the mixture work of Laetz, Baldwin et al. (2009) and Scholz, Truelove et al.
(2006) to construct mixture dose-response relationships predicated on additivity (see mixture
analyses in the Risk Characterization section).
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