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

Dose-addition assumes the cumulative toxicity of the mixture can be predicted from the sum of
the individual toxic potencies of each component of the mixture. Within the past decade,
government regulatory bodies started to use dose-addition models to predict toxicity for those
chemicals that share a common mode of action. In California, the CVRWQB used dose-addition
models (based on the toxic-unit approach) to develop TMDLs for the OP insecticides diazinon
and malathion. NMFS Biological Opinions have also recognized the environmental reality of
co-occurring pesticides in species’ aquatic habitats and applied additive toxicity models to
predict potential responses of salmonids (NMFS 2004; NMFS 2005; NMFS 2005; NMFS 2005;
NMFS 2008).
In salmon, dose-additive inhibition of brain AChE activity by mixtures of OPs and carbamates
was demonstrated in vitro (Scholz, Truelove et al. 2006). More recently, it has been found that
salmonid responses to OP and carbamate mixtures vary in vivo; some interactions were
synergistic, rather than just additive (Laetz, Baldwin et al. 2009). We used the dose-addition
method to predict responses by applying the modeling exposure estimates and measured
concentrations of carbaryl, carbofuran, and methomyl presented in the Exposure Analysis.
Effects of the three carbamates individually and in combination on AChE inhibition Figure 42
(A) and survival (B), are shown below. Based on additivity, the mixture is expected to be more
toxic than the individual carbamates for both endpoints. Due to the steep slopes of the two doseresponse
curves, and especially the mortality slope, small changes in concentrations elicit large
changes in observed toxicity. The exposure values represent concentrations from EPA PRZM-
EXAMs 60 d average modeling estimates for surface waters (carbaryl: 4 aerial applications at 5
lbs a.i./acre, citrus in FL; carbofuran: 1 ground application at 2 lbs a.i./acre, artichokes in CA;
methomyl: 3 aerial applications at 1.8 lbs a.i./acre, peaches). We recognize that this approach is
likely to under-predict toxicity for mixtures that produce synergistic rather than additive
responses (Laetz, Baldwin et al. 2009).
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