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

exposure concentrations. Moreover, certain combinations of OPs were lethal at concentrations
that were sublethal in single chemical trials. Concentrations of each insecticide are listed in
Table 63. Based on a default assumption of dose-addition, the five pesticides were combined in
all possible pairings to yield target levels of AChE inhibitions in the brains of exposed coho
salmon.
Table 63. Concentrations (μg/L) of insecticides used in mixture exposures. EC50s were
calculated from dose-response data of AChE activity using non-linear regression. Coho salmon
exposed to 1.0, 0.4, or 0.1 EC50 treatments had an equipotent amount of each carbamate or OP
within the treatment e.g., to attain the 1.0 EC50 treatment for diazinon and chlorpyrifos, 1.0 μg/L of
chlorpyrifos (0.5 the EC50) was combined with 72.5 μg/L of diazinon (0.5 of the EC50).
Concentration of each ingredient in binary
combination to achieve treatment level
Insecticide Measured EC50
0.40 EC50 0.10 EC50
1.0 EC50 units
units
units
Carbaryl 145.8 72.9 29.2 7.3
Carbofuran 58.4 29.2 11.7 2.9
Chlorpyrifos 2.0 1.0 0.4 0.1
Diazinon 145.0 72.5 29.0 7.3
Malathion 74.5 37.3 14.9 3.7
As determined by the regression, these levels of enzyme inhibition would result from exposure to
0.1, 0.4, and 1.0 EC50 units, respectively. Two thirds (20/30) of pesticide pairs yielded AChE
levels that were significantly lower, i.e., indicative of synergism, than would be expected based
on additivity i.e., dose-addition (t-test with Bonferroni correction, p < 0.005). The number of
combinations that were statistically synergistic increased with increasing exposure
concentrations. All pairings analyzed at 1.0 EC50 showed synergism in AChE inhibition.
The combination of carbaryl at 79.9 μg/L and carbofuran at 29.2 μg/L to achieve 1.0 EC50
produced synergistic toxicity, however no incidences of coho mortality were observed. At 0.1
and 0.4 EC50 levels with the two carbamates, synergistic toxicity was observed, although the
deviation from the predicted additive response was not statistically significant. In binary
mixtures containing an OP and a carbamate synergistic toxicity occurred with some of the
combinations at 0.1 and 0.4 EC50 treatments. The mechanism for synergistic toxicity in
salmonids is unknown.
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