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

Temperature and toxicity
We found no consistent correlation with temperature and toxicity of the three N-methyl
carbamates.
Studies with mixtures of AChE inhibiting insecticides
Because the three carbamates share a common mechanism of action, are registered for use and
applied in the same watersheds, and have demonstrated additive and synergistic effects in aquatic
organisms, we evaluate the response of salmonids and their habitat not just from exposure to
single carbamates, but also to common mixtures of N-methyl carbamates. We therefore include
an analysis of combinations of carbaryl, carbofuran, and methomyl based on additive toxicity
observed in recent publications with Chinook and coho salmon (discussed in the Risk
Characterization section) (Scholz, Truelove et al. 2006; Laetz, Baldwin et al. 2009). Because
NMFS identified mixture effects to listed salmonids as a critical data gap in understanding the
occurrences of multiple insecticides in salmonid habitats, a number of the experiments described
below were conducted by researchers at or associated with NOAA’s Northwest Fisheries Science
Center.
One of the earliest mixture studies available evaluated bluegill survival following a range of
exposure durations (24, 48, 72, or 96 h) to binary combinations of 19 insecticide mixtures
(Macek 1975). Carbaryl and several of the OP pesticides were tested. Macek (1975) used the
equation AB/(A+B) = X to calculate mixture toxicity; where AB was the number of dead fish
from a mixture of pesticides A and B, and A + B was the sum of dead fish from A and B alone.
The resulting ratios, X, were designated by the author as less than additive for a ratio of less than
0.5, additive when the ratio fell between 0.5 and 1.5, and synergistic for a ratio of more than 1.5.
Carbaryl containing mixtures resulted in additive toxicity for some compounds (DDT,
methoxychlor, parathion, methyl parathion) and synergistic toxicity for other compounds
(malathion, copper sulfate). Antagonism is when the cumulative toxicity of a mixture is less
than additive. In this study, mixtures containing carbaryl were not antagonistic. Differences in
classification between additive and synergistic combinations should be interpreted cautiously, as
the threshold for synergism was arbitrarily set at 1.5 by the authors. Mixture results with DDT
and toxaphene were 1.31 and 1.14, respectively. The binary combination of diazinon and
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