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

expected to result in toxicity were chlorpyrifos (OP), azinphos-methyl (OP), and endosulfan, the
N-methyl carbamates carbaryl, carbofuran, oxymyl, and fenobucarb all showed clear, likely or
assumed relationships between exposure and effect (Schulz 2004). No studies involving
methomyl were evaluated. It should be noted that these studies were not designed to establish
effect thresholds and in our review, are not sufficient to define thresholds. However, the data do
provide information on concentrations of insecticides known to cause biological and ecological
effects under field conditions.
Schulz (2004) noted that for all of the studies “that seem to establish a clear link between
exposure and effect, the pesticide concentrations measured in the field were not high enough to
support an explanation of the observed effects simply based on [laboratory bioassays] acute
toxicity.” Some authors have suggested differences in field measured exposure and actual
organismal exposure (aquatic, sediment, dietary; environmental variables that affect exposure) as
a reason for higher mortalities in situ than predicted by laboratory toxicity data. Schulz
concluded that on the basis of present knowledge, it cannot be determined whether the measured
concentration in the field regularly underestimates the actual exposure or if a general difference
between the field and laboratory reactions of aquatic invertebrates is responsible; both are
reasonable assertions. The review by Schulz shows a body of evidence that natural aquatic
ecosystems can be adversely affected by AChE inhibitors including carbaryl and carbofuran. We
expect a similar relationship for methomyl because it shares the same mechanism of action as
carbaryl and carbofuran (AChE inhibition) and concentrations that lead to death of aquatic
invertbrates are similar to carbaryl and carbofuran.
We reviewed multiple studies that showed direct, adverse effects to salmonid prey species
following exposures to N-methyl carbamates (Table 67). These included reduction in individual
aquatic species, changes in community abundance, richness, and diversity with salmonid prey
items.
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