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

effects of thiodicarb and other pesticide mixtures containing carbaryl, carbofuran, and methomyl
were not addressed in the BEs.
Cholinesterase inhibiting insecticides, including carbaryl, diazinon, chlorpyrifos, and malathion
are the most frequently detected mixtures in urban streams across the U.S. (Gilliom, Barbash et
al. 2006).
Gilliom and others (2006) suggested that assessment of pesticide mixture toxicity to aquatic life
is needed given the widespread and common occurrence of pesticide mixtures, particularly in
streams, because the total combined toxicity of pesticides in water is often greater than that of
any single pesticide compound. Exposure to multiple pesticide ingredients can result in additive
and synergistic responses as described below in the Risk Characterization section. It is
reasonable to conclude that compounds sharing a common mode of action cause additive effects
and in some cases synergistic effects. CDPR’s most recent pesticide use report indicates
6,857,530 lbs of cholinesterase-inhibiting insecticides were applied in California during 2006.
Over 60 cholinesterase-inhibiting a.i.s are currently registered in California (CDPR 2007).
Exposure to these compounds and other baseline stressors (e.g., thermal stress) was not a
consideration in the BEs. Therefore, risk to listed species may be underestimated.
Exposure Conclusions
Pacific salmon and steelhead use a wide range of freshwater, estuarine, and marine habitats and
many migrate hundreds of miles to complete their life cycle. Carbaryl, carbofuran, and
methomyl are commonly detected in freshwater habitats within the four western states where
listed Pacific salmonids are distributed. Because the proposed action of registration of the three
a.i.s for the next 15 years authorizes many of the same uses, these three a.i.s will continue to be
present in the action area. Therefore, we expect some individuals within all the listed Pacific
salmon and steelhead ESUs/DPSs will be exposed to these chemicals and other stressors of the
action. Carbaryl can exceed several mg/L in coastal estuaries based on measured environmental
concentrations. Carbaryl and carbofuran concentrations in off-channel habitat can also exceed
several mg/L where buffers are lacking. Peak concentrations of methomyl are expected to be
significantly lower given label restrictions that require buffers to aquatic habitats and a relatively
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