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

expect co-occurrence of the three insecticides, we expect synergism may occur if high enough
concentrations exist. Generally, these concentrations must exceed individual LC50s for the three
compounds, which is most likely to occur in areas with extensive crop uses where applications
overlap in space and time. In these areas, even more fish would die from synergistic effects than
predicted based on the additive toxicity for carbaryl, carbofuran, and methomyl. Juveniles and
returning adults may experience synergistic toxicity. Whether or not death occurs is dependent
on exposure duration and concentrations of the three insecticides. Typically, adults are less
sensitive than early lifestages, however it is very difficult to conduct toxicity assays with prespawn
adult salmoninds. Pre-spawn adults have used up most of their accumulated fat,
converting it into gamete production, and will die soon after spawning. We are unsure how
sensitive these adults are to toxic pesticides, but expect in their physiological state that they will
be as or more sensitive than juveniles. We expect that some adults that occupy or spawn in
shallow, low flow systems will be impacted from synergistic toxicity. Therefore, populationlevel
effects could be more pronounced, depending upon the number of individuals and the
importance of those individuals to the survival and recovery of the population. We conclude that
based on the expected environmental concentrations of the three insecticides, synergism is likely
in many off-channel habitats resulting in increased rates of death to juveniles and to adults.
Toxicity from other stressors of the action
We identified inert ingredients, adjuvants (NP), tank mixtures (recommended on pesticide
product labels), degradates (1-napthol, 3-hydroxycarbofuran), and other pesticide a.i.s
(malathion, bifenthrin, rotenone, metaldehyde, captan, and cupric sulfate) as toxic to salmonids
and their prey. There remain substantial data gaps on the expected concentrations of these
chemicals in salmonid habitats. However, some chemicals are detected at concentrations that
pose substantial risk to listed salmonids and their prey (e.g., malathion, NP). The risk posed by
these other stressors to salmonid populations is complicated by the same factors we discussed for
carbaryl, carbofuran, and methomyl (i.e., the numbers of individuals exposed, the uncertainty
surrounding the temporal and spatial uses of these chemicals, etc.). Severe population reductions
of Atlantic salmon in Canada were attributed specifically to the use of NP within a pesticide
formulation (Fairchild, Swansburg et al. 1999; Brown and Fairchild 2003). We conclude that
given the use and co-application of these chemicals with carbaryl, carbofuran, and methomyl,
exposed individuals are at increased risk of the suite of toxic effects expected from these
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