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

the fish to determine the cause of death. EPA characterized carbofuran as a “possible” cause of
the incident. It seems likely that the three cholinesterase inhibiting insecticides may have
resulted in a cumulative exposure that was sufficient to cause the fish kill due to mixture toxicity.
Incident I000108-001: DuPont reported a fish kill in 1992 in southern Georgia that occurred
following an application of Lannate LV (methomyl). One hundred and twenty-five fish were
found dead in a pond located 50 to 75 yards from a large field of sweet corn. The 200 acre field
was treated with Lannate LV five times between June 1 and June 16. Additionally, Lorsban
insecticide (chlorpyrifos) was applied four times during the same interval including coapplications
of four fertilizer treatments. On the day of the fish kill (June 16), applications of
both Lannate and Lorsban were made. Lannate LV was applied at rates of 1.5 pints/acre and
Lorsban was applied at a rate of 1 pint/acre. During the first 16 days of June a total of 10.52
inches of rain were received. The State of Georgia took water samples on June 17. Analysis
indicated 136 µg/L of methomyl in the pond. Chlorpyrifos concentrations were not reported and
it is not known if it was an analyte. Water temperatures were relatively high (reported to be
about 90ºF) but dissolved oxygen was reported to be “normal (7.2-10)”. DuPont suggested the
high temperatures, fertilizers, and suspended solids could be stressful to fish and low oxygen
content at night might have played a part in the deaths. The species involved in the incident
(carp and bluegill) were warm water fishes. EPA characterized methomyl as a “probable” cause
of the fish kill. Given the reported rapid dissipation of methomyl it is likely that the
concentrations measured do not represent peak concentrations that occurred in the pond. It
seems likely that chlorpyrifos may have also been a contributing factor in this incident.
Mixture Analysis of Carbaryl, Carbofuran, and Methomyl
As noted earlier, pesticides most often occur in the aquatic environment as mixtures. In our
review and synthesis of the available exposure and response information, we find the three Nmethyl
carbamates carbaryl, carbofuran, and methomyl share the same mechanism of toxic
action (AChE inhibition) and are expected to co-occur in salmonid habitats. Therefore, we
employ a simple mixture analysis derived from empirical data with Pacific salmonids to predict
potential effects to individual salmonid’s AChE activity and their survival from short-term
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