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

same additivity model described in the previous Mixtures section to predict the cumulative
percentage of death in an exposed population. Table 78 shows the model’s prediction from three
scenarios. We did not use the data from ambient monitoring programs to devise a scenario
because the programs were not designed to capture peak concentrations from drift or runoff into
juvenile salmonid rearing areas. We did run a few modeling runs with median values of the
three insecticides taken from ambient monitoring data. The results showed no statistically
significant reductions in lambda for the four populations. In scenario 1, 5% of exposed
individuals are expected to die following 4 d exposures to the estimated concentrations from
PRZM-EXAMS 24 h averages from EPA’s BEs. For carbaryl, we selected a 2 lbs/acre applied
aerially with four applications to apples in Oregon which resulted in a concentration of 19 μg/L.
For carbofuran, we selected a 2 lb/acre ground application to artichokes in California which
resulted in a concentration of 35 μg/L. For methomyl, we selected 0.9 lbs/acre applied ten times
to lettuce in California which resulted in a concentration of 88 μg/L. If these events were to
occur in a watershed, 5% of the individuals are expected to die, which would not lead to a
population-level effect (i.e., reduction in lambda), based on juvenile mortality.
In contrast, the other two scenarios showed substantial and severe percent reductions in lambda
for each of the modeled populations; reductions in lambda ranged from 27-52% (Table 78) In
scenario 2, we selected 90 d GENEEC-modeled scenarios for corn from Table 50.
Concentrations of the three insecticides were as follows: carbaryl = 229 μg/L, carbofuran = 53
μg/L, and methomyl = 49 μg/L. The toxicity is largely a result of the carbaryl concentration, as
it was close to the 96 h LC50 of 250 μg/L. In scenario 3, concentrations from drift into off-
channel habitats (0.5 m deep) were calculated using the AgDrift model and included a 100 ft nospray
buffer for methomyl. For carbaryl, 5 lb/acre applied once aerially with a fine-medium
spray droplet size resulted in a concentration of 335 μg/L. For carbofuran, 1 lb/acre applied once
also with a fine-medium spray droplet size resulted in a concentration of 67 μg/L. For
methomyl, 0.9 lbs/acre applied once resulted in a concentration of 17.1 μg/L. Cumulatively,
these three concentrations would result in 89% mortality of exposed juveniles which reduce
lambdas of the four salmonid populations by 41-52%, a substantial effect.
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