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

BE provides a peak EEC of 5.5 μg/L in water following an application rate of 10 lbs
a.i./acre. Yet three simulations conducted at a 10-fold lower application rate produced
greater estimates of exposure due to differences in site-specific assumptions (Table 49).
NMFS also questions whether input assumptions were adequate to represent the range in
variability among sites throughout the action area. Site-specific meteorological and soil
conditions vary greatly throughout the four states where listed salmonids are distributed
and crops are grown. The BEs did not indicate site-specific input assumptions of each
scenario nor did they put these assumptions into perspective with regard to the range of
conditions throughout the four states. This makes it difficult to determine the
representativeness of scenario estimates for the complete range of crop uses.
Crop scenarios do not consider application of more than one pesticide. The pesticide
labels NMFS reviewed had few restrictions regarding the co-application (i.e., tank
mixture applications) or sequential applications of other pesticide products containing
different a.i.s. Also, there were few restrictions for those pesticides containing
ingredients that share a common mode of action (e.g., cholinesterase-inhibiting
insecticides). For example, we saw no restrictions that would prevent either coapplication
or sequential application of products containing carbofuran, carbaryl, and
methomyl. Examples of fish kill incidents discussed in the Risk Characterization section
of the Opinion indicate combinations of cholinesterase-inhibiting insecticides are
sometimes applied on the same day or over a short interval, increasing the likelihood of
salmonid exposure to chemical mixtures that may have additive or synergistic effects.
Some labels encourage the use of more than one product. The Sevin Brand XLR Plus
Carbaryl Insecticide (EPA registration No. 264-333) advises that 8 applications of
carbaryl at 3 d intervals “may not provide adequate levels of protection under conditions
of rapid growth or severe pest pressure. The use of an alternative product should be
considered in conjunction with this product.” Multiple applications of pesticides increase
the likelihood of cumulative exposure. We considered cumulative exposure based on
generated 60 d time-weighted average concentrations to simulate situations where
pesticide products containing the three a.i.s were applied at separate times during the
growing season (Table 49). To address potential variability between sites, we generated
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