Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 199

Fig. 9 Percentage of bird
species affected versus
percent mortality for fl owable
chlorpyrifos applied airblast
to orange orchards at a rate of
6.28 kg ha −1 (5.6 lb A −1 )

may be overestimating the risk CPY poses to birds, particularly in those crops with
the highest application rates (i.e., grapefruit and orange). There are several potential
reasons why LiquidPARAM may be overestimating acute risks of CPY to birds.
For example, acute effects metrics were based on single-dose, oral gavage studies
that likely overestimate the toxicity that birds would experience when consuming
small amounts over the course of a day, as typically occurs in treated fields
(see Sect. 5.3 ). In addition, the exposure model assumed that proportion time in
treated fi elds equates to proportion diet obtained from treated fi elds. However, it
may be that many bird species obtain a relatively higher proportion of their diet
from higher quality edge habitats.
The results of fi eld studies consistently demonstrated that fl owable CPY has
negligible effects on birds at rates well above the application rate of CPY predicted
by Mineau ( 2002 ) (i.e., 0.19-0.26 kg ha −1 (0.17-0.23 lb ai A −1 )) to have a 1/10
probability of an avian kill.

6.5 Strengths of the Refi ned Risk Assessment

for Flowable CPY

LiquidPARAM explicitly accounts for factors affecting exposure of birds to fl ow-
able CPY in the fi eld. These factors include: application rates, number and types of
applications, foraging patterns, preferred diets, CPY concentrations on dietary
items over time and space, rates of metabolism, and avoidance behavior.
In several instances, LiquidPARAM refi ned the approach used by EPA's TIM
(USEPA 2005 ). For proportion of time foraging in treated fi elds, TIM uses data that
represent inter-fi eld variability as intra-fi eld variability. In estimating food intake
rate, TIM uses distributions for several minor input variables (e.g., gross energy and
assimilation effi ciency of dietary items), but treats the input variable with the greatest
uncertainty (i.e., free metabolic rate, the amount of calories consumed by free-living