Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 184

5.3 Derivation of Effects Metrics

The most realistic route of exposure for acute effects is the dietary exposure pathway.
This pathway is preferred over oral gavage exposures because the latter are only
relevant to situations where active ingredients are ingested rapidly in a single expo-
sure or gorging situation (ECOFRAM 1999 ). Most, if not all, bird species found in
agroecosystems are much more likely to continuously forage for food during the
daylight hours (Best 1977 ; Fautin 1941 ; Kessel 1957 ; Kluijver 1950 ; Pinkowski
1978 ). Birds foraging near agricultural areas are likely to ingest a mixture of con-
taminated and non-contaminated food items throughout a day. Studies in which birds
were exposed through the diet were available for only one of the focal species, the
northern bobwhite (Beavers et al. 2007 ). However, when dietary exposures for northern
bobwhite were converted to dose ingested, there was little evidence of a dose-response
relationship. In that study, birds reduced their food intake rates at higher dietary
concentrations. Because of this issue, the results of oral gavage studies were used to
derive the acute effects metrics in this assessment. Using results of studies that dosed
birds by a single oral dose via oral gavage is highly conservative because:

• Doses are administered as one large dose. In the fi eld, most birds feed continuously
throughout the day.
• Chlorpyrifos is rapidly biotransformed by birds to less-toxic metabolites.
The half-life for metabolism and elimination of CPY is approximately 1-d
(Bauriedel 1986 ). When feeding throughout the day, birds have the opportunity to
detoxify and/or eliminate CPY before it accumulates to internal doses that result
in lethality.
• Repeated exposure to CPY in the diet leads to avoidance (Bennett 1989 ; Fink
1978b ; Kenaga et al. 1978 ; Stafford 2010 ). In the fi eld, birds can switch to
sources of food that are not contaminated with CPY or avoid feeding for short
periods of time. There can be no avoidance with large single doses administered
by intubation during a gavage study.
• In oral exposures, CPY is generally administered in corn oil or gelatin capsules.
Such carriers have been shown to result in greater toxicity with other insecticides
than occurred when the insecticides were adsorbed to food items consumed by
birds in the fi eld (Stafford 2007a , b ). Use of corn oil or gelatin carriers maximizes
the potential for a pesticide to be absorbed rapidly, more so than would occur in
the fi eld where the pesticide is bound to food items. When pesticides are mixed
with food, or when consumed at a time when the gastro-intestinal (GI) tract has
other food items present, they are absorbed less effi ciently than when dosed as a
bolus in pure form into an empty GI tract (Lehman-McKeeman 2008 ).

In this assessment, the preferred effects metrics were dose-response curves for
the focal species of interest. However, acute dose-response curves could only be
derived for two focal species, the northern bobwhite ( C. virginianus ) and the
red-winged blackbird ( A. phoenicieus ). For other focal species, a Species Sensitivity
Distribution (SSD) approach was used. With this approach, the 5th, 50th and 95th