The Quotient Method (QM), a laboratory-based risk assessment methodology
used by the Environmental Protection Agency to evaluate pesticides for registration and
use, has not been thoroughly field-tested and its performance has not always been
reliable. My objective was to determine if variation in vegetation structure or diet of
exposed animals could result in adverse ecological effects that were not predicted by
the QM. In April and early May 1993, I established populations of herbivorous
gray-tailed voles (Microtus canicaudus) and omnivorous deer mice (Peromyscus
maniculatus) in 24 0.2-ha enclosures planted with alfalfa (Medicago sativa). Alfalfa in
12 enclosures was mowed on 22 June to reduce vegetation height. Small mammal
populations were monitored by live trapping from May through August 1993. On 14
July, an organophosphorus insecticide, azinphos-methyl, was applied at 0, 0.88, and
3.61 kg/ha. Insecticide residues were measured on canopy-level spray cards, soil
samples, and alfalfa. I compared the observed residue concentrations with predictions
based on the nomogram used to estimate exposure for QM risk assessments. I also
compared QM predictions of risk with observed effects on population size and growth,
survival, reproductive activity, recruitment, body growth, movements, and diet of the
small mammals.
Much of the insecticide reached ground level in mowed enclosures, but dense
alfalfa intercepted most of the spray in unmowed enclosures. The mean half-life of
azinphos-methyl on alfalfa was 3.4 days and was not affected by mowing. Mean
residue concentrations on mowed alfalfa and the top 15 cm of unmowed alfalfa were
underestimated by the QM exposure nomogram. Therefore, pesticides may pose greater
risk to organisms inhabiting sparse vegetation or the tops of plants than predicted by the
QM.
Treatment with azinphos-methyl at 3.61 kg/ha caused severe effects in both
mowed and unmowed enclosures on population size and growth, survival, recruitment,
and body growth of voles. Effects of azinphos-methyl on vole recruitment and body
growth and on survival of female voles were greater in mowed than in unmowed
enclosures. However, I did not find that population-level responses of voles to the
chemical differed between mowing treatments. Most effects on voles were of short
duration (<27 days) but vole densities in 3.61 kg/ha enclosures remained depressed >6
weeks after spraying. The 3.61 kg/ha application rate resulted in a 42% decrease in
deer mouse densities in mowed enclosures during the week of spraying, but the
insecticide had no adverse effects on deer mice in unmowed enclosures. In addition,
the insecticide may have reduced recruitment of deer mice in mowed enclosures.
Analysis of deer mouse feces indicated that consumption of arthropods increased in
insecticide-treated enclosures just after spraying occurred. Survival, reproductive
activity, body growth, and movements of deer mice were highly variable and not
significantly affected by azinphos-methyl.
Mowing resulted in greater residue concentrations than predicted and,
consequently, the insecticide adversely affected voles and deer mice in mowed
enclosures at application rates characterized as low risk by the QM. However, food
aversion or selective feeding on alfalfa tops may have resulted in similar exposure of
voles to the 3.61 kg/ha treatment in mowed and unmowed enclosures. I did not find
that insectivorous feeding behavior of deer mice made them more susceptible than
predicted. Although residue concentrations on alfalfa did not follow predictions, the
gross pattern of effects on small mammals was consistent with QM risk
characterization. However, the QM may underestimate exposure and risk when
pesticides are sprayed on sparse vegetation. / Graduation date: 1995
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/35201 |
| Date | 28 September 1994 |
| Creators | Schauber, Eric M. |
| Contributors | Edge, W. Daniel |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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