Environmental Engineering Reference
In-Depth Information
regulators) for carbofuran was 0.21 mg/kg bw, based on data from 18 species. This value is within
the 95% confi dence interval measured for the LD
50
in the fulvous whistling duck, the most sensitive
species tested to date. Unlike the situation that prevails with the majority of cholinesterase-inhibiting
pesticides (Mineau, Collins, Baril et al. 1996), carbofuran does not show very much scaling of toxic-
ity in relation to bodyweight. Hence, the value of 0.21 mg/kg bw can be used as a 'reasonable worst
case estimate' for risk assessment purposes for any avian species regardless of body weight. A 20g
songbird at that sensitivity level would have an Aspirin
®
™ index of approximately 46 800.
The toxicity of carbofuran can also be expressed as the calculated concentration (C) in the feed that
would kill half of the test organisms exposed over a standardised feeding (often 5 days) period, referred
to as the LC
50
. The LC
50
for the technical grade material ranges between 21 and 746 ppm (FMC
1972; FMC 1976; Hill, Heath, Spann, et al. 1975), depending on the species tested and the age of the
birds involved. Mallard in these tests proved more sensitive than ring-necked pheasants or northern
bobwhites. However, these data are perhaps of limited use. Laboratory based, short-term feeding
tests, with the more acutely toxic carbamate and organophosphate insecticides generally appear to
underestimate the hazard posed, and the test itself is faulted and has been largely discredited (Mineau,
Jobin and Baril 1994).
2.2 Exposure routes for the liquid formulation
Wildlife can be exposed to liquid carbofuran formulations in different ways. Oral exposure through
the ingestion of contaminated food is an obvious route conceptually and, indeed, dietary exposure
has historically received most of the attention (e.g., Kenaga 1973; Urban and Cook 1986). It is the
only route currently considered by regulators in routine assessments (e.g., EFSA 2008, US EPA
2011). However, wildlife can also ingest residues directly, when preening feathers (or grooming fur)
and when drinking from contaminated water sources. In addition, dermal exposure can occur via
direct contact during spraying, contact with contaminated surfaces after spraying, or through inha-
lation of fi ne droplets or vapour. Work on the organophosphates fenitrothion and methyl parathion
(Mineau, Sundaram, Sundaram et al. 1990; Driver, Ligotke, Van Voris et al. 1991) indicates that
dermal uptake and inhalation can be signifi cant routes of exposure in wild birds. However, the rela-
tive importance of each route of exposure is likely to vary tremendously in relation to the chemical
involved and the particular exposure scenario (Mineau 1991). Hayes and Laws (1991) indicated that
ocular exposure to carbofuran has also caused death in rabbits; however, this route does not appear
to have been studied in birds.
Given the number of cases of poisoning noted for waterfowl grazing on sprayed vegetation (see
Chapter 8), it may be useful to consider common application rates for liquid formulations trans-
formed into convenient units of poisoning potential. Based on LD
50
values available for mallard,
and application rates registered in Canada for various crops, we have calculated the area of spray
deposit which would represent the LD
50
(assuming negligible drift or volatilisation; Table 2.3). This
is a relevant value, whether the dose is ingested with sprayed vegetation, via drinking from a puddle
of water, or when a sprayed bird preens. Two scenarios are given: one for a 3-day-old mallard
duckling, and the other for an adult mallard. We assume that the relative toxicity of the formulated
material to technical material documented by Hill (see Section 2.1) applies to all species and age
groups equally.
Most impressive in the calculations outlined in this table is that, at the highest rate registered in
Canada, a mallard duckling's LD
50
would be applied to a surface of less than 1 cm
2
. The mallard is
also the least sensitive of the two waterfowl species tested by Hudson and colleagues (1972). Whilst
these calculations are rather crude/simplistic, they do give a simple indication of just how potentially
toxic carbofuran is to wildlife.
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