Environmental Engineering Reference
In-Depth Information
Table 2.5
Frequency distribution of carbofuran granules in the gastrointestinal tracts of 555 birds
poisoned in Utah cornfi elds
Number of granules in the gastrointestinal tract,
and % of those dead containing that number
Sample
size
Species
0
1
2
3 to 5
6 to 10
>10
Horned lark
(
Eremophila alpestris
)
18.0
20.3
15.4
23.8
12.8
9.70
479
Brown-headed cowbird
(
Molothrus ater
)
64.5
16.1
6.5
6.5
6.4
0
31
Yellow-headed blackbird
(
Xanthocephalus xanthocephalus
)
50.0
10.0
15.0
15.0
10.0
0
20
All birds
22.2
18.9
14.4
22.7
12.3
9.50
555
to be biased toward indicating a smaller number of granules, since it does not include granules that
passed through the birds' digestive tract (although, given the speed of onset of toxicity, this may not
be a serious bias).
The maximum number of granules found in one horned lark was 53. The median number in
horned larks was two, and 22.5% individuals of this species had ingested more than fi ve granules.
From a diagnostic point of view, it is interesting that
no
granules were found in almost one quarter
of all the casualties.
A study by the Bayer Corporation (Fischer and Best 1995) investigated the ingestion of blank
(pesticide free) silica granules in birds. These authors found no relationship between the number of
granules on the soil surface and the mean gizzard count in a group of blackbird species. Such a rela-
tionship did, however, exist for a group of sparrow species. They found that a reasonable 'worst case
estimate' for silica granule consumption by small songbirds was 25 granules per day.
2.3.2 Contaminated soil invertebrates
It has been recognised for many years that carbofuran is extremely toxic to earthworms, and that
dead worms can contain enough residue to kill fl ies attracted to the carcass (e.g., Kring 1969). Also,
carbofuran granules can adhere to earthworms. Even when granules are washed off the worms,
whole body residues can range between 0.3 and 670 ppm (in 11 of 12 worms analysed following
a 1.1 kg a.i./ha in-furrow application to corn; Balcomb, Bowen, Wright et al. 1984b). The average
residue in this study was 84.7 ppm. At the maximum level reported, ingestion of a single large
mature worm (
Lumbricus terrestris
: 15 cm, 5 g; Cathey 1982) would result in a dose of 43.5 mg/kg
bw for an average-sized American robin (
Turdus migratorius
: 77 g; Dunning 1993). At the aver-
age residue level reported, a single worm would represent a dose of 5.50 mg/kg bw. Given that this
value is higher than most LD
50
values reported for songbirds, we can conclude that a worm with an
average body burden is likely to be lethal to an adult robin. The risk would obviously be greater if
granules physically adhered to the worm. The risk to nestlings would also be higher because of their
small body size and high food consumption rate relative to their size. A similar hazard is predicted
for foraging American woodcock (
Scolopax minor
; Eisler 1985). In the United Kingdom, a buz-
zard (
Buteo buteo
) found dead with earthworms in its beak has also tested positive for carbofuran
(Fletcher, Hunter, Greig-Smith et al. 1989). No details were given regarding any pesticide applica-
tion involved; it is therefore unclear whether the granular or fl owable formulation was implicated.
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