Carbaryl, Carbofuran, and Methomyl - National Marine Fisheries ...

The second line of evidence is whether field level reductions in aquatic invertebrates correlate to
N-methyl carbamate insecticide use and/or concentrations in salmonid habitats. We found
several examples supporting this line of evidence. The available laboratory and field data show
reductions in aquatic invertebrate taxa and reductions in invertebrate abundances following
applications of carbofuran and carbaryl (Table 67). Furthermore, field studies explicitly
investigated whether real world applications and subsequent pesticide drift and/or runoff into
aquatic environments affected receiving water aquatic communities. One compelling study
measured pre- and post-application aquatic community responses from field applications of
carbaryl at 0.75 and 1 lb/acre (these rates are at the lower end of approved maximum application
rates, up to 12 lbs/acre) (Courtemanch and Gibbs 1980). Applications correlated to substantial
drift of salmonid prey type aquatic invertebrates, of which most were dead or dying. More
striking was that population reductions of plecoptera (stoneflies) persisted for sixty days
(Courtemanch and Gibbs 1980). Runoff from fields treated with carbofuran at 2.68 lbs a.i./ acre
contained concentrations as high as 264 μg/L in the field ditches that drained to a stream where
carbofuran was measured at 27 μg/L (Matthiessen, Shearan et al. 1995). Caged amphipods
present in the stream all died during peak runoff following a rain event one month after the
carbofuran application. This indicated that carbofuran can persist in soil and mobilize by rain
into subsequent runoff and ultimately elicit toxic responses from stream invertebrates
(Matthiessen, Shearan et al. 1995). Although we found no field studies with methomyl, we
expect that it would similarly reduce aquatic invertebrate populations at higher concentrations
than carbaryl and carbofuran based on its toxicity profile.
The third line of evidence we evaluated was whether salmonids showed reduced growth in areas
of low prey availability, particularly those areas coinciding with use of carbaryl, carbofuran, and
methomyl. An evaluation of this line is complicated by multiple factors affecting habitat quality
i.e., water quantity, quality, riparian zone condition, etc., which in turn affects prey items and
salmonids. We were unable to locate information attributing reduced growth in salmonids to
specific insecticide exposures that reduced prey, as most studies focused on measuring direct
effects on salmonids or direct effects on invertebrates (see review by Schulz 2004). However,
there are multiple field experiments and studies that demonstrate reduced fish growth resulting
from reduced prey availability (Brazner and Kline 1990; Metcalfe, Fraser et al. 1999; Baxter,
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