Effects of the cultivation of genetically modified Bt crops on nontarget soil organisms - Microbial Ecology in Sustainable Agroecosystems - page 208

shown to have more than 50 times the level of Cry1Ab found in Bt maize Events MON810
and Bt 11 (EPA, 2001a). When the EPA registration of Bt maize Event 176 expired in 2001,
it was not renewed by the producing seed companies, primarily as a result of concerns
about pest resistance, and is thus no longer grown in the United States (EPA, 2011).
Insect larvae in aquatic ecosystems may also be impacted by the cultivation of Bt
crops as a result of the transport and accumulation of transgenic Bt biomass in rivers
and streams (Douville et al., 2007; Rosi-Marshall et al., 2007; Prihoda and Coats, 2008;
Swan et al., 2009; Chambers et al., 2010; Jensen et al., 2010; Tank et al., 2010; Wolt and
Peterson, 2010; reviewed by Viktorov, 2011). In the Midwestern part of the United States,
where most of the nation's corn crop is grown, Bt plant biomass is often deposited into
aquatic ecosystems by wind and transported downstream, where it can accumulate in
the bends or edges of the waterways (Tank et al., 2010) and in sediments (Douville et
al., 2007). Invertebrate consumers are abundant in agricultural streams (e.g., Moore and
Palmer, 2005; Menninger and Palmer, 2007) and are likely to feed directly on decaying
plant material, including Bt maize, in waterways. Tank et al. (2010) reported that 86%
of 217 stream sites surveyed in Indiana, United States, in 2007—when 75% of all maize
cultivated in the United States was engineered to express single or stacked combinations
of insecticidal or herbicide resistance traits (USDA, 2010)—contained plant material from
maize, including leaves, cobs, husks, and stalks. Thus, it was not surprising that Cry1Ab
protein was detected in the water column at 23% of these sites. What was unexpected,
however, was that Cry1Ab was detected in streams even in areas where no maize fields
were present within 500 m, indicating that Bt toxins may be more widely distributed
in aquatic ecosystems than previously realized. As these stream sites were sampled 6
months after the corn fields were harvested, levels of Cry1Ab (and other Bt proteins)
would probably be even higher immediately following harvest when plant biomass is
fresh. Douville et al. (2007) reported that Cry1Ab gene from Bt maize cultivation could
be detected in surface water and sediments for more than 21 and 40 days, respectively.
The Cry1Ab protein was found in higher concentrations in the clay and sand-rich sedi-
ments than in surface water and could still be detected at least 82 km downstream from
the Bt maize plot, although the Bt concentration tended to decrease the farther away
from the plot it was detected.
Some of the first evidence of the potential sensitivity of some aquatic insects to Bt bio-
mass was reported in a laboratory experiment; caddisfly larvae fed Bt maize residue grew
less and had higher mortality than larvae fed non- Bt maize residue (Rosi-Marshall et al.,
2007). Caddisflies are closely related to target pests, and the larvae are an important food
source for fish and other marine life. Chambers et al. (2010) reported that the aquatic leaf
shredder Lepidostoma liba grew more slowly when fed Bt maize residue in laboratory trials.
However, no negative effects were detected on the abundance or diversity of natural popu-
lations of nontarget aquatic invertebrates, including Coleoptera, Diptera, Ephemeroptera,
Hemiptera, Odonata, Plecoptera, and Trichoptera in a field study, possibly because the Bt
plant biomass was already highly degraded at the time of sampling (Chambers et al., 2010).
In a laboratory feeding study, differences in the composition of plant tissue from different
maize cultivars, rather than a direct toxic effect of Cry protein in the Bt maize varieties,
were found to affect the growth and survivorship of nontarget aquatic detritivores (e.g.,
leaf-chewing shredders), including crane fly larvae and the aquatic isopod Caecidotia com-
munis (Jensen et al., 2010). Until recently, most risk assessment research has focused on the
nontarget effects of Bt crops in the terrestrial environment. Thus, it is largely unknown
what long-term effects, if any, could be expected on nontarget aquatic invertebrates and