Contents - Faperta

Transgenic Resistance to Insects: Interactions with Nontarget Organisms 341
measure the ecological effects of transgenic plants on nontarget organisms. Endogenous
production of Bt toxins in all parts of the plant throughout the crop-growing season may
lead to a different mode of exposure of herbivores, and thus their parasites and predators.
Also, altered metabolism of plants (by virtue of production of toxin protein or insertion of
the novel gene) may also affect the interaction between the plants, insects, and their natural
enemies. Therefore, there is a need for long-term studies involving insect-resistant
transgenic plants, including lethal, sublethal, and chronic effects in nontarget organisms.
Bt Sprays, Transgenic Plants, and Nontarget Organisms
The Bt spores and toxins are degraded quickly. Under UV light, the toxins decay within a
few hours to a few days, while the spores need soil for germination (Schutte and Riede,
1998). In comparison, the toxin proteins in transgenic plants are produced throughout the
crop cycle in all parts of the plant. Formulations based on Bt have shown toxic effects
against some nontarget insects (Wagner et al., 1996), but only a few of the parasitoids and
predators may be directly harmed by Bt sprays (Flexner, Lighthart, and Croft, 1986; Deml
and Dettner, 1998). Application of Bt as foliar sprays does not cause any harmful effects to
microorganisms or to vertebrates (Kreutzweiser et al., 1996). Bt sprays have shown negative
effects against 24 species each of parasites and predators, but no adverse effects have
been observed against 78 predators and 87 parasitoids (Deml and Dettner, 1998). Indirect
effects through nutritional quality of the prey have been observed on 12 predators and
eight parasitoids. High levels of Bt toxicity have been observed only against two species,
but low levels of toxicity have been observed against 29 species. Toxins from B. thuringiensis
var. tenebrionis have shown no effects against 9 of the 11 species examined, but high
mortality and reduced fecundity have been observed in two species. Under fi eld conditions,
Newleaf (Cry3Aa) potato plants have been found to be highly effective in suppressing
populations of Colorado potato beetle, Leptinotarsa decemlineata (Say) and provided
better control than weekly sprays of a microbial Bt-based formulation containing Cry3Aa,
without reducing the number of the generalist predators, and thus is compatible with integrated
pest management (IPM) programs (Reed et al., 2001).
Transgenic plants express semiactivated proteins, which are ingested by the insects. In
this process, no crystal solubilization and protoxin-toxin conversion is necessary in the
insect midgut. Increased activity has been observed in some herbivorous insects when fed
with toxin instead of the protoxin, suggesting that such insects lack the appropriate
enzyme composition or pH that converts the protoxin to toxin (Moar et al., 1995). The truncated
and activated toxins expressed in transgenic plants constitutively in high doses kill
the insects much faster than the Bt sprays, which need to be activated by the insect enzymes.
Arthropod species that have the binding sites but cannot activate the toxins could be
harmed by the toxins from the transgenic plants. However, Bt toxins can be activated in
different ways, leading to a change in their specifi city (Haider, Knowles, and Ellar, 1986).
While Bt sprays might affect all the insects in an ecosystem wherever they are sprayed, the
toxins from the transgenic plants affect only the insects that ingest the toxins from the
plants through feeding or indirectly through the insect host or prey (Figure 11.1). Therefore,
toxicity assays from Bt sprays cannot be substituted for the toxins expressed in transgenic
plants. The Bt protein molecule is quite complex, and its mode of action on nontarget
organisms is not understood properly. Little information is available on mode of action in