In general, GM trees should be less directly damaging to natural enemies of pests than are traditional pesticides (Raffa et al. 1997). However it seems inevitable that any human intervention to protect crops, reducing the pest population, will entail some disruption of tritrophic systems. Resistant transgenic plants are likely to lead to a reduction in host quality and so may affect larval parasitoids indirectly: this can occur both with Bt plants and with plants expressing enzyme inhibitors and lectins, which are generally less resistant to pests but affect a wider range of species (Schuler et al. 1999). Two reports showing adverse effects on lacewings feeding on larvae that had ingested Bt corn (Hilbeck et al. 1998) and on the monarch butterfly through pollen consumption (Losey et al. 1999) received considerable attention, but both reports were criticized for their inappropriate design, methodology and interpretation (Hodgson 1999; US EPA 2000). These studies demonstrate the difficulty in performing laboratory assays that have relevance in the field, as interactions in laboratory, although dramatic, may not be realistic in the field (Shelton et al. 2002). Laboratory test species should be selected, considering their ecological role in the agro-ecosystems and tests should be carried out under ecologically realistic laboratory 'worst case' scenarios (Lovei and Arpaia 2005). Also, the risk assessment on the potential adverse effects on soil organisms of GM crops is especially difficult because our basic understanding of nutrient cycling, detritivore taxonomy and ecology, and soil biochemical and biophysical processes is limited (Raffa et al. 1997). The effects depend on the gene product and the soil properties: Bt toxins and proteinase inhibitors active against Coleoptera pose some concern, as beetles are important components of the soil fauna. The consequences deriving from the presence of a stable gene product must also be considered, as buildup in the soil over long periods may represent a problem (Raffa et al. 1997). A comparison between soil organisms found in fields cultivated with Bt and non-Bt crops and between soils amended with Bt or non-Bt biomass showed no significant differences (Saxena and Stotzsky 2001).
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