Abstract
Concerns about possible undesired environmental effects of transgenic crops have prompted numerous evaluations of such crops. So-called Bt crops receive particular attention because they carry bacteria-derived genes coding for insecticidal proteins that might negatively affect non-target arthropods. Here we show a remarkable positive effect of Bt maize on the performance of the corn leaf aphid Rhopalosiphum maidis, which in turn enhanced the performance of parasitic wasps that feed on aphid honeydew. Within five out of six pairs that were evaluated, transgenic maize lines were significantly more susceptible to aphids than their near-isogenic equivalents, with the remaining pair being equally susceptible. The aphids feed from the phloem sieve element content and analyses of this sap in selected maize lines revealed marginally, but significantly higher amino acid levels in Bt maize, which might partially explain the observed increased aphid performance. Larger colony densities of aphids on Bt plants resulted in an increased production of honeydew that can be used as food by beneficial insects. Indeed, Cotesia marginiventris, a parasitoid of lepidopteran pests, lived longer and parasitized more pest caterpillars in the presence of aphid-infested Bt maize than in the presence of aphid-infested isogenic maize. Hence, depending on aphid pest thresholds, the observed increased susceptibility of Bt maize to aphids may be either a welcome or an undesirable side effect.
Highlights
With the rapid expansion of the commercial use of genetically modified (GM) plants, there is an increasing demand for information on their possible impact on non-target organisms
To date several studies on the direct and indirect impact of GM plants on these beneficial insects have been conducted, whereby most emphasis has been on so-called Bt plants, which are crops into which a gene has been incorporated from the entomopathogenic bacterium Bacillus thuringiensis
There were no differences in the mean relative growth rate (MRGR) of individual R. maidis within each of six pairs of transgenic and near isogenic lines tested (T = 212, p = 0.407 for N4640Bt/N4640; T = 258.5, p = 0.290 for TXP138/EXP138; T = 223.5, p = 0.724 for Novelis/Nobilis; T = 255, p = 0.351 for Valmont/Prelude; T = 253, p = 0.395 for Navares/Antares), except for the pair MEB 307Bt/Monumental, where the aphids did not survive on the near isogenic line (T = 345, p,0.001)
Summary
With the rapid expansion of the commercial use of genetically modified (GM) plants, there is an increasing demand for information on their possible impact on non-target organisms. To date several studies on the direct and indirect impact of GM plants on these beneficial insects have been conducted (reviewed by [1,2,3]), whereby most emphasis has been on so-called Bt plants, which are crops into which a gene has been incorporated from the entomopathogenic bacterium Bacillus thuringiensis. An impact on entomophagous insects resulting from this transformation could be due to direct effects of the toxin, indirect effects via reduction in host or prey quantity and quality, or through unintended changes in plant characteristics caused by the insertion of the transgene. The first two potential effects have been widely investigated [1,2,3], but very few studies have looked at the impact of other plant characteristics that may have unintentionally been altered as a result of transformation
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