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Tracking the Movements of Transgenic Toxins Through Complex Food Webs
Harwood, J. D., J. J. Obrycki
Department of Entomology
The planting of transgenic crops has increased rapidly since the commercialization of Bacillus thuringiensis corn in the mid-1990's. These crops convey significant benefits to natural enemy communities in agroecosystems through reduced input of broad-spectrum insecticides. However, questions remain on the movement of Bt endotoxins through arthropod food webs and potential subtle effects on fitness parameters in some arthropod species.
This project examines the movement and potential consequence of Bt-endotoxins through complex arthropod food webs in the field. Therefore the purpose of this study is to quantify the uptake of Bt-endotoxins by non-target organisms in transgenic crops and thus aid in the risk assessment of transgenic organisms in the environment.
2009 Project Description
Ongoing training of a graduate student in the agricultural sciences is an integral part of this project. Five presentations have been given at scientific meetings during this reporting period, including national and international conferences.
(1) Quantification of endotoxin exposure pathways in Carabidae. A study published in 2009 quantified exposure pathways in non-target arthropod food webs across multiple transgenic events. Adult ground beetles (Coleoptera: Carabidae) were collected from transgenic corn fields expressing lepidopteran-specific Cry1Ab, coleopteran-specific Cry3Bb1, and both Cry1Ab and Cry3Bb1 (stacked event), as well as a non-transgenic isoline. Significant numbers of carabids tested positive for Cry1Ab from the lepidopteran-specific field, including Harpalus pensylvanicus, Stenolophus comma, Cratacanthus dubius, Clivina bipustulata, and Cyclotrachelus sodalis. The highest proportion of Bt-endotoxin uptake was 4-6 weeks postanthesis. Only one species, H. pensylvanicus, screened positive for Cry1Ab from the stacked line, despite similar expression of this endotoxin in plant tissue harvested from both lines.
(2) Identification of pollen interception frequencies by webs of Linyphiidae. Linyphiidae represent a major component of ecological food webs in agroecosystems; their webs have the potential to intercept pollen grains during anthesis. Over 150,000 corn pollen grains and 5,000 prey items were intercepted at simulated websites. Dates of peak anthesis resulted in pollen counts up to 4,000 grains per website in the interior of the cornfield. SADIE (Spatial Analysis by Distance IndicEs) indicated significant temporal and spatial variability in pollen interception within and outside the corn field. These results revealed potential for dietary supplementation with indicating that pollenivory may be an important component of the feeding biology of linyphiids and risk-assessment of genetically modified crops.
(3) Molecular analysis of generalist predator food webs. In 2009, approximately 1000 Linyphiidae spider webs from each of the four fields were located. Gut-contents will be analyzed using the denaturing gradient gel electrophoresis (DGGE) technique. This molecular tool will determine the composition of prey found in the guts of spiders. Current optimization for DGGE techniques using spider predators and their insect prey is ongoing. These techniques will test the hypothesis that linyphiid spiders have a strong trophic connection with the detrital food web, with Collembola constituting the majority of their prey.
(4) Quantifying interaction pathways in the laboratory. Major interaction pathways are continuing to be inferred using laboratory feeding assays with Cry1Ab corn. Major predator and prey have been established in laboratory facilities. These will be used to identify the movement of endotoxins within the decomposer food chain, and identify levels of compartmentalization and exposure pathways within these systems.
(5) Winter-active foraging in corn agroecosystems. The trophic interactions of winter-active predators and their prey in corn agroecosystems are being examined. As Bt-endotoxins have been known to persist in the soil for up to several months, the winter foraging of predators presents additional opportunities for Bt-endotoxin uptake and exposure.
Peterson, JA, JJ Obrycki & JD Harwood. 2009. Quantification of Bt-endotoxin exposure pathways in carabid food webs across multiple transgenic events. Biocontrol Science & Technology, 19: 613-625.