Research Accomplishment Reports 2008

Tracking the Movements of Transgenic Toxins Through Complex Food Webs

Harwood, J. D., J. J. Obrycki
Department of Entomology

 

Non-Technical Summary

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.

2008 Project Description

(1) Role of zoophytophagy in Coccinellidae. A study was published in 2008 (Environ. Entomol.), quantifying the significance of zoophytophagy in Coccinellidae. A series of studies were conducted to examine the probability of feeding on corn seedlings by 3rd and 4th instar coccinellid larvae, the regularity of these feeding events and the effect of leaf feeding on development. Tissue feeding regularly occurred by Harmonia axyridis and Coleomegilla maculata, even though larvae had constant access to water and prey, identifying a new exposure pathway. Furthermore, when given access to corn seedlings, development time increased following Bt-hybrid corn compared to non-Bt corn.

(2) Collection of Carabidae from transgenic corn. Adult carabids collected during 2007 were screened by ELISA to elucidate uptake and transfer of transgenic endotoxins in complex food chains. Significant levels of uptake were documented, with clear differences between transgenic events. These findings have major implications for potential exposure pathways and are examined below.

(3) Collection of Araneae from transgenic corn. Throughout 2008, approximately 800 spiders (Linyphiidae, Lycosidae, Araneidae, Tetragnathidae, Salticidae, Thomisidae, and Theriidae) were collected from four transgenic fields. All will be screened by ELISA to indicate exposure pathways and understand food web processes in transgenic corn.

(4) Identification of pollen interception frequencies by webs of Linyphiidae. A potential exposure pathway in spider food webs is consumption of corn pollen, either directly or via re-ingestion of the web. A 20x20 m grid was established during 2008 and mini-sticky traps were deposited in the field every 24 h and collected. Corn pollen and potential prey on each trap is currently being quantified.

(5) Molecular elucidation of food web processes. Arthropod natural enemies forage within dense and tangled microhabitats, making observations of their interactions with prey or host species, and interpretation of the data thus obtained, difficult. Molecular detection systems are being developed to identify exposure pathways within carabid and spider food webs, to understand mechanisms of foraging and elucidate movement of transgenic toxins in these food webs. COI sequences for prey have been obtained and species-specific primers are being designed. DNA decay rate experiments have been completed for key pathways. Adult carabid beetles were collected each week from May to October 2008 and are currently stored for PCR-based examination of gut content.

(6) 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.

(7) Optimization of ELISA protocols. ELISA protocols have been fully optimized and have been used to examine the movement of transgenic toxins through complex food webs.

2008 Impact

The planting of transgenic crops has increased rapidly since the commercialization of Bacillus thuringiensis (Bt) corn in the mid-1990's. These crops convey significant benefits to natural enemy communities in many agroecosystems but questions still remain regarding the consequence of movement of Bt-endotoxins through arthropod food webs and potential effects on fitness parameters in some species. Field studies have documented temporal variability in Bt-endotoxin uptake in coccinellid food chains and the lack of a direct correlation between anthesis and Bt-endotoxin concentrations in predator guts. These data provide clear evidence for the need for future risk assessment of transgenic crops to non-target food chains in the field, specifically identifying trophic linkages through which endotoxins are most likely to flow and the retention time of Bt-endotoxins following the consumption of Bt-containing food items.

Given that B. thuringiensis var. kurstaki is a ubiquitous and widely distributed bacterium found in the soil, it is possible that some detectable Cry1Ab endotoxins were transferred from native Bt in the soil or from plants. However, the absence of Bt proteins in natural enemies and non-target herbivores from non-transgenic habitats and overwintering sites makes this scenario unlikely. Ultimately the incorporation of laboratory exposure experiments, field population surveys and quantitative assessments of Bt-endotoxin movements through non-target food webs can provide accurate information upon which the safety of bioengineered crops can be assessed. Furthermore, molecular gut-content analysis techniques will be incorporated into field research to elucidate the structure and function of food webs in transgenic corn, ultimately inferring levels of connectedness and exposure pathways in the field.

2008 Publications

Moser, S.E., Harwood, J.D. & Obrycki, J.J. (2008). Larval feeding on Bt-hybrid and non-Bt corn seedlings by predacious coccinellids. Environmental Entomology 37, 525-533.

Published Abstract: Peterson, J.A. & Harwood, J.D. (2008). Fate and exposure pathways of Bt-endotoxins in terrestrial food webs. Chemical & Engineering News vol. 86, issue 26. June 30, 2008.