Research Accomplishment Reports 2007

Tracking the Movements of Transgenic Toxins Through Complex Food Webs

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

 

Project Description

Quantifying the update of Bt-endotoxins by coccinellid predators: A study has been completed and published in 2007 (Bull. Entomol. Res.) quantifying the uptake of Cry1Ab-endotoxins in coccinellid predators from fields of Cry1Ab corn. Using ELISA technology, the movement of Bt-endotoxins was correlated to the phenology of the corn, identifying potential routes of transfer. Adult Coleomegilla maculata, Harmonia axyridis, Cycloneda munda and Coccinella septempunctata all contained significant concentrations of Bt-endotoxins, indicating direct or indirect exposure to Bt-containing food. This was most evident in C. maculata, with 12.8% of 775 individuals testing positive for Cry1Ab-endotoxins. Interestingly, the presence of endotoxins in gut samples was not confined to periods around anthesis, but varied significantly throughout the season (C. maculata: P<0.001). Such variation, peaking at around 40% of individuals containing endotoxins 3-4 weeks after anthesis, suggested tri-trophic linkages facilitate the transfer of endotoxins into these predators. Further experiments are underway to quantify the movement of these proteins, and identify long-term exposure patterns in the field. This contrasts with adult coccinellids entering overwintering sites where no Bt-endotoxins were detected.

Collection of Carabidae and potential prey from fields of Cry1Ab-corn: Fields of Bt-corn (Yieldgard Rootworm, Yieldgard Corn Borer, Yieldgard Plus, and a non-Bt isoline) were planted and separated by commercially recommended distances. From May 2007, carabids and potential prey have been collected using fixed-area traps within corn fields. To date, Harpalus pennsylvanicus (n=348), Evarthrus sodalis (n=9), Cratasarnthus dubius (n=5), Scarites subterraneus (n=35), Chlaenius tricolor (n=13), Pterostichus lucublandus (n=11), Agonoderus pallipus (n=568), Anisodactylus baltimorensis (n=2) and Harpalus caliginosus (n=2) have been collected and are being screened by quantitative ELISA to characterize the exposure to Cry1Ab-endotoxins (results to be presented by graduate student at Entomological Society of America Annual Meeting, December 2007). Major prey (slugs and earthworms) are being collected and analyzed to document the existence and potential trophic pathways for Bt-endotoxin within fields of corn (results will be used to aid in feeding trials with carabids and spiders (see below). Monitoring will continue throughout the fall and winter, examining long-term exposure patterns in the field.

Quantifying interaction pathways in the laboratory: Major interaction pathways are being inferred using laboratory feeding assays with Cry1Ab corn. Prey populations (Mollusca and Collembola) are established in laboratory facilities; predators (Carabidae and Araneae) are being collected and will be used to identify the movement of endotoxins within the decomposer food chain. Results will be used to determine the 2008 field sampling effort.

Optimization of ELISA protocols: ELISA protocols are being optimized to ensure accurate detection of Bt-endotoxins in herbivores, detritivores and predators. This research in ongoing and the results will be used to analyze 2008 populations.

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.