More Research Needed on the Effects of Bt Crops on Aquatic Ecosystems


Dear Friends and Colleagues

More Research Needed on the Effects of Bt Crops on Aquatic Ecosystems

‘Bt crops’ is the collective term for crops which have been genetically modified to include a gene (or genes) sourced from Bacillus thuringiensis (Bt) bacteria, which code for insecticidal Cry proteins. Bt toxin-producing crops have been used for controlling pests of Lepidoptera, Coleoptera, Diptera and Hymenoptera, as well as nematodes.

Despite growing recognition that aquatic ecosystems near agricultural fields receive significant amounts of run-off and crop residues that contain Bt toxins, environmental risk assessments of transgenic crops tend to neglect aquatic ecosystems as a relevant context for testing.

A recent paper reviews literature related to exposure, spread, break-down rates and effects of various types of Bt crop material on non-target organisms and aquatic communities. It finds that there are significant knowledge gaps on the fate of Bt crops and their potential effects on aquatic systems and identifies several important issues for further research:

  • The effect of other Bt genes/toxins and crops on aquatic organisms besides just single gene events (mostly Cry1Ab) in maize or rice.
  • The possible effects of stacked events.
  • The patterns and concentration levels of both endogenous proteins and Bt toxins in Bt crops, especially stacked events.
  • The combinatorial effects of Bt and herbicide co-technology.
  • Determining which aquatic species are relevant to test.
  • Acute dose-response toxicity testing of Bt on relevant aquatic species.
  • Conducting pulsed, chronic exposure studies rather than single exposure studies.
  • Conducting field studies is necessary.

The Abstract and Conclusions and Recommendations of the paper are reproduced below.

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by Hermoine J. Venter and Thomas Bøhn
Environmental Toxicology and Chemistry
DOI 10.1002/etc.3583


Bt crops collectively refer to crops which have been genetically modified to include a gene (or genes) sourced from Bacillus thuringiensis (Bt) bacteria. These genes confer the ability to produce proteins toxic to certain insect pests. The interaction between Bt crops and adjacent aquatic ecosystems has received limited attention in research and risk assessment, despite the fact that some Bt crops have been in commercial use for 20 years. Reports of effects on aquatic organisms such as Daphnia magna, Elliptio complanata and Chironomus dilutus suggest that some aquatic species may be negatively affected; while other reports suggest that decreased use of insecticides precipitated by Bt crops may benefit aquatic communities. In the present study, we consider the literature regarding entry routes and exposure pathways by which aquatic organisms may be exposed to Bt crop material, as well as feeding trials and field surveys which have investigated the effects of Bt-expressing plant material on such organisms. We also discuss how Bt crop development has moved past single gene events, towards multigene stacked varieties which often contain herbicide resistance genes in addition to multiple Bt genes, and how their use (in conjunction with co-technologies such as glyphosate/Roundup) may impact and interact with aquatic ecosystems. Lastly, we make suggestions for further research in this field.

Conclusions and Recommendations

Bt and HT crops are in many ways the pioneers of the genetically modified crop movement. The reasons for highlighting the gaps in monitoring and risk assessment are not simply to determine whether these specific crops themselves are problematic, but to consider gaps in the risk assessment of GM crops generally. Almost 10 years of planting Bt crops passed before aquatic ecosystems were seriously considered within risk assessment, and another decade has passed since then with limited improvement. Decline of aquatic biodiversity due to pesticides and agrochemicals is well-documented, but despite this, approximately 90% of major agricultural areas worldwide have not been included in investigations of pesticide concentrations in surface water, and the possible contribution of Bt crops to this situation has not been considered. There are significant knowledge gaps for the fate of Bt crops and their potential effects in aquatic systems. We recommend the following issues to be further investigated:

I. The “other” Bt genes/proteins: The research relating to aquatic organisms which has been done up to now has primarily focused on single gene events (mostly Cry1Ab) in maize or rice.

Other crops and Bt genes/toxins should also be investigated. This includes the amount of plant material that reaches the aquatic system, the rates of toxin release and break-down, and the influence of factors such as temperature, biotic and abiotic factors in the environment, and potential effects on aquatic organisms.

II. Effects of stacked events: Risk assessment is currently operating under the assumption that a stacked event is no more than the sum of the parts of the single genes which were combined to make the stack, and that there are no interactions between the components. However, this is disputed. Instead of relying on assumptions, the possible effects of stacked events (some of which have yet to be investigated as single events, see point above) should be researched.

III. Bt-expression and unintended effects: Patterns and concentration levels of both endogenous proteins and Bt toxins in Bt crops, especially stacked events, merit further investigation. Variability in expression may be altered by the event, the environmental conditions, co-technologies used (e.g. herbicides), climate, stress, etc. Cases of adverse effects when the Bt toxin was determined to be inactive may indicate possible unintended (pleiotropic, insertion and position) effects in GM Bt crops. High throughput sequencing, proteomics and metabolomics may all shed light on such effects, which are not only of interest in terms of environmental effects, but for the refinement of the genetic engineering process.

IV. Combinatorial effects of Bt and herbicide co-technology: Though the present study has focused on Bt crops, in many cases the stacked nature of Bt HT crops makes the issue of their potential environmental effects inseparable. The potential additive or synergistic effects of Bt and herbicide co-technologies should be investigated.

V. Determine which aquatic species are relevant to test: Several perspectives on which organisms to test exist, but what is still missing for aquatic ecosystems are exposure surveys such as those performed by Yu et al. which measured the amount of Bt protein present in terrestrial arthropods in Bt soybean fields. The advantage of this approach is that it provides baseline data of which organisms are exposed, illuminates possible tri-trophic relationships and food web interactions, and may also indicate community-level responses. Investigations of the levels of co-technology herbicides present would be an interesting expansion of such studies.

VI. Acute dose-response toxicity testing of Bt on relevant aquatic species: Dose-response experiments including additional control groups fed non-transgenic material with purified Bt toxin added, would assist in distinguishing between effects of Bt-toxin from other effects of the genetic modification process. This may also assist in recognising potential alternative modes of action of Bt-toxins. Modes of action of Bt toxins in adversely affected non-target organisms outside of the target order are largely undescribed.

VII. Pulsed, chronic exposure studies: For crops such as maize, the entry of plant material and Bt protein into the aquatic system is likely to take place fairly consistently, with spikes at times of pollen shed, harvest, or after rain. Studies which reflect this pattern are more likely to reflect natural conditions than single exposure regimes. The consideration of plant material as a delivery system for herbicides and pesticides (in addition to Bt toxins) to aquatic ecosystems, and the role of riparian buffers in limiting this, should also be investigated.

VIII. Field studies. In nature, individuals and populations often live under stress by competitors, predators, parasites, etc. so that even a small additional stressor may turn out to be critical for their survival. Amphibians die from much lower levels of certain pesticides when they are under stress (predator present) than without the predator. Laboratory experiments are thus not representing ‘worst-case-scenarios’, since the natural stress factors and variation in the environment is changed to stable and mostly favourable conditions in the laboratory. Therefore, an integrated approach based on test protocols in the laboratory should be linked to food web characteristics and trophic roles as well as early warning systems and computer simulation models. Field studies under realistic conditions are difficult but necessary.

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