THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE
Dear friends and colleagues
Re: Aphids Attack GM Cotton
Many ecologists and scientists have long warned about the potential of secondary pests emerging from the mass cultivation of crops genetically engineered with Cry proteins derived from Bacillus thuringiensis (Bt) in order to kill target pests. Bt cotton farmers have for some time now reported the emergence of secondary pests, such as aphids. This has been attributed to reduced pesticide use by various quarters.
A recent study has shed new light on this problem by establishing a link between the rise in aphids in Bt cotton and a weakened plant defence system. The study, published in the journal Proceedings of the Royal Society B in March 2013, found that Bt cotton plants had lower levels of defence compounds known as terpenoids that are normally released in response to caterpillar attacks. However, as Bt cotton plants kill the caterpillars, the lowered terpenoid levels make the plants an easy target for aphids which are Bt-insensitive.
The researchers studied the correlation between Bt and non-Bt cotton plants, defence compound levels and aphid populations under both glasshouse and field conditions. They found a positive correlation between reduced terpenoid levels and aphid populations under the former and a lesser link under the latter.
The implications of this study are particularly important─ the findings have provided evidence of non-target herbivores developing into (secondary) pests when primary pests are addressed in isolation, such as in Bt cotton crops. This phenomenon is similar to that which first arose (and continues to arise) with the persistent use of chemical pesticides and industrial agricultural systems that fail to take into account ecological balance.
The full report can be downloaded here: http://www.scidev.net/en/agriculture-and-environment/gm-crops/news/use-of-gm-cotton-linked-to-rise-in-aphid-numbers.html.
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Item 1
http://rspb.royalsocietypublishing.org/content/280/1758/20130042.abstract
Pest trade-offs in technology: reduced damage by caterpillars in Bt cotton benefits aphids
Steffen Hagenbucher[1], Felix L. Wäckers[2], Felix E. Wettstein[1], Dawn M. Olson[3], John R. Ruberson[4] and Jörg Romeis[1]
[1] Agroscope Reckenholz-Tänikon Research Station ART, Reckenholzstrasse 191, 8046 Zurich, Switzerland
[2] Lancaster Environment Centre, Lancaster University, Lancaster LA14YQ, UK
[3] Crop Protection and Management Research Unit, USDA-ARS, Tifton, GA, USA
[4] Department of Entomology, University of Georgia, Tifton, GA, USA
e-mail: joerg.romeis@art.admin.ch
Abstract
The rapid adoption of genetically engineered (GE) plants that express insecticidal Cry proteins derived from Bacillus thuringiensis (Bt) has raised concerns about their potential impact on non-target organisms. This includes the possibility that non-target herbivores develop into pests. Although studies have now reported increased populations of non-target herbivores in Bt cotton, the underlying mechanisms are not fully understood. We propose that lack of herbivore-induced secondary metabolites in Bt cotton represents a mechanism that benefits non-target herbivores. We show that, because of effective suppression of Bt-sensitive lepidopteran herbivores, Bt cotton contains reduced levels of induced terpenoids. We also show that changes in the overall level of these defensive secondary metabolites are associated with improved performance of a Bt-insensitive herbivore, the cotton aphid, under glasshouse conditions. These effects, however, were not as clearly evident under field conditions as aphid populations were not correlated with the amount of terpenoids measured in the plants. Nevertheless, increased aphid numbers were visible in Bt cotton compared with non-Bt cotton on some sampling dates. Identification of this mechanism increases our understanding of how insect-resistant crops impact herbivore communities and helps underpin the sustainable use of GE varieties.
Item 2
Use of GM cotton linked to rise in aphid numbers
Richa Malhotra
8 April 2013
In an unexpected trade-off, the cultivation of cotton that has been genetically engineered to reduce caterpillar damage by producing its own insecticide has been linked to higher numbers of another pest — aphids.
Previous studies had linked the increase in aphids to reduced insecticide use by farmers cultivatingBt (Bacillus thuringiensis) cotton.
Now researchers have for the first time tied the suppressed production of natural defence compounds in Bt cotton — caused by the lack of caterpillar attacks — to a surge in non-target pests such as the cotton aphid.
Non-Bt plants respond to caterpillar infestation by producing defence compounds, which also protect the plant against other pests.
The researchers studied the correlation between Bt and non-Bt cotton plants, defence compound levels and aphid populations in both glasshouse and field conditions.
In the glasshouse, they artificially infested Bt and non-Bt cotton plants with caterpillars and monitored the levels of various defence compounds known as terpenoids that are released in response to caterpillar damage. The plants were then artificially infested with aphids.
In the field, one set of Bt and non-Bt cotton plants were artificially infested with caterpillars and another was left to natural infestation by caterpillars. Terpenoid levels were measured and both sets were then exposed to natural aphid infestations.
In the glasshouse, caterpillars on the Bt cotton plants died. As a result, the plants were less damaged and therefore contained less defence compounds than their non-Bt counterparts.
The researchers thus attribute the resulting increase in aphid populations on these plants, compared with non-Bt plants, to lowered terpenoid production.
Although a relative rise in aphid numbers was noted on a few occasions on Bt cotton plants in the field, the scientists found no correlation between aphid populations and terpenoid levels in this part of the experiment.
Jörg Romeis, one of the paper’s authors and head of the biosafety research group at the Agroscope Reckenholz-Tänikon Research Station ART, Switzerland, tells SciDev.Net, "Our study shows that a technology like Bt cotton should not be used in isolation".
"To control other herbivores we have to use the technology as part of integrated pest management approach."
T. M. Manjunath, a consultant in agrobiotechnology and integrated pest management from India, where Bt cotton is cultivated, says: "What this study shows with Bt cotton can happen with any other insect-control measure. Infestation by aphids and other non-target pests has been seen before the introduction of Bt cotton".
The study was published in the Proceedings of the Royal Society B last month (13 March).
REFERENCES
Proceedings of the Royal Society B doi: 10.1098/rspb.2013.0042 (2013)